EMC basics: I/O

EMC Basics #5: I/O as critical circuits article gives some useful tips on the EMC issues related to inputs and outputs.

Digital inputs/outputs — The key concern for digital interfaces is ESD. A secondary concern is radiated emissions. Radiated susceptibility is rare with digital I/O, although possible at very high RF levels. The solutions for both radiated problems include filtering at the interface and/or or shielding of external cables.

Analog inputs/outputs — The key concern for analog interfaces is RF. High RF levels can cause rectification in the I/O circuits causing errors and/or noise. Typical solutions include high frequency filters and/or shielding of the external cables.

Relay outputs — Since relay drivers are usually digital, the regular digital concerns apply. In addition, inductive transients from the relay coils may pose a self-compatibility problem. Snubber circuits may be needed at either the relay (best) or at the driving circuit on the boards.

Contact inputs — Since the receiving circuits are usually digital, the regular digital concerns apply.

When designing or reviewing circuit boards for EMI, ALL of the I/O circuits deserve EMI attention!

I have some additions to those suggestions:

Opto-isolators (also known as optocouplers) work to protect the receiving system at the expense of the sending system needing to drive the cables/interconnects. They are a great way to isolate digital from power circuits but have limited bandwidths. Fairchild Application Note AN-3001 Optocoupler Input Drive Circuits gives some implementation tips for optocoupler based input circuits.


Using a balanced line interface for sensitive and/or fast signal is a very good idea. Using balanced interface reduces EMI pickup and radiated EMI considerably compared to single-ended signals. Applications like telephone lines, analogue instrumentation, professional audio signals, fast serial bus standards and Ethernet all use balanced interfaces to get good noise performance.

Be careful on the grounding of cable shield when they enter the cabinet. The cable shields should be grounded at the point where they enter the metal cabinet. This will stop the RFI from entering inside the device. This advice applies especially to sensitive analogue circuits like audio interfaces. Proper grounding is essential in keeping RFI and ground loop noise away.

In many power controlling applications you can’t beat a relay for isolation or low on-resistance, as well as low cost. For relay outputs you need to carefully consider the need for snubber circuits. When talking about snubber circuits there are two kind of applications for them: Snubber cuircuit in parallel with the relay coil and snubber circuits in parallel with the relay output.

For the relay coil driven with DC voltage at known polarity an inexpensive diode in parallel with the coil works well. If the relay is switched with AC, the DC polarity is not known or you need very fast operation (parallel diode can slow down relay release time).

You need to consider snubber circuit also at the relay contact side especially if you are switching anything that is even slightly inductive. Relay contacts can arch. The end result of Contact Arc Phenomenon is shortened contact life. In addition to that arching causes lots of electromagnetic interference.

Relay Contact Life article tells that perhaps the most popular method of quenching an arc between separating contacts is with an R-C network placed directly across the contacts. Contact Protection and Arc Suppression Methods for Mechanical Relays gives information how to design a suitable R-C network for quenching an arc.


Some relay users connect a diode across the inductive load to prevent counter-voltage from reaching the contacts. In some application zener diodes are used. The MOV performs in a manner similar to back-to-back zener diodes, and can be used in both AC and DC circuits.

An added benefit of arc suppression is the minimization of EMI. An unsuppressed arc between contacts is an excellent noise generator. Arc may radiate energy across a wide spectrum of frequencies. By suppressing the arc, electromagnetic interference is held to a minimum. By quenching the arc quickly, this action is held to a minimum. The result often is a considerably lessened amount of electromagnetic and radio frequency interference. Contact arc noise can be troublesome to sensitive components in a circuit. In worst-case conditions, EMI can cause unwanted turn-on of IC logic gates, SCRs, and triacs, and can cause damage to other semiconductor devices.


  1. Tomi Engdahl says:

    You auto know

    “I understand the purpose of RF input capacitors, but I have never believed that they should be near the power-output pins. I found it hard to believe that EMC radiation would corrupt low-impedance, high-power outputs. Nevertheless, we were religiously placing those capacitors, according to standard automotive-design tenets, at all field connections.”

    “There’s another reason to put those small-value bypass caps near the connector. It has nothing to do with operation in the vehicle. It has to do with ESD protection in the manufacturing process and in handling/storage before being installed and when serviced. Having some extra capacitance on those signal pins (and power also; electrolytic caps have too much internal inductance to help for fast-rise ESD discharges) prevents low to medium ESD events from raising voltages to the level of being destructive to chips. That’s why it’s a “best practices/Lesson learned” item in many automotive OEM suppliers. It’s not something you would be aware of in a high-humidity environment like South India, but if you start getting unexplainable field failures from cars services in, say, a Chicago winter, don’t be surprised!”

  2. Tomi Engdahl says:

    Interference-cost tradeoffs and cheating

    I figure that the manufacturer of the power supply submitted one unit to a testing lab with a filter installed, then after getting certified as meeting the specs for conducted and radiated noise, modified the BOM to remove the filter. Yeah, it saved a buck or two of cost, and most customers would never know the difference. Does your company do this?

    This power supply manufacturer broke the rules, cheated, and caused the kind of interference to a licensed service (in this case, the Amateur Service) that Class B computing devices are not supposed to cause.

  3. Tomi Engdahl says:

    EMI problems? Just the facts, please

    EMI or RFI sources continue to become more prevalent in our world. This type of noise can invade even the low frequency analog circuits. The source of this radiated noise interference can be found wherever electric or magnetic fields exist.

    Take care to create boards and use components that are EMI-resilient, regardless of your analog or digital circuit’s bandwidth. When an EMI source is present in the vicinity of your application circuit, it may create a response to the radiating source.

    How did the radiated noise from the phone get into the measurement with such a low-frequency board? In EMI terms, three elements are at work with this type of problem: a radiation source, a coupling path for the radiation signal to travel through, and a receptor.

  4. Tomi Engdahl says:

    Optocouplers: EMI/RFI mitigation in industrial communications ports

    Optocouplers have most commonly been used to provide safety isolation for compliance with domestic and international regulatory requirements.

