I am dealing with a bunch of relays that have burnt out contacts: Mechanics works fine but contacts measure 3 - 15 Ohms between them instead of zero. I opened several - the contact pads turned dark gray, almost black. It is not coal, still looks like metal surface. The dark surface is very thin, several passes of sandpaper clean it and relay works fine after that.
I wonder if there is a way to clean the contacts without opening relays (the relays are sealed, opening destroys them)
Appreciate any wisdom
Andrey
PS: the relays are used to switch RF power at 2 - 30 MHz
It sounds like you either have el-cheapo relays or that they're being abused -- are they actually switching when the power is on, or should they only be switching with power off?
If the system is designed to switch with power on then it'll take a special relay to not break down under that treatment; I don't even know if such a thing exists at all.
If the system is _not_ designed to switch with power on then either something is wrong and you _are_ switching with power on or something (like way high SWR) is wrong and the relays are seeing RF voltages high enough to arc the contacts when they're open.
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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Hi, Andrey. I'll leave the wisdom to others, but here's some advice.
Sandpaper is pretty brutal. The idea is to clean the metal surface of the contact without decreasing the surface area by gouging it. Contact cleaners are available to do this job. They are a little longer than a matchstick, about 1/8" to 1/4" wide, and flat. They have very fine abraisive material, and are stiffer than sandpaper, which will tend to misalign the contacts. When using them, insert the flat between the two closed contacts and gently rub back and forth a few times, being careful not to twist the contacts out of alignment.
However, if you can't open the relays, you have to find some way to break through the surface oxidation (the dark gray/black indicates you probably have silver contacts). If nothing else is wrong, it's possible that switching a higher voltage (AC, 50/60 Hz) at something below the rated current of the relay will do the trick. The heat of a small contact arc is sometimes enough to refresh the contacts, if they're not too far gone. Can't tell you exactly without looking at the relay contact ratings, but I'll usually use either a 24V or 120V light bulb. Hook up the AC, switch your bulb on and off a couple of times, and see if that improves your contact resistance. Note also that the cold resistance of the bulb is much less than when heated, so you're actually goosing the contacts with several times their rated current current for a few milliseconds. Note also that this does not work and should not be tried with the bifurcated contacts often used in signal switching, which will tend to stick when you use them in this manner. And that can't be fixed without opening the case.
This trick may work for relays and switches which are being used for dry contact (low voltage and low current) service, too.
Could also be oxidisation if they dont have some 'wetting' current which is not easy clean off without physically touching them. Are they located in good conditions ?
You didn't ask, but here is my suggestion to prevent the problem. Ideally, the relay should be energized before RF power is applied and de-energized following the removal of RF power. It may be arcing between the slightly open contacts causing your problem.
Two words: Ragnar Holm. Hard to find but worth the effort.
Failing that, read the book Schrack published on relays. very interesting stuff. here are 2 great ways to ruin relays PDQ:
1) use a low current relay to switch a large inductive load - that'll splat the hell out of the contacts, quite possibly welding them closed.
2) use a whopping great relay (say the one that should have been used for the load above) to switch a thermocouple. The contacts will go intermittent, and perhaps stick open or closed.
relays which are designed to switch high currents have really grunty contacts, carefully designed by hordes of fanatical metallurgists. Often a current of 200mA or so is required to blast the inevitable accumulation of crud off the contacts - using such relays for signal apps fails to clean the contacts, and problems ensue. I personally have overseen the replacement of a few dozen "big" (well puny really, 10A
230V) relays used to switch signals in a low voltage tester. Rather than getting the requisite gazillion switching operations, they crapped out after a few tens of thousand. So I replaced them (OK, got a tech to replace them) with a suitably chosen low-current "signalling" relay
Conversely low-current relays use totally different contact materials, and invariably rely on some mechanism other than a good belt of amps to keep the contacts clean. Try to switch an inductive load with one of these, and count the hugely reduced number of operations - hundreds, maybe thousands, but nowhere near millions.
I once looked at RF relays, and man were they *expensive*. I suspect you just have the wrong relay for the job.
Some of us distinguish between the two based on the construction.
Best regards, Spehro Pefhany
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I read in sci.electronics.design that Spehro Pefhany wrote (in ) about 'How to clean relay contacts?', on Fri, 4 Feb 2005:
What are the constructional differences? I have relays that look like small contactors. Or....
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Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
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They are contactors iff they can truthfully be so categorized under US Harmonized Tariff Schedule Item 8536.41.45 or 8536.41.65
Best regards, Spehro Pefhany
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"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
AFAIUI, contactors have bridging contacts (two pairs per circuit). Relays have one pair of contacts per circuit and a current-carrying wire, braid or flexure of some kind. Admittedly, this distinction gets stretched pretty thin with liquid state relays/contactors.
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Yes, the pusher type of relay. From what I've seen, that's a European & telecom thing. Relatively inefficient electrically, and contact life is reduced for a given coil wattage, but superior coil-contact isolation.
If you feel like it, Google on "definite purpose contactor" and see what you find.
But it could be a Pondian thing.
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
I *always* open them up, and always life-test them under load before using them in a new application. Even some very cheap ones if you look carefully you can see signs that they were manually tweaked in production. Not what you want for high reliability.
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
I admit to not seeing any WWII era US radar altimeters. The only WWII surplus stuff I saw (when I were a lad, of course) was probably RCAF and really looked like it had been through a war.
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
"Spehro Pefhany" schreef in bericht news: snipped-for-privacy@4ax.com...
Well, if I were asked about the difference between relays and contactors,
*I* would make something up about contactors for heavy stuff like large motors and what have you, and relays for the smaller and milder tasks. Talking about double contacts, I think about vibration issues, different lenghts of blades, each contact looks like a fork, where each contact has a different resonance frequency. For use in nasty industrial environments. This 'trick' Ive only seen on the smaller relays up to a few amps rating.
Copper blade fork, with different lenghts of contacts, top view: ######################.### ##### ###################.#### Two of the above needed to make a switch.
Some 'Relay' contacts are a sandwich of contact blades and insulation spacers, hold together with popnails or tiny screws. The larger contacters have those typical thick braid wires. +25Amps.
One of us has it totally reversed, relays and contacters. Not that it is terribly important... what's in a name. Use what fits the requirement.
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Thanks, Frank.
(remove 'q' and 'invalid' when replying by email)
I read in sci.electronics.design that Spehro Pefhany wrote (in ) about 'How to clean relay contacts?', on Fri, 4 Feb 2005:
I still have two out of a WW2 AN/APN1 radar altimeter. They claim US citizenship. (;-)
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
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