Cleaning Relay contacts by Electrical Current.

Discussion in 'General Electronics Chat' started by FrancescoC, Mar 2, 2016.

  1. FrancescoC

    Thread Starter Member

    Nov 22, 2014
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    Hi all,I have been asked to investigate the possibility to clean relay contacts by applying a voltage and a measured current to them.

    I have been trying to find some some information on the subject but i had no luck.
    The idea is to energize the relay whilst applying a voltage to the contacts.
    I have tried this method on several relays and it seems to work.
    However i cannot decide if is better to first energize the relay and then apply voltage to the contacts, or do it the other way round.

    And then there is the question of how much current and for how long to apply it to the contacts.
    I would appreciate any comment please.

    regards
    Francesco C
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Never heard of cleaning by applying voltage, are these dry contact applications?
    Max.
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Isn't that how a relay is normally used to switch a load?

    Can you describe exactly what you do? Maybe a schematic would help.

    John
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
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    There's some merit to the idea.

    Some early electronic ignitions were doomed to early failure because the circuitry didn't pass enough "wetting current" to keep the points faces clean.

    Just a little sputtering burns away oxide and tarnish - but each and every sputter loses a tiny bit of contact material.

    Some contacts have a hardened surface, its like the enamel on teeth, once its gone the pulp underneath wears rapidly.

    Within reason - switching with wetting current may cause less damage than cleaning with abrasive strip.
     
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  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Any data for that?

    Off hand, it seems polishing removes the high spots in preference to pits, arching probably accentuates surface irregularities. When the OP started his question, I though immediately of electropolishing. But I do not think that is his intention. I am not aware that can be done in air.

    John
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    And are we talking of dry contacts or power switching contacts?
    Max.
     
  7. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Yes. relays have a minimum current that can be switched. It is typically called the wetting current.

    I had a Keithley power supply that I "repairs" at work just by writing a program to operate the relay on/off overnight. The power supply was fixed by morning.

    I did at one time use mercury wetted relays in a low current application,.
     
  8. FrancescoC

    Thread Starter Member

    Nov 22, 2014
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    5
    Hi,
    I think the conversation is going into the wrong direction.
    I will clarify my original question.

    Where I work we deal with equipment that use some relays. (Dry relays)
    These relay have contacts rated from 1A to 12A at 230Vac.
    Very often these relay are difficult to replace due to fact that are no longer manufactured, or they have a long lead time (6 month).
    Add to that the fact they are sealed relay, you can see that many time we current current to 'clean' the contacts
    and reuse the relay (the alternative would be to declare the equipment not usable anymore).

    Because everybody has different opinion how this should be done. I am trying to get a 'formal'
    way of doing this. Normally our technicians look at the relay specs ant apply a voltage and a limited current
    within those specs. Then the relay is energized and ed-energized until the contact resistance
    is low enough to meet compliance, usually few m Ohms.

    The above system is also used when relays have been in storage for a long time.
    Under this condition a patina of oxidization will form on the contact point thus increasing contact resistance.

    I would like to formalize the operation by something like: a) Set voltage at 30V
    b) Set current limit at 2A
    c) Energize relay for 1s
    and so on.

    It is true that some relay do have a wiping action when in operation but many time that is not enough.

    The reason to formalize the operation is that some technicians apply too much current for too long time.
    This has caused contacts to weld together making the relay unusable.

    Regards

    FrancescoC
     
  9. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Are you useing AC to clean the relays?

    Are they staying within the specs of AC/DC ratings as well as maximum currents and voltages that can be switched?

    Thinking about it, I'd be more inclined to try:

    Close relay
    Apply current limited voltage. e.g 80% of rating
    measure contact resistance using I//V method.
    Break the contact
    Turn off the voltage
    Repeat.

    I somehow think that breaking the connection to be more beneficial than making it in cleaning, but I also see potential free cleaning also working.

    Some reasoning: A poor contact will generally result in heating. A high voltage should result in vaporizing and breaking down the contact resistance. I don't think you really want to heat the contact.

    I'm also wondering if plasma etching https://en.wikipedia.org/wiki/Plasma_etching could be used for non sealed relays?
    It's an off the wall idea.
     
  10. FrancescoC

    Thread Starter Member

    Nov 22, 2014
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    5
    Hi, 'KeepItSimpleStupid'
    I would agree to first close the contacts then apply voltage to them.
    You quoted 80% of rating. Is this a calculated number or is it the result of your experience?
    Also, you suggested to open the contacts then turning off voltage. Any reason for that?