    In addition to safety isolation, however, optocouplers also provide another benefit that is often overlooked: isolation from electrical noise. The devices can be used to reduce the effects of noise at both the board and system level.

    Optocouplers separate noisy circuitry from more sensitive circuitry by allowing signals to cross boundaries without requiring the sensitive circuit to share a common ground reference with the offending circuit’s noisy ground plane. The ability to easily and effectively separate sections of PCB boards can greatly reduce the design risks in systems that require sub-circuits of varying levels of noise generation and sensitivity.

    Optocouplers are available to isolate I/O com ports ranging from low-speed RS-485, RS-232, I2C, CAN Bus, and 4 mA to 20 mA current loop applications, up to higher Mbit communication ports such as Profibus and other high-speed communications requiring a high level of noise immunity. This requirement for noise immunity is not limited to the harsh environments of industrial applications. Increasingly, optocouplers are being used in automotive applications, as well as less severe commercial and consumer applications. As coupler data rates increase, so do the number of communication applications to which they provide ideal EMI and RFI mitigation solutions.

  5. Tomi Engdahl says:

    You’re old. Get over it.

    “We’re looking for someone just like you, who can do all you can do, except young.”

    Had I turned 35, 45, 65, or 85, was not the point. These attributes are not defined by one’s age on a driver’s license but by mindset and dedication to one’s career.

    My friend’s reply was short but not sweet: “You’re old. Get over it.”

    I worked with a career strategist and wrote resumes for some very experienced people. “Mask their experience level,” I remember being told because, although this was sugarcoated, age discrimination exists. If you list 20 years of experience on a resume, it’s presumed the candidate is either at too high a salary level or out of touch — just plain old.

  6. Tomi Engdahl says:

    The Secret to Eliminating ESD Damage? Read Your Datasheet

    Manufacturers will inject an ESD pulse into the device they’re selling, and then show a plot of it on the datasheet. But if you don’t read the fine print underneath that plot, you as a designer could be misled. That plot may not apply to your device.

    Know your standards. There are two different standards for ESD design.

    Is it applicable? Many protection device datasheets call out 2kV and 4kV pulses, even though the majority of product manufacturers use 8kV as a benchmark. “If I’m using 8kV, then I don’t care what a 2kV plot looks like,” Marak said.

    Use the right layout. “Even if you’ve purchased the best circuit protection device in the world, it won’t matter unless you use the proper printed circuit board layout,” Marak said.

  7. Tomi Engdahl says:

    Differential vs. single-ended data transfer: What’s the difference?

    The primary function of an isolator is to pass some form of information across an electrical barrier while preventing current flow. Isolators are constructed from an insulating material that blocks the current flow, with a coupling element on both sides of the barrier. Information is typically encoded before being transmitted across the barrier by the coupling elements. iCoupler digital isolators use chip-scale micro-transformers as the coupling element to transfer data across a high quality polyimide insulation barrier.

    Two primary methods of data transfer have been used in iCoupler isolators: single-ended and differential. Selection of a data transfer scheme involves engineering trade-offs to optimize the desired characteristics of the end product.

    One of the main benefits of the single-ended approach is lower current consumption at low data rates.

    Differential transfer has two other advantages over single-ended: propagation delay and noise immunity.

    The LEDs used in optocoupler designs are single-ended in nature, which contributes to the poor CMTI performance generally exhibited by optocouplers.

    Differential data transfer provides an additional performance boost for iCoupler digital isolators over optocouplers.

  8. Tomi Engdahl says:

    Design Decisions: What Designers Need to Know About EMI Shielding

    For OE designers of today’s advanced electronic equipment, two growing equipment protection concerns are thermal management and shielding from electromagnetic interference (EMI). Unfortunately, protection in one area often means forfeiting some performance in the other.

    A completely sealed, shielded enclosure would ward off EMI across a wide range of frequencies. However, the need for airflow means there must be openings in the enclosure, which means opportunities for EMI.

    enclosure designers can help OE designers protect their equipment with common-sense solutions. As electronic equipment becomes more demanding and is subjected to harsher environments, it’s becoming increasingly important for OE electronics designers to collaborate with enclosure designers throughout the design process.

  9. Tomi Engdahl says:

    Use optocouplers for safe and reliable electrical systems

    Electrical systems, no matter what their purpose, share three primary requirements: they should be reliable, safe, and deliver a long operating life. To ensure safe operation, users must be insulated from any dangerous high voltages the equipment employs. To ensure reliable and long-life operation, control electronics must also be protected from hazards such as electromagnetic interference and voltage spikes. Although there are multiple technologies–capacitive, magnetic, RF, and optical–that can provide electrical isolation, optocouplers deliver safety and protec­tion unmatched by any other isolation technology.

  10. Tomi Engdahl says:

    Check Contact Resistance to Diagnose Relay Problems

    Even the best relays can fail at some point, but what causes them to fail?

    Conventional wisdom lays the blame on worn-out contacts. And there is some truth to that view. Every electromechanical relay has a finite number of cycles it can endure before the contacts call it quits.

    The truth about relays, however, is that they sometimes fail to last as long as they should because of overload or contamination.

    Fortunately, both of these failure modes can be diagnosed by measuring the relay’s contact resistance. The same measurement can also help you predict when relays are reaching the end of their expected lifecycle.

    Two methods are generally used to measure contact resistance are:

    Digital multimeter method (DMM): uses a multimeter to directly measure resistance across the contacts.
    DMM can produce misleading results whenever the contacts’ surfaces aren’t clean.

    6V1A method: This method applies 1A through the contacts and derives a resistance value using Ohm’s Law
    The 6V1A method produces a more accurate contact resistance value than DMM because the heat going through the contacts removes oxidation and other contaminants.

    Keep in mind that contact resistance specifications on data sheets represent an initial value. This value can change over time, depending on operating conditions.

