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EMI Protection best practices

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gary36

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Hi
I am designing an interlock logic for safety system, which receives potential free relay contacts as digital inputs from remote location which is noisy. What is best practice to remove this noise. Literature recommends ferrite beads and MOVs. It is not clear if this would suffice and how should we connect them.
 

As the inputs are from relay contacts I assume the rate of opening and closing is relatively slow. That should allow you to use an RC filter. Without knowing your exact input circuitry it's difficult to advise but my first guess would be that a resistor in each leg (~47 Ohms) and a capacitor across the input to your circuit (~47nF) should be enough to isolate and filter out common mode and differential noise. Ferrite beads work well for high frequency noise and MOV are good for high voltages but for more ideas we need to know the voltage and current at the input and some idea of the kind of EMC you require.

Brian.
 

I am still working towards getting the magnitude of hostile voltages that will hit the circuit. In the meantime, I was interested in the circuit connections. There are 3 things proposed, RC filter, ferrite bead and an MOV. How should I connect them ?
 

Hi,

for very noisy environment consider to use POF = plastic optical fiber.

***
If you use copper cables:
* Use twisted pair. Maybe shielded cables (shield= earth signal)
* treat both wires as equal "signal lines". Ie: don´t connect one wire to GND and the other to a high impedance input.
* current loop signals are better than voltage signals.
* use common mode filters
* use difference signal recievers (and or galavanic isolation)

Klaus
 
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