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:

    How to Read Data Sheets: Relays

    When you are used to working with semiconductor devices, the data sheets for relays may pose some problems. What do terms like ‘Operate Voltage’ and ‘Release Voltage’ mean, for example?

  2. Tomi Engdahl says:

    How to Read Data Sheets: Latching and Automotive Relays

    Latching relays latch mechanically or magnetically so you don’t have to maintain current running through the coil to keep them on.

    Latching Relays

    you can buy latching relays. These relays latch mechanically or magnetically so you don’t have to maintain current running through the coil to keep them on. This sounds like a great solution for my battery cutoff switch

    Looking at this data sheet, we see that this beauty is rated for 60A at up to 250VAC, and it will break a maximum of 15kVA. The only downside is that the initial voltage drop is 1mΩ at 3A. While the 1mΩ is very good, the 3A gives me pause. This implies that it might not work so well if I have a low power radio.

    Moving on, the coil resistance is much lower than for the non-latching relay, which means that the power required is higher (which makes sense if it has to latch). The two-coil version has a resistance of 48Ω, which is 0.25A at 12V and will dissipate 3W. No wonder there’s a maximum duration.

    irst, Coil data followed by Contact data and other specifications.

    The Set and Reset voltages are presented a bit differently than in the previous data sheet in that they are given as a 75% of the rated voltage. For a 12V coil this is 9V. The coil resistance for the two-coil version is 72Ω. The contacts are rated at 50A with the minimum load specified as 100mA. So far, this part is looking better than the other latching relay for my purposes.

    Because a relay is also an electrical device, there is another component of its life relating to the load that is being switched. If you are switching a resistive load of 50A, for example, you’ll get 1 x 104 operations, but if you switch 25A, they you’ll get 1×105 operations.

    Automotive Relays

    The Panasonic Electric Works CB1A-M-12V looks similar to ones I’ve used in the past. This part is rated at 40A and I should be able to get something very similar at my local automotive store…

    …or maybe not.

    According to the data sheet, in addition to the bracket mount with tabs that I’m familiar with, these parts also come in PCB and plug-in types. They also come in heat-resistant types for use in the engine compartment.

    The Coil data section is a bit different for this relay. Instead of just giving guaranteed Pick-Up and Drop-Out voltages, we are given a range. This is extremely useful information if we want to reduce the power consumption required to keep the relay on. For example, the 12V part has a 103Ω coil. Since the dropout voltage is 4.2V or less, we can use a 150Ω resistor in series to hold the relay on with 4.8V. Of course, this assumes that the Pick-Up and Drop-Out voltages are constant over temperature.

    Moving along, for the standard 12V type, Contact Resistance is 2mΩ (measured at 1A), while the heat resistant type is 15mΩ. Both of these are good numbers. The minimum and maximum switching capacity is given for 14VDC

  3. Tomi Engdahl says:

    Relays Are Great, But There’s No Need to Let Them Waste Power

    The relay remains one of the most useful components in the designer’s kit, but its power-hungry operation can be overcome with some non-static drive and specialized ICs.

    And why not? Yes, the relay is “ancient” and may not get a lot of respect or consideration from novice engineers, yet the countless models on the market covering an enormously wide range of specifications attest to its value, and I am sure millions of units are shipped every year (and we’re not even considering at RF relays).

    Let’s face it: the relay is easy to use; easy to size; the input coil and output contacts can have wildly different ratings (current, voltage, AC, DC); the contacts are signal agnostic to a large extent; the contacts are easily “floated” (not grounded); and the contact arrangement is offered in many problem-solving variations, including normally open (NO), normally closed (NC), both NO and NC, and multiple independent poles, and various mounting styles, to cite a few. The absolute isolation between the coil drive-side circuit and the contact-closure side is another major benefit. In short: they do lots of things well, and don’t give headaches or unexpected surprises; further, if used within specifications, a quality relay has a million-plus cycle life. What’s not to like?

    Actually, there is one thing, a shortcoming that is inherent in the relay’s physics (and its close cousin, the solenoid): they can use a lot of power. As a device based on magnetic-field energy, it takes current to energize the coil, and using current means dissipating power (obviously, a waste) and self-heating (which has important consequences).

    Fortunately, there are ways around this dilemma. First, it’s important to know that the relay “hold” current is typically about half the pull-in current.

    IC vendors know this, and also know that relays are a big opportunity. That’s why there are application-specific ICs such as the Texas Instruments DRV110 PWM current controller which offloads the microcontroller, (Case 3). It allows the user to set initial activation current, time at that peak current, hold current, and other factors,

    It’s important to have independent control over the activation and hold currents due to that unavoidable irritation of circuit design: high temperature. As the relay coil heats up due to the applied current, the resistance of its copper windings increases substantially

    This, in turn, reduces the current to the coil, and thus the ampere-turns which develop the magnetic field.

    So, don’t be afraid of the relay, it can be a viable solution – and often, the best or even the only solution –to a situation where the controlling signal and the load are very different or need to be isolated.


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