  11. Tomi says:

    Utilization categories

    In electrical engineering utilization categories are defined by IEC standards and indicate the type of electrical load and duty cycle of the loads to ease selection of contactors and relays

  12. incense cones says:

    Hey there! This post could not be written any better! Reading this
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    I will forward this article to him. Fairly certain he will have a good read.
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  13. Tomi Engdahl says:

    Time bomb: The case of the invisible failure mode

    There is a failure mode that is worse than intermittent; no tests, measurements, or parts replacement will directly reveal the cause. You can test the circuit on the bench and in the field for months, and it will work perfectly. Then, within a year, a transistor fails. When you replace it, the device works well for months; then, the same transistor fails again.

    I once had to fix an oil-pipeline-monitoring device with a relay driver that behaved this way. One after another, the transistor that switched the relay coil would fail within a year of installation. The relay-coil resistance was about 240Ω (12V dc at 50 mA)—by no means an excessive load for a small TO-92 transistor.

    But a little 12V relay coil can produce a 200V spike. I was amazed when I saw it for the first time, and, with the scope that I had 30 years ago, it was hard to see.

    A transistor with a collector-to-emitter breakdown voltage of 80V will not last long under those conditions, and, if you don’t protect it from the inductive spike, the transistor will fail after some number of switching operations—in minutes or months.

    The usual way to protect a transistor from inductive spikes is to place a diode across the inductive load (cathode to VCC). Such an arrangement slows the relay release, but this circuit had no speed requirement so would have been satisfactory.

    But I think the designer had placed a 10-nF capacitor from the transistor collector to ground to suppress the inductive spike, thus perpetrating another failure mode that looks exactly like the inductive-spike-failure mode—invisible! Only the transistor collector’s on-resistance and the resistance in the capacitor when the transistor switches on limit the current. Now, instead of an inductive-voltage spike, there is a capacitive-current spike, the amplitude of which is independent of the capacitor size. The current spike produces approximately the same result to the transistor as the voltage spike: It continues to operate for some time and then shorts.

    I decided to protect the transistor with a different modification: I placed a 12Ω resistor in series with the emitter to limit the collector-current worst-case peak to 1A. A small plastic transistor, such as the MPS8099, can easily tolerate such a peak if it is short. Then, with a 50-mA normal relay-coil current, the drop across the emitter resistor was only 0.6V

  14. Tomi Engdahl says:

    Don’t be the victim of electrical noise and EMI

    Electromagnetic interference (EMI) and radio frequency interference (RFI) in automated machinery can negatively affect operations, and there are options to reduce or remove it. Learn the consequences of electrical noise and benefits of quieting things down.

    Even with proper precautions, some electromagnetic interference (EMI) and radio frequency interference (RFI) will be present in automated machinery, so components should have appropriate shielding and filtering such that interference will not negatively affect operations.

    There are common mode and differential mode components to EMI noise. Common mode EMI noise is transmitted on multiple conductors at the same time and in the same direction on all conductors from source to receiver. Most pulse width modulated (PWM) ac drives produce high frequency common mode noise. Differential mode noise is induced on a conductor and travels in the opposite direction as it does on the grounded conductor. This is similar to a complete circuit with a separate supply and return path for the EMI.

    Often depending on the frequency, EMI is emitted as conducted or radiated EMI.

    There are many sources of EMI in industry

    There are several types of components often affected by EMI in industrial applications (see Table 2). Encoders rely on low-level signals from rotating equipment and are thus susceptible to EMI.

    Electrical noise near analog signals and measurement instrumentation often can cause symptoms including unexpected voltage spikes and ripple or jitter causing incorrect or nonrepeatable readings. This occurs more often in voltage-based signals such as 0-10 V dc. The integrity of a current-based 4-20 mA signal is less susceptible to noise.

    With communication networks and components, electrical noise symptoms almost always include loss of communication or errors in reading or writing data. And with programmable logic controllers (PLCs) and other microprocessor-based components, symptoms can include loss of communications, faults or failure in the PLC or processor, discrete inputs or outputs triggering unexpectedly, and analog inputs or outputs reporting incorrect values.

  15. Tomi Engdahl says:

    Understanding isolator standards and certification to meet safety requirements

    Electrical products use numerous components, some of which are important for protection of the product user. Isolator components are commonly used to protect/separate product users from dangerous voltages, but knowing how to select the correct isolator for safety protection can be confusing. Is it the isolation voltage, the working voltage, the standard, the certification, or … what’s most important? This article will clear up the confusion surrounding the selection of isolators for safety applications in situations where hazardous voltage circuits need to be isolated from product users.

  16. Tomi Engdahl says:

    Wet and dry switching

    You may have come across two terms about using relays for switching. One term is “dry” and the other is “wet.” Where these two words come from is beyond my understanding

    “Dry” switching means that the goodie current being switched is “negligible”, that the current being interrupted and resumed is negligibly small. What constitutes negligibility may be open to debate or even to dispute, but one relay company I came across defined dry switching for their products as applying for contact currents of no greater than 1 mA.

    “Wet” switching, by the obvious reasoning, means that the goodie current is substantial where that word might mean an ampere or more, perhaps a tenth of an ampere or more, perhaps 10 mA or more , but you get the idea. Again, this may be open to debate and dispute as well.

    The choice of contact materials makes the difference between contacts that are suitable for wet switching service or for dry switching service.

    Relays rated for wet service will have contact materials that can withstand arcing when their contacts open up or when they bounce a bit when closing.

    In fact, a relay that is rated for wet service that gets put into dry service can have its contacts develop surface contamination that may over the course of time interfere with low resistance contact closure. If that happens, you have an operational failure mechanism. We don’t want that.

    Conversely, relays rated for dry service will maintain their low resistance contact closure capability as long as they never get called upon to do wet switching. Not even once! Gold is often used as the contact material and that material is set up to remain clean without having to depend on arcing. Reed relays for example

    Thus, if you have a dry switching rated relay that you use for wet switching, that relay may (or may not) suffice for wet switching purposes, but it will no longer be suitable for dry switching service afterward.

