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OhmsLaw
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Gold
The Case of the Intermittent Relay
OhmsLaw   8/6/2011 7:37:28 PM
NO RATINGS
I once had designed a box with 96 high power relays for a custom SCADA control system for remote launchpad rocket control when I started in Aerospace Design in the late 70's. Nothing but the most reliable parts were used for Aerospace. But these Relays were failing after very little useage.

They had secondary 1A contacts for sensing closure.  Then one day, a month later into R&D, they started to fail intermittently open circuit on the low current contacts. So what's the deal? These were not cheap relays and they looked clean.

Project background info ... http://www.designnews.com/messages.asp?piddl_msgthreadid=240429&piddl_msgid=429892#msg_429892

Trying to sleuth an intermittent problem that only happens once in a while is about as hard as it gets. Anticipating the problem takes experience.

But this was not that tough a problem.  After all I was not exceeding the rating current and I allowed for contact bounce time in sensing.

Being a "green weenie" engineer and realizing they don't teach you this stuff in university, I had to find out what made these contacts fail some of the time.

So I wrote a program on the HP9825 in HPBasic to toggle each 25A relay and sensed the voltage on the 1A auxillary contacts.

The high power contacts worked flawlessly. So the program looped  turn on relay .. check result.. then turn off... check result and repeat for the next one of 96 Relays.

You should have heard this box.. It sounded like a gattling gun firing 20 rounds a second and then getting stuck on one relay. I'd tap the relay and it would fire away in sequence like machine gun in spurts. It seemed vibration would correct the situation temporarily and it would continue.

So then I researched relay technology and failure modes and read how low current contacts needed to be gold plated. I was only loading them with the equivalent of TTL current input and a pull up resistor on a 1A contact. Whetting should be around >5% or >10% of the rating depending on voltage. Lower current is ok with high voltage, but low voltage is a problem.

Well the high current contacts could not be gold plated as that would burn off pretty quickly.

So that clued me into that oxidation might be causing the open circuit in the low current contacts. But how could I fix this?

Well it turns out and all you pro's out there are already thinking..

Did you check the whetting current specs for the 1A contacts before you selected them? ie. the minimum reliable current for contacts.  (errhmm no, it was not listed in those days)

So what would you do? Swap vendors?  Search for gold plated Aux contacts? 

Nope.

I decided I could create an arc on each contact closure to clean it for next operation. That could satisfy the contact closure whetting current specs.

Solution:

Add one small Tantalum electrolytic (high rel) across each Aux Contacts for all 96 Relays. They connected to TTL input on a rack mounted sensor card.  The 5V discharge cleaned the contacts from the capacitor discharge on the 1st run and then it ran like a sewing machine errr... machine gun non stop with the software reliability loop test.

Long term aging and oxidation of the contacts could be a problem so I recommended WD-40 on the open contacts  to extend the shelf life between usage. For space, vacuum sealed silver contact relay$ are often used. I thought that might be overkill.

 Ultimately Gold Contacts must be used for any low current/ low voltage contact switches. I used Capacitor discharge instead. Not too big nor too small. Some switches may wipe the surface on closure are better.

Both relay aux contacts and toggle switches need this to prevent the oxidation that causes high resistance on the contacts.

Normal use with high voltage breaks down the oxide layer, but Low voltage , Low current does not.

Gold plating is golden but Caps are a quicker fix.

Another Murphy's Law episote thwarted with a little help from Sherlock Ohms. Another day in the life of R&D electronics.

Anthony Stewart

EE since 1975

 



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