Where to place TVS diode

Thread Starter

jmoffat

Joined Jul 18, 2012
42
I want to use a TVS diode to protect the output of a PLC from inductive spike generated by the relay. This has become the subject of some discussion. Where would you place the diode. D1, D2 or D3?

What would be your preferred method of protecting the PLC?
 

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AlbertHall

Joined Jun 4, 2014
9,301
If it really must be a TVS diode then D3 is the place to put it to minimise the voltage going back to the PLC.
But I would use an ordinary diode in the D1 position, cathode to +24V.
 

crutschow

Joined Mar 14, 2008
24,345
The lowest voltage spike will be generated if you place a regular diode from the PLC output to V1 (cathode to V1).
That way you will suppress the inductive spike from the wire inductance as well as the relay inductance.
 

Thread Starter

jmoffat

Joined Jul 18, 2012
42
Thanks for the replies AlbertHall and Crutshow. Now just to be clear the TVS conducts no current until the threshold is reached and limits the voltage to the rated clamping voltage. Correct?
 

crutschow

Joined Mar 14, 2008
24,345
Thanks for the replies AlbertHall and Crutshow. Now just to be clear the TVS conducts no current until the threshold is reached and limits the voltage to the rated clamping voltage. Correct?
Correct.
But note that, for example, if it were connected directly across the relay, then the voltage at the PLC input at the instant the relay is turned off would be the TVS voltage plus the supply voltage.
 

Thread Starter

jmoffat

Joined Jul 18, 2012
42
Correct.
But note that, for example, if it were connected directly across the relay, then the voltage at the PLC input at the instant the relay is turned off would be the TVS voltage plus the supply voltage.
Wow I didn't think of that. The TVS we were going to use has a clamping voltage of 47 volts plus the 24 volt supply. That just might exceed the breakdown voltage of the output transistor. If it was up to me I would have put the TVS in parallel with the relay because that's where a normal diode would be placed. It seems as if the TVS should be placed in parallel with the PLC output.

The whole TVS question came about because the designer wanted to use a non polarized component. I guess he doesn't trust the wireman to get the polarity right. That being the case I guess a resistor and capacitor could be used. I say use a diode and be done with it.
 
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