  17. Tomi Engdahl says:

    One component for switching high voltage DC (which is hard):

    G7L-X PCB Power Relays
    Compact Power Relay Capable of Switching 1,000 VDC Loads

    Compact design that achieves high-capacity DC breaking and switching. (52.5 × 35.5 × 41.0 mm (L×W×H))
    Two poles wired in series to break or switch 600 to 1,000 VDC.
    Complies with solar inverter safety standards (UL and EN).
    Designed for safety with 6.0-mm contact gap (two-pole series wiring).

    G7L-2A-X: 25 A at 600 VDC / 25 A at 1,000 VDC
    G7L-2A-X-L: 20 A at 600 VDC / 20 A at 1,000 VDC

    The relay contacts are polarized. Incorrect wiring may cause a failure to break the circuit. Wire the Relay with care

    The Relay is designed and manufactured under the assumption that it will be used with 2-pole series wiring. Do not use just one pole only.

    The Relay weighs approx. 100 g.

    These Power Relays are suitable for switching and breaking high-capacity DC. Do not use them for switching minute loads, such as signals.

    These Relays must be used for high DC voltages. The final
    failure mode is failure to break the circuit. In a worst-case
    scenario, burning may extend to surrounding components. Do
    not use these Relays outside of the specified ratings and
    service life, or for any application other than high DC voltages.

    The electrical durability of these Relays is specified as the
    number of load switching operations under a resistive load

    The coil drive circuit, ambient environment, switching
    frequency, or load conditions (e.g., inductive load or capacitor
    load) may reduce the service life and possibly lead to failure to

  18. Tomi Engdahl says:

    Back EMF and snubber

    Electrical engineering students are taught about magnetics and that energy is stored in an inductor when it is energized. This can be presented in a theoretical sense and may be applied to topics like switch mode power supplies. What does not get a great deal of attention is the fact that there are inductors embedded in many devices and that the rules of energy and energy dissipation still apply there. The real world is bristling with relays and it is quite likely that the nascent engineer will be faced with activating some. Success! It turned on just fine, but wait a minute… when it turned off the micro went nuts. What happened?

    Well the relay, like the solenoid, the contactor, the motor, and who knows what else are inductors (aka coils) and they store energy when energized. But what happens to that energy when the coil is de-energized? The coil produces a back EMF (electromotive force) that is given by V= -L(di/dt). First of all, note that it is negative and secondly early in the process dt is small, so V will be a large number. Translated this means that you get a voltage spike at the coil terminal that is being driven by the electronics. Two things can happen: The voltage may be high enough to damage or destroy the electronic driver; and the spike may radiate and upset the operation of the micro (and other electronics) running in the vicinity. In extreme circumstances it may even cause a hard fault. It may also generate enough emissions to cause an RF emissions certification problem.

    Since getting rid of all the inductors is not possible, we have to suppress this back EMF. The most common technique with DC switching is to use a diode in parallel with the relay coil (aka freewheeling diode)

    the current flowing through the coil is the maximum current expected to pass through the diode. The freewheeling diode technique is so common that you can buy relays with the diode embedded (did you ever wonder why the pins to a coil were polarised?), or semiconductor drivers like the ULN2803 that have the diode integrated onto the silicon.

    However the freewheeling diode is not a panacea. In some cases, depending on the load, the flow of current in the reverse direction may keep the coil activated until the energy has sufficiently dissipated.

    It is also possible that the freewheeling diode can damage the relay. The app note “Coil Suppression Can Reduce Relay Life” from TE says that a diode and zener in series work better than the diode alone.

    I have also seen MOVs, TVSs and fast Zener diodes used

    Until now this was all about DC powered coils. What about AC? Well, there is a technique that works for both AC and DC and it basically involves connecting a resistor and capacitor across the inductor as shown in Figure 2a. This combination is known as a “snubber”. It is not unusual to find a snubber connected across the energizing switch

    However there is one drawback with the snubber across the switch – whenever there is a capacitor there is an AC path. Aside from the fact that there is a voltage that may surprise the unsuspecting engineer or service technician, there will be a leakage current across the switch and in some cases it may even activate the load.

    One counter-measure is to add a bleed resistor or capacitor across the load to sink the leakage.

    what values do you use? In the field you will find that panel makers and other proponents of the industry have rules of thumb and will stick to those values, no matter what different experts say. (These are the suspiciously convenient 120R and 0.1uF.)

    Coil Suppression Can Reduce Relay Life

  19. Tomi Engdahl says:

    Control System Basics – Open Collectors

    In the fourth installment of this five-part series, Jon Titus explores the basic elements of a control system. In previous installments, Jon has covered relays, sink vs. source control and PNP vs. NPN logic.

    Q: It looks like NPN and PNP sensors each provide an “open” collector contact. Is this what “open collector” means in data sheets?

    A: Yes, but the topic needs a thorough explanation. You already learned that PNP transistors can source current and that NPN transistors can sink current.

    Open-collector PNP current source can provide a wide range of voltages depending on the manufacturer.

    if you specify equipment with open-collector NPN current sinks, you could provide a 12V power supply and use the sinks to control a direct connection to ground.

    As an aside, in the early years of transistor-transistor-logic (TTL) circuits, IC manufacturers produced many devices with NPN open-collector outputs. This arrangement let designers connect many outputs to a “bus” signal. A pullup resistor to +5V placed the bus in a logic-1 state. But any open-collector device on the bus could connect the bus to ground, thus a logic-0. The open-collector ICs also could provide current sinks of up to 40 mA for LEDs, numeric displays, small indicator lamps, and reed relays

    Control System Basics – PNP vs. NPN Logic

    As a memory tool think about the PNP transistor as a direct switch connected to the Positive voltage supply and the NPN transistor as a direct switch connected to the Negative (ground) voltage.

    Q: I understand how a PNP sensor sources current and how an NPN sensor sinks current. In my equipment some sensors will control relays and others sensors must supply logic levels to equipment. Will the PNP and NPN connections still work?

