Mercury Relay Contact resistance using SCXI 1127

[Vijay shah]

I am trying to measure contact resistance of Mercury Relay on PWBA. I am using SCXI 1127, 1331 terminal and DMM in 2 wire mode. The Mercury Relays have 0.25 Ohm contact resistance but it measures 0.7 to 1.5 Ohms and it is unstable every time with repeat test. I would like to know what is the contact resistance of SCXI Relays. Is there any ground need to be connected to stabilize readings or any other suggetions to measure these resistance using DMM. The PWBA I am using has 20 Mercury relays. Thanks

[Raecine Meza]

The path resistance of the SCXI-1127 is less than 1 Ohm.

What is most likely happening is that you are measuring the protection resistance on the paths of the switches you are using to connect the measured resistor to the DMM. Try using the 4-wire measurement mode, rather than the 2-wire mode. It will separate the current path from the voltage path, and it will allow you to measure the voltage drop on your resistor directly.

[Vijay shah]

I don't think the 4 wire measurement will make much difference,I will try it but it may stabilize the readings but since SCXI relay contact is about 1 Ohm it wan't be much help. Typically Mercury Relays have 0.25 ohm resistance and in my measurement it has a very tight tolarance requirement and that is the reason we have to use Mercury Relays. Is there any other way to measure the contact resistance of Mercury Relays without using SCXI and use DMM by other means. Any suggetion. It looks like PXI system in this case has limitation and only this can be solved by other idea. I would like if some one has other idea to explore. Thanks.

[Raecine Meza]

It is always possible to measure the resistance directly with your DMM. Using a switch module in conjunction with your DMM is not necessary, but they make a great combo for getting a higher channel count from a single-channel DMM. To measure with the DMM, just use your standard banana-plug cables to connect to Hi and Lo connectors of your DMM module. You can run the DMM Soft Front Panel to verify your signal.