MOVs or RCs?

Recently, a problem was experienced with noise due to OW-16 relay outputs from SLCs. Looking at the AB info, they suggest surge suppression. In the Modular Hardware Style User Manual (1747-UM011E-EN-P) there are a couple of notes on page 2-29 that say Varistors are not recommended on relay outputs and RC are not to be used with triac outputs. Has anyone had experience using MOVs on relay outputs? Can anyone justify ABs recommendations?

I had thought it was a lot of personal preference. No you have gotten me thinking (that's scary).

AB, despite their other problems, would have given this a lot of thought. It does make some sense to me.

I have used both on both, with out any problems that I am aware of, but I do lose track of the majority of my projects over time.

I generally use relay outputs, being that they are universal (AC or DC), plus typically higher current rating. I general use mov's on AC, but when customer is footing the bill big time, will use GE's RC units.

The more I think about it, I will only use RC on DC in the future. I like the cutoff that mov's provide. RC being more of a filter.I have seen some pretty impressive spikes coming off of solenoids and relays, may just scrap the RC thingies all together.

Wish I had time and money to go play with this stuph in the lab for a few months.

regards.....casey

the only problem I had with an RC was across a crydom DC SSR when I was cycling a small dc asco 1/8" solenoid at 700 cycles /minute. The RC was messing with the valve by "ringing" the coil voltage and keeping the valve half open. I called Crydom (excellent TEch support) and they specked a TVS (transient voltage surge suppressor)

Also with respect to a triac. If you try to switch DC with a Triac it will never shut off so along the same lines an RC circuit will maintain voltage until decay so the output won't switch off right away.

For ac I just use a standard 130V MOV. Closer is better so usually you have an LB or handy-el at the solenoid valve and right in that across the wire-nuts I put the MOV. (ps I don't use wire nuts I cut off 2 off a 12 terminal barrier strip of those nylon moulded type. Much better than wire nuts Grainger p/n 6yh99) Expecially handy for 4-20 loops where you can just remove wire and screw down on the lead of your fluke and not fight twisting and untwisting of leads all the time.

The reason why they may recommend against MOVs is that over time and repetetive surge "hits", the MOV breaks down and loses its capacity to shunt the surge voltage. In extreme cases they may fail dramatically.

That's why in any homebrewed surge suppressor you may build look for one w/ the max. joule rating available for the voltage rating you need.

I had some good app notes from Motorola and Harris semiconductors that went into pretty good depth on this from a design side, unfortunately I could not find soft copies of them. Instead I found this which explains MOVs and others...
http://www.extremetech.com/article2/0,1558,1155237,00.asp