    A: Yes, but you have two different situations; direct control of an on-or-off device or the need to produce two voltages to represent a logic-0 and a logic-1. Let’s look at both. You can use a simple current sink or current source to control a relay coil, for example, because it either gets current or it doesn’t. The flow of current through the coil can go in either direction. (Note: You want relays specified for a DC coil current. Relays that use an AC coil current differ in construction and might not be suitable for a DC coil current.)

  20. Tomi Engdahl says:

    Upgrade your Loxone with 16A SSR outputs

    This short project is about upgrading the Loxone with solid state relay instead of the normal mechanical relay.

    I bought a Loxone and I am happy with the product. Good software, plenty possibilities and easy to use. But I hate the relay outputs. The click-clack sound is annoying. That was my main reason to replace the relay with solid state relay (SSR). SSR is a semiconductor part and has no moving parts. This is also the reason why a SSR will last longer than a normal mechanic relay. But because it’s a semiconductor, it’s more sensitive to ESD (electrostatic discharge). This will be solved by adding a varistor. So these are the advantages:
    * Current output up to 16A
    * No mechanical switching parts, the makes the SSR also better for environments with flammable gasses
    * Zero crossing switching reduces signal noise
    * Short switching time vs mechanic part
    * No contact bouncing
    * No sound from SSR.

    * It’s not possible to switch a small analog signal. This is possible with a mechanical relay.
    * The SSR can only handle the AC OR DC voltage it is made for.

  21. Tomi Engdahl says:

    Meaning of AC15, DC1, AC1, and DC13 in relay durability curves

    In the datasheet of the relay OMRO G7SA from the durability curve they specify the number of operations as a function of the contact current.
    Each curve is for a specific current and voltage. The curves are labeled DC13, AC15, DC1 and AC1. Can anyone tell me the meaning of these labels?

    Those are just the IEC Utilization categories.

    Utilization categories: Contacts will last longer under different load characteristics. A relay/contactor that is continuously starting, reversing and stopping an AC motor , for instance, will have contact wear different than the same contactor that controls a resistive heating element. Straight resistive load vs an inductive load, etc. AC vs DC load. Switching off a large DC load is more difficult than switching off a similar size AC load. Contact material and the gap between contacts, protective circuits and space between contacts and the speed at which contacts open will all be factors in the suitability of a contactor for the job. Selecting the right contactor for the application will result in higher dependability and might save money in manufacturing cost. IEC utilization categories can be helpful in selecting a suitable contactor but real-world application data is more useful.

  22. Tomi Engdahl says:

    Utilization categories

    In electrical engineering utilization categories are defined by IEC standards[1] and indicate the type of electrical load and duty cycle of the loads to ease selection of contactors and relays.

    IEC Utilization Categories

  23. Tomi Engdahl says:

    NEMA contact ratings

    NEMA contact ratings are how much current at a rated voltage a relay or other pilot device can switch. The current rating of smaller NEMA contactors or their auxiliaries are defined by NEMA ICS 5: Industrial Control and Systems, Control Circuit and Pilot Devices[1] standard.

  24. Tomi Engdahl says:

    Control System Basics – Isolation

    In the final installment of this five-part series, Jon Titus explores the basic elements of a control system. In previous installments, Jon has covered relays, sink vs. source control, PNP vs. NPN logic and open collectors.

    Q: I don’t like the idea of control signals going from a field device into my rack of expensive controllers. A short circuit in a processing plant could cause a lot of damage. How do I protect my investment?

    A: I don’t like that situation either. You already know relays isolate the coil-control current from the electrical circuits switched by the mechanical contacts. But relays don’t solve every isolation problem. The simplest isolation technique involves an optical “link” between a light transmitter and a light receiver. Think of an optical isolator as a short piece of optical fiber between a light-emitting diode and a phototransistor. No electrical path connects these two devices.

    The SeaI/O-420U module provides 16 optically isolated inputs that offer 300-volt isolation. The input signal may range from 5 to as high as 30 volts.

    The fiber-optic “gap” electrically, or galvanically, isolates the LED circuit from the transistor circuit. That small distance–a millimeter or so–between an LED and photo-transistor can isolate a 1000V signal on one side from the device on the other side. Special optical isolators “separate” 10′s of thousands of volts.

    Q: OK, I understand how an optoisolator lets me control high-power devices, but can I also isolate analog signals coming into my apparatus from remote sensors and switches?

    A: Yes, but equipment rarely requires such isolation. Specialized analog-isolation devices and modules are available. Companies such as Silicon Laboratories, Analog Devices, Avago Technologies, and Maxim Integrated sell different types of isolation devices that use optical, transformer, and capacitive techniques. Application notes from these companies describe how to use their analog-isolation products. Figure 17 illustrates an analog-isolation circuit that I built and tested. It exhibits excellent linearity across its input range.

  25. Tomi Engdahl says:

    Sinking and Sourcing Concepts

    When choosing the type of input or output module for your PLC system, it is very important to have a solid understanding of sinking and sourcing concepts. Use of these terms occurs frequently in discussion of input or output circuits.

    Sink/source I/O circuits combine sinking and sourcing capabilities. This means that the I/O circuitry in the PLC will allow current to flow in either direction

    The common terminal connects to one polarity, and the I/O point connects to the other polarity (through the field device). This provides flexibility in making connections to your field power supply.

    Common terminals and how to use them

    In order for a PLC I/O circuit to operate, current must enter at one terminal and exit at another. This means at least two terminals are associated with every I/O point.

    If there was unlimited space and budget for I/O terminals, then every I/O point could have two dedicated terminals. However, providing this level of flexibility is not practical or even necessary for most applications. So, most input or output points on PLCs are in groups that share the return path (called commons).

  26. Tomi Engdahl says:

    What Is the Difference Between the Terms Sinking and Sourcing?

    Sinking and Sourcing are terms used to define the control of direct current flow in a load. A sinking digital I/O (input/output) provides a grounded connection to the load, whereas a sourcing digital I/O provides a voltage source to the load.

