Wetting Current Measurement

[shanjey]

How can I measure wetting current cross the switch contact?

We set up a test circuit with 7 mA, and then integrate the switch contact with series and measure the current.
Our spec is 1-7 mA. With this method we always get 6.95-6.99 mA and believe is high and the method we are doing not right.
Please advise. Thanks for your time and advise.

 

[davidbeach]

If you put 7mA through a closed contact you ought to measure 7mA through the contact.

Current through the contact is strictly a function of the voltage and the impedance of the circuit the contact completes. The contact has no impact on the current.

 

[shanjey]

Is there is any procedure available for wetting current measurement?

The way we do is set max current(7mA), And add contact to circuit and measure.

But wetting current is the minimum current needing to flow through a newly-closed mechanical switch or relay in order to break through any film (contact oxidation) that may have been deposited on the switch.

So if there is any standard procedure is available, may helpful.

Thanks

 

[itsmoked]

You are talking about current. The issue with switch contacts is voltage.

The contacts develop oxidation which is a thin layer of non conductive molecules. They, like any insulator, have a stand-off voltage. Depending on what materials the switch contacts are made of, that layer of oxidation will be chemically different. That leads to the various punch through voltages different switches need to reliably switch.

By varying the current you are actually varying the voltage. Once the voltage is great enough you get punch-through.

With most non-gold contacts you need about 10V to reach reliable punch-through. If you try to characterize a switch you are in for a long process. Why? Because you need time between tests for the oxide to form. Also any scuffing of the contacts as they come together alters this response. Furthermore any pitting of the contacts over their life will greatly change the response again.

Generally, if you are a switch user, you need to run 10V or more thru your switches unless;
1) The switches are designed to be self cleaning as they operate

or

2) They have gold contacts.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[analogkid2digitalman]

Keith:

I thought it was much lower than 10V, more on the order of a few tenths of a volt(no references). How do 1.5, 3V, and 9V battery operatted devices work reliably? Do they typically use gold in those applications, especially with units manufactured in China at minimal cost?

and also 3) Mercury switches. (Not very PC with ROHS)

Thanks

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 

[shanjey]

I get you point. But our Test voltage is 12.5V constant and contacts are gold/gold or carbon/carbon.

 

[shanjey]

I get your point. But our Test voltage is 12.5V constant and contacts are gold/gold or carbon/carbon.

 

[MacGyverS2000]

Not my field, but contact wetting should only matter for items such as relays. Slide contacts, like what are found on even the cheapest of products, provide a surprisingly high amount of self-cleaning friction.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

I thought it was much lower than 10V, more on the order of a few tenths of a volt(no references).

Nope. It just gets worse at those lower voltages. I have had switches fail due to contact oxidation up to about 15V, though, very rarely.

How do 1.5, 3V, and 9V battery operated devices work reliably? Do they typically use gold in those applications, especially with units manufactured in China at minimal cost?

Yes they either use gold, which is not particularly expensive in the quantities needed, or they have a good enough scrubbing design to allow fairly reliable operation for the first year or so.

I get your point. But our Test voltage is 12.5V constant and contacts are gold/gold or carbon/carbon.

And you have 12.5V / 7mA = 17,000 ohms of contact resistance? This is horrendous. Which probably means we are not understanding the point of your test. A normal switch has a contact resistance of about 0.010 ohms. That would result in a current of 12.5V / 0.010 = 1,250A, most likely flattening your 12V supply.

You need to fill us in more on what you are trying to accomplish. Why are you even doing this test? What are these contacts used for?


Keith Cress
kcress -

http://www.flaminsystems.com

 

[zeusfaber]

Could I check whether I have understood the test you are doing, and that we're not losing something in the language barrier?

As I understand it, your test procedure is:

1. Connect 12.5V power supply, resistor and ammeter in series.

2. Adjust either the voltage or the resistor (please tell us which) to achieve a current of 7 mA

3. Without adjusting any further, break the circuit and insert the set of contacts under test

4. Measure the current in the new setup.

If the test is being done this way, I suspect there's an element of trying to find out how wide a patch of oxide layer gets burnt past before the current rises to the point where the series resistor drops too much voltage for any more punching through to happen.

This could be relevant for applications where the contacts are switching a discrete signal driven by a high-impedance source - in which case the thing that is missing from the test protocol could be a measurement of the voltage across the contacts, and a comparison of that with the threshold voltages of whatever sort of logic is being switched.

Does that make any sort of sense?

A

 

[shanjey]

Exactly we doing the same way as you mentioned.

1. Connect 12.5V power supply, resistor and ammeter in series.
2. Adjust the resistor (1.785K) to achieve a current of 7 mA
3. Without adjusting any further, break the circuit and insert the set of contacts under test
4. Measure the current in the new setup.

But I feel we setting up (7mA) high end. Our spec is 1-7 mA.
It should more lower than 7mA. right now we are measuring around 6.96-6.99 mA depends on contact type. How to improve current set up?

 

[davidbeach]

You're trying to measure something that has no meaning. If the 12.5V and the 1.78k[Ω] are both completely stable, I'd be worried about any contact that showed less than 7mA. A contact that shows 6.96mA has a contact resistance of almost 16[Ω] and that would be totally unacceptable. If you measured only 1mA, that would mean a contact resistance of 10.72k[Ω]. How can your specification allow that kind of contact resistance?

What you need to be doing is using a very precise voltage source, starting at zero volts and slowly increasing the voltage until current begins to flow through the contact. That will give you your punch-through voltage. The current, once punched-through, will give you your contact resistance.

"Wetting current", per se, has no meaning or use. The only parameters of this type that have any meaning are the punch-through voltage and the contact resistance. Also useful are make, carry, and break current ratings, but that would be a completely different set of test equipment and/or set up.

 

[itsmoked]

This sounds like a perverted specification that was originally for measuring optical inputs which are only current sensitive. You would want them to trip between 1 and 7mA or call them dead. Not mechanical switches.

Keith Cress
kcress -

http://www.flaminsystems.com