    Regards

    FrancescoC
     
  11. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The kind of contact 'wiping' you are thinking of does clean the contacts, but is SUPPOSED to be done on a regular basis during the operating life of the equipment. In such an application it extends the life of high current relays and maintains low resistance in signal relays.
    Using it as a repair scheme is not proper. The relays have reached an end of design life limit. Your repair is unreliable and mis leading to the customer.
    You should put energy and money into a replacement. Develop a universal mount you can attach to the various models you repair and install new relays that give your customer a unit which you can warranty as "good", and not a unit you can only promise that "it's working right now" but could fail again any day.
    See attachment. Lots of good info in the back pages
     
  12. FrancescoC

    Thread Starter Member

    Nov 22, 2014
    30
    5
    Hi Kermit2,
    Believe me, our customer are fully informed on matter. When the ask to have an old equipment repaired
    they are always told what are the alternatives. They are also aware of our current system for restoring old relays.
    If a relay reaches the end of life, we would not be able to repair the equipment, and the customer has to trow away their equipment
    because no replacement for the relay is available. They rather prefer we use any means to 'restore' the old relays.

    The idea is to formalize/change the method so that is used consistently and therefore can be monitored.
    Thus we could then try to find how effective is our system.

    If you had to restore old relays contacts, how would you do it?

    Regards

    Francesco C
     
  13. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    I've only ever repaired vibrator contacts(to power tube radios in mobile installs)
    It was done by opening the metal cans and removing the relay so the contacts could be cleaned and polished.
    This restored a full operating life.
    You should be generating sales of new equipment by these customers with old end of life units.
    If you cannot access the contacts the equipment is just one more relay activation away from another failure.
    Cleaning the way you do should be a "free" service to keep a customer going TEMPORARILY until his replacement unit comes in.
    If you cannot do this, then I am out of ways to offer help.

    I would suggest the use of AC current and a relay operating freq that insures breaking action occurs randomly so high and low currents of both polarity pass through the contacts. Use the lowest current from the range that relay is designed for. Measure contact resistance by way of a millivolt meter while passing a known DC current through the contacts. That's all I got
     
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  14. Roderick Young

    Member

    Feb 22, 2015
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    One small tidbit, for what it's worth. When I was a teen in a shop repairing pinball machines, sometimes a relay would get its contacts fouled, and stop working. It was invariably a relay that was switching DC. Relays that had AC running through the contacts never seemed to fail. So maybe AC, or reverse bias DC, will clean the contacts?
     
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  15. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    That is what I oft do. We have one automated test set where the guy who designed it used 10A relays in the sense point controls thinking they could also do some power switching for him. Every year or so at least one relay we use goes open.

    I do something similar to what you do, but with less current and voltage, usually 1A at 2 or 2 volts from a regulated lab bench supply. An amp is more than enough to burn off a mild oxidation layer, and I keep the voltage low because when closing the contacts you get a large rush of uncontrolled current from the output cap of the power supply.

    We have a shorting jumper and a simple program to cycle thru all the relays (we have 100) and burn each in turn. Usually one or two passes does the trick.
     
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  16. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    AC is much easier to switch than DC due to AC always going to 0 current every half cycle..
    DC is just ON all the time and develops a much larger arc.. arcs cause pitting,etc...
     
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  17. ian field

    Distinguished Member

    Oct 27, 2012
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    If the contacts have developed irregular surface, closing them with voltage applied is more likely to blatt the irregularities away - applying voltage while they're closed stands a better chance of spot welding them.

    Some relays are designed so the contacts wipe slightly on closing, hooking the coil up to a multivibratorer and running them in with no voltage/current applied might do some good - it would be better if you can gain access to the contacts and blow out any dust that evolves.
     
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  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If these are dry contact relays as you mention, then I assume it is just oxidization over a certain period.
    Therefore they should show limited contact wear, If I were faced with this as a preventative maintenance issue, I would ensure a small stock as spare and at a pre-determined preventative maintenance period change out the relays with ones previously taken out of service where the contacts have been re-polished in the appropriate manner, Far better than the procedure of passing current through a dry contact to clean it, also a more definitive approach. IMO.
    Max.
     
  19. FrancescoC

    Thread Starter Member

    Nov 22, 2014
    30
    5
    Hi all,
    thank you for the all the advice and opinions.
    I am now more aware of the task ahead.
    I will talk now to my colleagues and decide how to proceed in creating a
    a viable procedure to restore the relays.

    Regards

    Francesco C
     
    KeepItSimpleStupid likes this.
  20. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    One thing we didn't talk about was the environment where the failed relays come from and is that a factor?

    I agree that you should be looking into correctly chosen alternatives.

    I worked in a lab and when there were recurring failures with a known cause, I would fix them forever.

    My predecessors would replace a drive transistor every year or so. I took me longer, but once fixed, the next failure was a circuit breaker that died of old age.
     
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