    Figure 1 shows a sinking digital output that is connected to a sourcing digital input. In this circuit, the load is pulled to ground because of the sinking digital input provided.

    Figure 2 shows a sourcing digital output that is connected to a sinking digital input. In this circuit, the load is pulled up to receive voltage because the sourcing digital input has been provided.

    Because both a voltage source and a ground reference are needed in order to create a complete circuit, you must have a sourcing input or output connected to a sinking output or input. If you wish to connect a sourcing input to a sourcing output or a sinking input to a sinking output, you will need to add an additional resistor.

    Connecting Two Sinking I/O or Two Sourcing I/O Together

    To Connect a Sourcing Input to a Sourcing Output

    When a sourcing input is connected to a sourcing output, a circuit with two voltage sources and one load is created. Since the input determines the “on” and “off” based on the voltage level at the input, a pull-down resistor needs to be added to pull the line to ground. Since having two voltage sources is a problem, there are two ways to configure the digital input’s voltage source. The first is to give it the same source as the digital output.

    In this case, the pull-down resistor must be sized so that when the sourcing Digital Output is off, the voltage at the digital input registers at a logic low. Use the load resistance of the digital input to calculate this value. Additionally, the resistor must be sized so as to not violate the current limits of the Digital Input device.

    The other case is to remove the supply voltage for the Digital input entirely.

    In this case, the pull-down resistor must be large enough to provide an adequate load when the line is connected to a voltage source by the sourcing output. The resistance of the resistor must also be small enough to prevent the line from “floating” (not being grounded) when the voltage source is disconnect by the digital output. Since there is not a voltage source attached to the digital input, the maximum resistance only needs to be high enough to prevent the leakage current from the transistor from pulling the line to V+. This value can be determined if the leakage current of the transistor is measured; however, since this current is so small, a resistance with a value less than 100 kΩ should be more than sufficient.

  27. Tomi Engdahl says:

    TI Designs
    8-Channel Digital Input Module for Programmable Logic Controllers (PLCs)

    Chapter 6 Digital Input (DI) Circuit

    The FBS-PLC provides the ultra high speed differential double end 5VDC inputs (i.e., single input with two terminals without common) and the single-end 24VDC inputs which use the common terminal to save terminals.

    DI circuit speccifications:

    Low speed 24V DC Single ended input
    ON current > 2.3 mA
    OFF current 4 mA
    OFF current 8 mA
    OFF current 11 mA
    OFF current < 2 mA

  28. Tomi Engdahl says:

    AN970: Design Guide for PLC Digital Input Modules Using the Si838x

    This application note serves as a design guide for selecting an input resistor network for
    the Si838x that is robust in regards to part and resistor tolerances. Also discussed are
    methods of selecting input resistor values to adhere to off, on, and transition region requirements,
    such as those in IEC 61131-2. The input network serves to map the input
    thresholds of the Si838x to the system-level current and voltage thresholds (e.g. 24 V dc

    In order to comply with the IEC 61131-2 standard for Programmable Logic Controller
    (PLC) Digital Input Modules, the Si838x LED emulator inputs are combined with a tworesistor
    input network and an LED indicator light. This application note provides a system
    of equations and guidance for selecting the bill of materials to meet system requirements,
    such as those defined in IEC 61131-2.

    Table 5.1. Example Design Requirements
    Requirement Number Description Value
    1 Input Signal Magnitude 24 V dc (30 V dc max)
    2 Input LOW voltage VIN 19 V
    4 Resistors Series E24 – 5% tolerance
    5 Input HIGH current IIN > 3 mA

  29. Tomi Engdahl says:


    Beckhoff I/O systems: Digital inputs for 2-wire and 3-wire sensors according to EN 61131-2

    This application example explains why the characteristic of the input is decisive in the selection of digital
    inputs for different sensors. The EN 61131-2 standard for current sinking digital inputs defines three types
    that differ in current consumption and logic level and are only compatible to one another to a limited

    Difference between 2-wire and 3-wire sensors
    There is a basic conflict of goals in the recording of digital input signals in the I/O field: as opposed to 3-wire sensors, 2-wire
    sensors reduce the amount of wiring and the use of materials; however, the current required to drive the sensor causes
    constant power loss with corresponding heat losses at the I/O modules. 3-wire sensors have an external voltage supply; the
    digital inputs for this type of sensor have low power consumption. 2-wire sensors conduct the signal and the voltage supply via
    the same wire. Therefore, in order to maintain the function of the sensor, a permanent minimum current, the so-called quiescent
    or residual current, must flow via the wires and the digital input. The residual current must be lower than the switching
    threshold of the following coil or switching element while at the same time ensuring the reliable operation of the sensor.
    The sensors and the digital inputs of the controller are chosen in accordance with the EN 61131-2:2003 standard, to which the
    manufacturers of operating equipment and peripheral devices of controllers are bound. The standard describes the operating
    equipment requirements and tests for the PLC in order to ensure the compatibility of peripheral devices and controllers.

    Why are there three types?
    The three types (1 – 3) distinguish the development steps in sensor technology. Type 1 is suitable for electromagnetic switching
    devices, such as relay contacts, push buttons, switches, etc. and dates from the time when mainly mechanical contacts were
    used and semiconductors were not so widespread. Type 1 is therefore only suitable to a very limited extent or not at all for
    the use of 2-wire sensors, for which a high quiescent current is required. For this reason, digital inputs for 3-wire and 2-wire
    sensors are separated, being represented by type 1 and type 2 in the standard.

    In comparison with the present state of the art, the power consumption of semiconductor-based circuits in earlier 2-wire
    sensors was several times higher. Therefore, according to EN 61131-2, a maximum current consumption of 30 mA is possible
    by the sensor; this value is adapted to the standard for 2-wire proximity limit switches, IEC 60947-5-2. If this range is fully
    exploited, then type 2 digital inputs are not technically implementable as multi-channel input modules: a current consumption
    of 30 mA per channel would result in a current consumption of 240 mA to 480 mA or even 960 mA per module with the
    present typical channel density of 8, 16 or even 32 channels. Even in the case of low voltage 24 V DC, the power consumption
    of 16 digital input channels would be around 11,520 mW.

