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:

    Review: Tool measures power-line EMI

    Ever since we’ve switched from incandescent to LED lighting and from linear to switch-mode power supplies, EMI on power lines has started to become an issue, especially for those of us who still enjoy AM broadcast radio and amateur radio in the MW and HF spectrum (0.54 to 30 MHz). Add defective power utility transmission line arcing and corona, and many amateur radio enthusiasts and military operators simply can’t receive national and international stations with the resulting hash on their receivers.

    I wrote an article in 2015, How cops are finding “grow ops” with AM radios, about how those who were growing marijuana illegally were purchasing poorly-filtered power supplies (“ballasts”) from Asia for their lighting. These power supplies were producing EMI over a wide range, up to 1 mile away, and the police in Oakland, California, realized they could hear the hash coming through their AM radios, which allowed them to zero in on these grow operations.

    Because this broadband high frequency EMI is both conducted and radiated, it’s very difficult to get rid of at this point in time; an unintended consequence of the desire for more efficient power supplies. In the meantime, those using these frequencies are suffering the consequences.

    OnFILTER recently released a power line EMI adapter that can be used with spectrum analyzers or oscilloscopes to evaluate conducted power line EMI (Figure 1). They have units that can plug directly into a wall socket or one that includes test leads.


  2. Tomi Engdahl says:

    Fields from specific power lines
    Use the links in the table to find the field for any specific power lines.

  3. Tomi Engdahl says:

    “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

    “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.

    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.


  4. Tomi Engdahl says:

    The Difference Between Contactors And Relays – ELECTROMAGNETIC SWITCHES electricians use

  5. Tomi Engdahl says:

    What is the difference between ac contactor and dc contactor?

    Main differences between AC contactor and DC contactor: Iron core of AC contactor would generate eddy current and hysteresis loss, while the DC contactor has no core loss. … AC contactor adopts a grid arc extinguishing device while the DC contactor adopts a magnetic quenching arc extinguishing device
    #contactor #modularcontactor #hvac #hecheng #hechengelectrical

  6. Tomi Engdahl says:

    How Optocouplers work – opto-isolator solid state relays phototransistor

  7. Tomi Engdahl says:

    Low noise solid state relays suitable for use in professional food equipment and commercial, industrial, and home appliances


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