Relay Output Protection


Thread Starter

Patrick Levinson

Hello list members, I am driving several 24 VAC solenoid valves with relay outputs from my PLC (SLC503 with 1746-OW8 output modules). I know that MOVs are recommended across the load or contacts to minimize wear on the contacts (noise is not an issue here). My question is, what would be a "typical" percent reduction in contact life if MOVs are not used? The reason for my question is the fact that the wiring on the machine complicate MOV installation. The valve coils are typically rated at 70 VA inrush and 40 VA sustained, at 24 VAC.

One more question: the supply for the valves comes from a 24 VAC transformer; I want a low cost option for surge suppresion on this supply, and was thinking on using MOVs also. Any better suggestions?

Any comments will be appreciated. Thanks in advance.
I'm not certain how easy it would be to rate the effects of skipping the recommended installation of MOVs for surge supression across the coils since electromechanical degradation of your I/O has a lot to do with the number of cycles your relays switch and the amount of noise ambient in the environment.

Here are a couple of ways you might want to approach your situation:

1) Select a solenoid or connector with integrated surge suppression. Your solenoid manufacturer may have a solution specifically designed into a product which suits your application. If not, companies like Lutze, Muerrelektonik and/or Lumberg make products that provide a cable and connector assembly which integrates appropriately sized MOV's, diodes and/or specialized integrated circuits that allow you to connect your solenoid directly to the I/O card without intermediate connections between terminal blocks with MOVs hanging.

2) Companies like Phoenix Contact, Weidmuller and Lutze make relay modules that are relatively inexpensive ($10 - $20)and consume about the same space as a terminal block. Instead of using a relay output card use a 24VDC digital output. Electromechanical interposing relay modules provide a high level of isolation. These relays employ surge suppression and electromechanical isolation to protect your I/O card. When the relay fails, unplug the failed relay and plug a new one in. This will dramatically enhance the life and reliability of your I/O while eliminating the cost of down time related to card failure.

I hope these suggestions are helpful.

Al Pawlowski

I can't answer the question asked, but this question brings up another that I would like to ask myself. Can anyone comment on the current state of MOV technology with regard to operating consistency, i.e. V/I curve changes for
repeated conductance?

I have always recommended, and used, RC snubbers for relay arc suppression because the last authoritative information I remember reading stated that a MOV's "on point" decreases relatively quickly with each instance of conductance so that they are better suited as guards against unusual occurrences.

Paul Wedding

Percentage wise I don't know how the life of the contacts would be reduced, you could get the specs on the relays they used, if its an Allen Bradley PLC, they more than likely use Omron relays. Unless it's a temporary installation, you really have to do it. You could also try a "free wheeling diode" across the coil, say a 1N4007 etc.

If you don't use suppressors for the solenoids as the coil circuit opens the transient condition often generates spikes of several hundred volts back into the contacts and transient emi will be generated inside the I/O chasis. Most solenoid valves now days offer supression either built into the vavle and/or cable. Another space saver is terminals with integrated supressor varistors/capacitors or component terminals where you solder in your own MOV.
I'd recommend using an RC network in parallel with the MOV. The varistor will possibly (likely) degrade depending on the frequency of transient
events, and their energy level. Look up the PDF file regarding the 'LA' MOV series at Intersil's web site, and examine the curves listed in the

Adding the RC network limits the rate of voltage change over time (dv/dt), and is better than using an MOV alone.

Also, I think A-B has a publication at their site than discusses RC network selection in some useful depth.

Alas, the wiring problems adding these doodads often brings about is one of the black holes of industrial control systems, and I am not aware of any great answers. However, perhaps someone like Phoenix Contacts makes terminal blocks with built-in snubber networks ...


William L. Mostia, Jr

There are a number of things that can effect contact life such as contact design, current ratings, materials, rated number of electrical operations, number of operations during useful life, voltage type, load characteristics,
operating environment, etc. Consideration of contact life and the need to protect the contacts is a system consideration and not just the load

Just because you are switching an inductive load does not necessarily mean that you need contact protection. There are a number of combinations of the above that would not require contact protection. A relay that is operated with a medium number of operations well within its inductive current rating might be one. On the other hand, a relay operated at its limits would
probably need contact protection.

The best source of discussion of contact protection(a whole chapter) that I have found so far is in the book:

Noise Reduction Techniques in Electronic Systems, 2 ed.
Henry W. Ott
John Wiley & Sons

There may be a later edition available.

In regards to the use of MOVs, while cheap, the MOV is generally subject to damage each time it operates, depending on the level of "surge" it sees. After a period of time depending on the frequency, voltage, and energy involved, the MOV could fail, either in the open or closed position. I would not recommend it for contact protection for that reason. I would probably recommend a RC network for this type of application if needed.

My comments regarding the MOV are also applicable to the transformer application. Again, I would analyze the system and see if I really need any
protection. If the system consists of only electrical and electromechanical components, they are fairly resistant to power disturbances and the
transformer will help mitigate any transient. If the problem is that you have had failures or misoperations due to power disturbances, then I suggest you use one of the surge protection terminals that have built-in transient protection. These are available from MTL, Entrelec, Weidmuller, etc. Going cheap with MOVs may backfire if you have MOV failures that reflect poorly on your equipment.

Bill Mostia

William(Bill) L. Mostia, Jr. PE
Principal Engineer
WLM Engineering Co
Independent I&E Consultant
P.O. Box 1129
Kemah, TX 77565 USA
E-Mail: [email protected]

These opinions are my own and are offered on the basis of Caveat Emptor.

Steve Jamison

> Hello list members, I am driving several 24 VAC solenoid valves with relay outputs from my PLC

I'm an ME by training so I can only tell you what I've used with great success. Electrocube makes RC networks that are self contained. I think I buy the 47 ohm 1 W 0.1 mFd rigs for everything from 24 VAC to 240 VAC. Install them as close to the solenoid as possible. I usually install a small terminal block close to solenoid valves for easy repair/troubleshooting or put the network right across the coil terminals of relays. Electrocubes are a bit pricey if you need 30 or 40 of them. I found some units that aren't as pretty , that look like mylar caps for around $0.60 each.

Steve Jamison, NJ Industrial Controls