    Beckhoff offers type 2 inputs in the packaging densities 2, 4, 8 and
    also 16; the inputs exhibit a typical characteristic in accordance with fig. 3 and a current consumption of at least 6 mA. In
    principle, type 2 digital inputs can be used as type 1 or type 3, but the power consumption is then unnecessarily high.

    EN 61131-2. The operating range consists of the “On range”, the “Transition range” and the “Off range”.

  30. Tomi Engdahl says:

    Connection of Encoder Types
    in Compliance with IEC
    61131-2 to DI Modules

    The Three Input Types in Compliance with the IEC
    The choice of digital inputs is based on the characteristics of the inputs and is
    significant for the different sensors. The IEC 61131-2 standard defines three types
    for current-sinking digital inputs. Current-sinking modules are those which have the
    characteristic of consuming current. The three digital input types are described
    Type 1: Mechanical switching contacts (2-wire connection) and semiconductor sensors
    (only 3-wire connection)
    Type 1 digital inputs convert signals from electromechanical switching devices
    (relays, pushbuttons …) with two states into a binary number (a bit). However,
    these inputs cannot be used for the 2-wire connection of semiconductor switches
    (sensors, proximity switches…). The definition of Type 1 in the standard was made
    at a time when mainly mechanical contacts were implemented.
    Type 2: Semiconductor sensors (2-wire connection)
    Type 2 digital inputs convert signals with two possible states of semiconductor
    switches into a binary number (a bit).
    Type 2 inputs:
     have increased power consumption and are more suitable for modules with a
    low channel density
     can be used for 2-wire proximity switches if connected in compliance with IEC
    Type 3: Semiconductor sensors (2-wire/3-wire connection) – reduced power
    Similar to Type 2 digital inputs, Type 3 digital inputs convert signals with two
    possible states of semiconductor switches (2-wire proximity switches) into a binary
    number (a bit).

    Current and Voltage Limits for Digital Inputs

    For 24V type 1:

    Signal 0:
    Voltage -3 to +15V
    Current limit 15 mA

    Signal 1:
    Voltage 15V to 30V
    Current limit 2 to 15 mA

    For 24V type 2:

    Signal 0:
    Voltage -3 to 11V
    Current limit 30 mA

    Signal 1:
    Voltage 11V to 30V
    Current limit 6 to 35 mA

    For 24V type 3:

    Signal 0:
    Voltage -3 to +11V
    Current limit 15 mA

    Signal 1:
    Voltage 11V to 30V
    Current limit 2 to 15 mA

    Due to the heat build-up in the control cabinet you should give priority to Type 3
    inputs for the control cabinet construction, because these modules have a low
    electrical power consumption and less heat dissipation.

    “P-schaltend” (“sinking”) and “M-schaltend”
    There are various terms and categories for characterizing the digital circuits, for
    example: “P-lesend” and “P-schaltend” (PNP), “M-lesend” and “M-schaltend”
    (NPN) in German and sinking/sourcing in English.

  31. Tomi Engdahl says:

    Current-limiting terminations for automation digital inputs
    Shrink the I/O module size and increase its EMI immunity

    The CLT3-4B is a quad termination circuit that runs as
    type 1 or 3, limiting its current to 2.7 mA typical, and
    transfers its logic state to an opto-transistor.

    The PCLT-2A is a dual termination circuit able to run as
    type 2 or 3 by its programmable limiting current from
    2.5 mA to 7.5 mA. Its output is CMOS compatible.

    The SCLT3-8B device further enhances system
    integration by cutting dissipation (1 W saved per SCLT)
    and reducing the count of opto-transistors on board.
    For its eight type 3 input sections, the SCLT3-8B
    embeds one adjustable digital filter and a LED status
    driver per input. It also serializes the input-state transfer
    using a 2 MHz SPI peripheral and a 5 V voltage regulator
    that supplies the three SPI transferring isolators.

  32. Tomi Engdahl says:

    Safety Input Units NX-SIH400/SID800

    Internal I/O common PNP (sinking inputs)
    Rated input voltage 24 VDC (20.4 to 28.8 VDC)

    Safety input current 4.5 mA typical 3.0 mA typical
    Safety input ON voltage 11 VDC min. 15 VDC min.
    Safety input OFF
    voltage/OFF current 5 VDC max., 1 mA max.
    Test output type Sourcing outputs (PNP)
    Test output load current 25 mA max. 50 mA max.
    Test output residual
    voltage 1.2 V max. (Between IOV and all output terminals)
    Test output leakage
    current 0.1 mA max.

  33. Tomi Engdahl says:

    Optimizing snubber design through frequency-domain analysis

    The use of a snubber circuit to damp the voltage ringing stemming from parasitic oscillation in switching voltage regulators (VRs) is often not an option. The components for this circuitry must be carefully selected to get the proper attenuation without wasting unnecessary power. This article presents an analytical method based on frequency-domain analysis that leads to simple, yet accurate, equations to determine the optimum value of the components for an RC snubber. Following the step-by-step procedure described in the conclusion, an optimal RC snubber can be designed to obtain the required ringing attenuation.

  34. Tomi Engdahl says:

    Filtering EMI And RFI Noise

    The increased complexity of modern electronic components, circuits, and systems often results in elevated EMI and RFI levels which must be brought into proper compliance.

    Noise can not only degrade the performance of electronic equipment, it can also prevent new electronic equipment designs from passing compliance testing. A combination of factors, such as increasing digital processing speeds, shrinking electronic package sizes, and more densely spaced electronic components, are contributing to increased amounts of electromagnetic interference (EMI) and radio-frequency-interference (RFI) noise, and boosting the needs to understand such noise sources and how to protect against them. Fortunately, as noise sources have become more complex, filtering solutions, such as filtered connectors, have been developed to help keep EMI and RFI in check.

