Location and type of inductive kickback suppression device. 2

All PLCs have built-in suppression. I never add any unless something odd is being driven.

1) Suppression at the already crowded PLC breakout also allows the lines running out to the field to radiate stuff that wouldn't be available if suppressed at the load.

2) Never heard of any advantage over a flywheel. (Again, even base PLCs have this covered too.)

Keith Cress
kcress -

Thanks for your reply!

Our PLC doesn’t have suppression on its semiconductor outputs. The manufacturer has said that all coils need suppression...

It’s an oddball fault tolerant system (ICS Triplex/AADvance)

Really I want to know if the suppression can be fitted at the PLC terminals, EMI issues aside.

I don’t see why it wouldn’t be effective if it was at the PLC terminals, or just at some location aside from the solenoid coil, which is encapsulated and hence can’t be accessed.

Thanks again.

Really I want to know if the suppression can be fitted at the PLC terminals, EMI issues aside.

Click to expand...

Yes. The suppression then uses the relatively high impedance field wiring as part of the suppression. Anywhere will likely work.


The line resistance is used to drop the voltage spikes.





Keith Cress
kcress -

Hi 123MB, I just want to say that in our company, in the recent years we always add supression diodes on solenoids. In our system this has nothing to do with the plc output itself, as output is switching a small relay, which then swith on the coils.

Some examples of problems we have had, before we made supression a standard:
-A lot of damaged small 24VDC power supplies. These where supplying the solenoids that did not have supression.
-When switching off solenoids this has caused analog input cards to fail for a second or two. I believe this has something to do with the wiring, shared cable for coil and sensors.
-Damaged analog output card...

Note that it in our case it took a year or so until power supplies e.g. failed due to voltage spikes from the solenoids.

We have added suppression diodes in serveral ways:
-At the coil, using e.g. "Hirschmann" plug with suppression diode built in.
-In the nearest junction box..
-At the power supply itself.


We have had systems working fine without suppressors also, but we find it beneficial to just add supressors as a standard.

Howdy 123,
We have always added free-wheeling diodes to any DC output point on a PLC or DCS. I have never heard of any issues having taken this path. Some solenoid manufacturers will offer a built-in FW diode with their coils. (Note: I believe that ASCO offers this option).
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

I think that phenomena "in the periphery" need more attention (and understanding). Coils, contacts and snubbers/free-wheling diodes are such periphery things. I have started a column in a Swedish paper where such things are dealt with. It is at
Sorry for the language, this column will appear in English later on but the pictures may already be interesting if a few short comments are added:

1 Shows a complete break from contact opening to final "power pulse" when contacts have separated so far that the arcing cannot be maintained any more. The break-down voltage increases as contact distance goes from a few microns up to around one millimeter. Break-down with higher and higher amplitudes occur during the process and the sharp discharge (almost completely vertical) voltages cause lots of HF interference that can be heard in radios and also upset electronic counters and registers. They also create problems in communication and can destroy insulation and power supplies. No good.

2 Shows a zoomed-in detail of 1. The repeated charging of coil and cable capacitance and the very fast discharges (BW inversely proportional to discharge time) can be seen. There are around 15 of them in this zoom - and 100-200 in the complete process.

3 Shows what an RC snubber across the coil does. The C (typically 100 nF) absorbs the current so that voltage increases slower than contact break-down voltage increases when contacts separate (just before the trig point). To the left, there is the 230 V/50 Hz across the coil, a few bounces occur - but no fast discharges - and finally there is a ringing starting at around 100 Hz and increasing to 200 Hz as air gap increases and inductivity of coil decreases.
The R in RC? It is there to protect contacts from welding, or pitting, when they close. Without the R, you would have literally hundreds of amps when the contacts close. A typical (100 ohms) R reduces that current to a few amps.

The next part will show what a MOV does (not always so helpful) and after that there will be a few things on DC coils, TVS and such things. I'll try and get better pictures for those. Not as bleak as these ones.


Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.

I always put an MOV across the coil of my relays/starters.

Seems to me that having the snubber directly across the coil minimizes the potential for noise being generated in the wiring to the PLC.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list

With a successful RC snubber, it does not matter if it is physically across the coil or in the panel. Simply because there is no interference and "antenna" length is without effect.

It is different with a MOV. It limits voltage but does not prevent arcing. And that is irrespective of where along the cable the MOV is situated. An RC snubber is effective in reducing RF interference and a MOV is effektive when it comes to overvoltage/kickback protection.

And then, we haven't mentioned DC. Diode/resistor/zener etcetera work well there.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.