# In measuring relay contact resistance, should I use 4 wire measurement or calculate it using the voltage across the relay and current?

I'm reading mixed things online where a current from a typical ohmmeter is too low to accurately tell us the true resistance of the relay contacts.

I've got a 30V 2.5A power supply, electronic load, and an Agilent 3458A meter at my disposal.

The relays under test have a max carry current of 1.3A

• The two things you mention in the title are not mutually exclusive. You do both: you measure voltage obtained from a known current and apply current in a different cable pair, hence 4 wire measurement. – vangelo Jul 30 '19 at 19:39
• Measuring relay contact resistance is going to be difficult. That's like measuring the weight of a feather with a historic scale. It's not impossible, just a tad difficult. - Do you care about the actual resistance, or do you care if it is less than X ohms? – Harry Svensson Jul 30 '19 at 19:45
• @vangelo that is true, but I've read from Panasonic that you should use higher currents for relays with higher carry current ratings. So I've got a relay rated for a carry current of 1.5A, and Panasonic suggests injecting a current of 1A to calculate the contact resistance. A TE pdf I found online also states that I should inject a higher current than your typical ohmmeter, and I'm not sure if 4 wire measurement would suffice – P P Jul 30 '19 at 19:57
• @HarrySvensson I want it to be less than 0.5 ohms – P P Jul 30 '19 at 19:58
• I'm not sure what you're thinking a 4-wire measurement is. Four wires is four wires. Two, supplying 1A, and two, reading some small number of mV. That's four. – TimWescott Jul 30 '19 at 20:05

In the image below, from the document you mention, if you connect a voltmeter close to the contact you are measuring, you are performing a 4-wire measurement. The high current does not circulate through the voltmeter cables.

If you don't want to do this with this crude setup (variable resistor) you can improve it by implementing a current source (which is quite simple) or use a bench power supply with this feature.

By using any of these 2 options with current sources you can use your very precise bench meter to measure both the current and voltage.

In measuring relay contact resistance, should I use 4 wire measurement or calculate it using the voltage across the relay and current?

They are the same thing. A 4 wire resistance measurement measures the voltage across the unknown, while applying a current through a different set of contacts to the voltage measurement.

You can do this manually on the bench with a current power supply, voltmeter and ammeter, or you can use a '4 wire resistance measurement meter' which integrates those elements, and bundles 'ease of use' into it as well.

With an unpackaged relay, you could measure the voltage directly across the contacts, and get the pure contact resistance. If it's a packaged relay, the best you are going to be able to measure is the contact resistance plus the connections from the contacts to the closest you can get the voltage leads, the package terminals.