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Measuring Contact Resistance

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Measuring Contact Resistance

Postby andywileman » Thu Mar 13, 2014 9:10 pm

Hi,

I'm trying to measure contact resistance across the contacts of a relay switching 6V through 6 Ohm load at a 1Hz frequency for reliability testing. I'm trying just to get the contact resistance which should be around 50 mV at 1 amp, but at the moment I'm getting a square wave of around 800mV. Is there a way of just measuring the contact resistant variation on the square wave instead of the voltage drop across the relay + the contact resistance. E.g. would a dc offset be of any use?

Any thoughts would be appreciated.

Thanks,

Andy
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Re: Measuring Contact Resistance

Postby Martyn » Mon Mar 17, 2014 1:37 pm

Can you post a drawing of your circuit ?
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Re: Measuring Contact Resistance

Postby andywileman » Mon Mar 17, 2014 10:02 pm

Hi Martin,

This is the circuit I'm using, it's the standard test manufacturers use.

http://www.pidtechinsights.com/2012/02/ ... of-relays/

All I'm seeing at present is around 800mV across the contacts, where the contact resistance should be in the order of 50mV (manufacturers data).

I wondered if it was possible to offset the level so the scope just reads the resistance?

Thanks for your help,

Andy
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Re: Measuring Contact Resistance

Postby Martyn » Tue Mar 18, 2014 7:27 am

If you don't use a square wave but just energise the contact, so it's permanently on, what voltage drop do you get ?

If it is not 50mV, or a value you would expect, then either the scope is setup wrong (x10 setting in the software) or the current drops when you switch the relay on. What sort of power supply are you using ?

Post a psdata file of your waveform and we can check the settings.
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Re: Measuring Contact Resistance

Postby andywileman » Tue Mar 18, 2014 4:39 pm

Hi Martin,

I've had a reconfigure of the circuit, I found this article on the web.

http://www.edn.com/electronics-news/438 ... t-Stations

I've attached the results below (2014318.0001), you can see the measurement across the contacts approx 900mV and the contact resistance (I think) superimposed on the top and bottom of the waveform at around 80mV. The trouble is, if I try to take a measurement say between ground and the bottom level of contact resistance at 5V range it still gives 80mV, but if I up the range to say +/- 100mV the contact resistance is only 8mV.

The power supply is a tti QL355tp if that helps and I'm using a 6 Ohm load.

What do you think?

Kind regards,

Andy
Attachments
20140318-0001.psdata
Measurement Results
(43.92 KiB) Downloaded 57 times
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Re: Measuring Contact Resistance

Postby PeterF » Tue Mar 25, 2014 12:02 pm

Hi Andy,
You say you are using the second test circuit that you mention. Can you confirm the actual current flowing through the relay contacts when they are closed, please? I am assuming that it is circa 1A.
The diode across the contacts will only allow about 700 to 800 mV across the points when they are open and there is no point in measuring that value, it is just the diode forward voltage. The voltage you should be measuring is the DC voltage when the contacts are closed and the total voltage then will be in the order of 30 to 50mV. If the current is exactly 1A that will give a value for contact resistance of between 30mohms to 50mohms. (1A through 50mohms gives 50mV).
You will need to set the oscilloscope voltage range to +/-50mV in order to get an accurate reading.
Your scope probe needs to be connected with both the ground lead & signal lead attached as close as possible to the contact terminals of the relay to remove volt-drops along wiring.
The contact resistance should be less than 50mohms when new as it will increase with use.
Please contact me again if you have any other questions.
Regards,
PeterF.
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