We have a product which is to do resistance measurement. The simplified schematic is as below. A 50mA DC current flows through the resistor, and we measure the voltage on the resistor's terminals. There are three relays from OMRON, G4W-2212P-US-TV5-24VDC. And because that relay has two contacts, so we use them in series. That is both RLY2 and RLY3 actually two contacts in series.
Today, we encountered an interesting problem. When all the relays closed, \$V_{CD} < V_{AB}\$, the 10 ohms standard resistor was measured to 9 ohms or so. It's not surprising that there must be different voltage drops on the two relay branches. What surprised us is if we break the current path and re-close it (either control RLY1 or break the loop manually), then the resistance measured is correct. Considering the relays have been in sock about two years, we doubt their contacts may not in good situation, so we break them and do a contact cleaning. Then all are OK.
The problem is resolved now. But i have some questions:
- Why breaking the current loop and re-closing it will work?
Is it normal for a relay without using for 2 years to behave like this? Or we choose a relay not suited for our applications?- Beside breaking them and clean the contacts, are there other methods to make them work?
Update:
Thanks to MarkU, he points out the doc states the relay is not suited to switch micro loads less than 100 mA, but i still want to known the answer to other questions, thanks.
simulate this circuit – Schematic created using CircuitLab
Wetting current
is what this minimum current is "officially" called (wikipedia)... has to do with breaking through the contact oxidation. \$\endgroup\$