    Filter inserts represent a low-cost, practical solution for many EMI/RFI problems.

    Filter connectors can be supplied in a wide range of configurations to control EMI and RFI.

    Filter modules are sophisticated EMI/RFI filtering approaches and can be supplied with additional components.

  35. Tomi Engdahl says:

    Use Current to Drive Solenoid, Relay from Array of Voltages

    Rather than use a fixed-rail voltage source, it’s more efficient and flexible to utilize a current source so that performance remains consistent while supply rail and coil resistance change.

    Relays and solenoids are available with many different voltage ratings. Most factory- and process-automation equipment operates from 24-V supplies. However, customers may have control signals for a wide range of ac or dc voltages, such as 12 V, 24 V, 36 V, 48 V, or even 120 V or 220/240 V for some valves and contactors. For each voltage, the coil designs of the variants must be different. Having coils to accommodate all of the possible voltage adds to inventory, BOM, and spare-part headaches.

    One solution could be to design one coil for 12 V, then use a resistor to limit the current into the coil for each voltage option. However, this wastes energy and dissipates heat from the resistor, especially if the 12-V coil is used for the 220/240-V design.

    A more energy-efficient solution is to use pulse-width modulation (PWM) driving and a freewheeling diode to regulate the current in the solenoid. In addition, you could add current-sense feedback with PWM and control the current in the solenoid.

    While PWM driving without current-sense feedback is easy to implement, variations in coil resistance, temperature, supply voltage, and the other factors can cause unintended solenoid de-actuation. As temperature increases, the resistance of the coil will increase. According to Ohm’s law, the increased resistance will cause the current to decrease. Since current is responsible for generating the magnetic force, when the current decreases, so will the magnetic force.

    Thus, using PWM driving with current-sense feedback is a more reliable driving method for the coils and relays. With current control, the driver regulates the coil current to the required value independent of resistance, which makes the system more robust over temperature.

  36. Tomi Engdahl says:

    How Do You Choose The Right Type Of NTC Thermistor To Limit Inrush Current For Capacitive Applications?

    Matt Cuhadar from the Ametherm team answers a reader question about choosing the right type of NTC Thermistor to limit inrush current for capacitive applications.

    Q: How do you choose the right type of NTC Thermistor to limit inrush current for capacitive applications?

    A: Inrush current refers to the maximum, instantaneous input current drawn when power is applied to an electronic system’s power supply. A dc system has an input capacitor and an ac input system has and input rectifier and capacitor that can exhibit high inrush current when the associated equipment turns on. If steps are not taken to minimize this inrush current, it can damage power devices and reduce equipment life. A safe and cost effective way to reduce inrush current is to use an inrush current limiter (surge limiter), which is a special type of negative temperature coefficient (NTC) thermistor.

    Inrush current occurs at the moment the power switch is thrown. This happens because the input filter capacitor acts as a short and its minimum equivalent series resistance (ESR) and line resistance are only a few milliohms, which can lead to a high inrush current.

  37. Tomi Engdahl says:

    Have Your Cake and Eat It, Too: Overcoming Conflicting Isolation and EMC Standards

    Sponsored by: Texas Instruments. Applying techniques to improve EMC performance plus integrating compact data and power isolators can bring together the often-at-odds benefits of small size and excellent emissions performance.

  38. Tomi Engdahl says:

    The Technology Revolution in Solid-State Switching Relays

    Evolving from magnetic coil actuators to capacitive-coupled isolation, the ever-shrinking relay continues to be an essential component within the automation arena.

    When comparing performance advantages of solid-state-relay (SSR) technology, one quickly realizes that these devices are not created equal. Optical MOSFET-based relays such as PhotoMOS have highly linear input and output characteristics that outshine alternatives such as triacs or optocouplers (Fig. 2). These SSRs with MOSFET output chips can also control small analog signals without distortion.

    SSRs that use output chips such as triacs or bipolar transistors have offset voltages that distort and clip signals.

    To turn on the relay, a current is applied to the LED on the input side, which will illuminate. The light is then absorbed by the photoelectric element that converts the light to electric power, similar to a solar cell. This electrical current subsequently passes through a control circuit and charges the gates of the two MOSFETs on the output side

    Taking this technology one step further is a capacitive-coupled relay, such as the Panasonic CC TSON (Fig. 4), which employs a control method that differs from the long-established LED-operated MOSFET relays. In this device, the LED from the input side has been replaced with a capacitive-coupling driver IC
    This allows the relay to be voltage-driven rather than current-driven.
    oscillated signal is then passed through a capacitor, which provides isolation between input and output.

    Removing the LED from the input side relieves some of the design limitations previously encountered by MOSFET relays—specifically, the LED takes up a lot of space

    Since temperature has a direct impact on the LED character, another advantage of removing the LED from the relay is the ability to withstand industrial ambient operating temperatures.

    In I/O modules for safety PLC devices, the ability to withstand high industrial temperature and small size can be crucial.

    As long as circuit design has the need to automate switching with electrical isolation, relays will continue to remain integral components. The evolution of relays has gone from using magnetic coil actuators, to LEDs that are used to drive triacs or MOSFETS, to the recent development of capacitive-coupled isolation.

  39. Tomi Engdahl says:

    One Man’s Tale Of EMC Compliance Testing

    If you turn over almost any electronic device, you should find all those compliance logos: CE, FCC, UL, TÜV, and friends. They mean that the device meets required standards set by a particular region or testing organisation, and is safe for you, the consumer.

    Among those standards are those concerning EMC, or ElectroMagnetic Compatibility. These ensure that the device neither emits RF radiation such that it might interfere with anything in its surroundings, nor is it unusually susceptible to radiation from those surroundings. Achieving a pass in those tests is something of a black art, and it’s one that [Pero] has detailed his exposure to in the process of seeing a large 3-phase power supply through them. It’s a lengthy, and fascinating post.



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