# Recommended Relay Wiping Current



## wildleg (Apr 12, 2009)

desolder the relay and jump out to a component that does what you want.


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## Moonshot180 (Apr 1, 2012)

Sounds to me that the relay and associated contacts within may be defective. If the current and voltage fall to within the rating of said relay then said relay should be replaced with a suitable replacement, since you are evidently having problems with the device presently installed.


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## JRaef (Mar 23, 2009)

Most good relays will give you that number, somewhere. I ran into this same issue on elevator controls once, the contacts were bifurcated, but the signal was TTL logic levels, so only 5VDC, 7 mA and I eventually found the relay contact info saying it required at least 10mA of current so it was too low to keep the contacts cleaning themselves. I was not able to increase the current by adding a series resistor because it would drop the voltage too low, but I was able to add contacts in parallel to increase the reliability. That may not be an option for you on PCB mounted relays, but do you have any range within your 24VDC to add a burden resistor?


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## RSmike (Jul 31, 2008)

JRaef - Yep that's my issue exactly... 

Lots of people look at me funny when I try to explain this problem to them. Most aren't familiar with the "too little current problem".

I can add a resistor on the other side of the contact (parallel) before it goes into the input module. When it closes the resistor will pull more current through the contact so it "wipes". 

I know the relay is rated at 10mA minimum. The input module needs 12mA. I know I'm right on the edge of being acceptable which is probably why this worked for a few years without any issues.

I was hoping someone had seen something official or something along the lines of "when I worked at NASA we always took the minimum relay contact current and increased it by 157.5% to overcome that type of problem"....or some stupid rule of thumb. For example...the world says to double the wattage rating on a resistor that'll be under constant load (unless you want to burn your finger)

I'm thinking about putting in a resistor and increasing the current by tenfold....going from 10mA to 100mA. Thoughts on that?

The original question still stands... I'd want to know the ideal value to maintain the "wiping" action as well as maintain the contact life. Surely there is a sweet spot for something like this. 

RSlater,
RSmike


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## JRaef (Mar 23, 2009)

Every relay contact is going to be different. Gold flashed contacts are best for low level circuits, bifurcated are next best, but silver-cad contacts can build up a film over time and need energy to burn it off, they actually REQUIRE a little bit of arcing. If the current is too low, they can't punch through the buildup.

Another option for you is to use the PCB mounted relays you have to drive interposing relays, and use ones that have lower minimum current requirements. For example there are what I call "terminal block" style interposing relays out there that are maybe 14mm wide, with relay contacts rated at just 20mW of minimum energy, as in 2.5mA at 8VDC. However their coils will pull enough current from your PCB mounted relays to keep them clean. The A-B 700-HLF relays with gold flashed contacts are what I just looked at.


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## Night Owl (Jul 14, 2015)

*parallel snubber cap*

I just read a wiki page that suggests using a snubber capacitor across the contacts to boost wetting current.


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## RIVETER (Sep 26, 2009)

RSmike said:


> I need some help...I'm running into a situation where the current through a relay is too low to "wipe" the contacts.
> 
> Based upon what I'm seeing the relay is too lightly loaded to sufficiently wipe the contacts. Mechanically the contacts close but electrically the resistance is too high to turn on the PLC input.
> 
> ...


Swap input cards from another slot. Possible weak optical isolator.


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