Design help needed for an interesting problem

[ljbdragon]

So here’s the problem, I'm designing a relay contact tester which automatically tests all contacts within special types of relays and provides a pass or fail indication based on the contacts resistance. The part that is giving me the most problems is the circuit that tests the contacts resistance. so here’s the requirements, the contact must be tested with a current of 40mA, the circuit must be able tell the difference between a contact normally open and a contact with a high resistance, and lastly the design calls for 4 resistance ranges, 1ohm, 5 ohm, 10 ohm, and 20 ohms which can be selected by the user. i.e. if 5 ohms is selected then all contacts which are 5ohms or less will get a pass indication.

The relays in question have around 16 contacts and come in different configurations i.e. 12 N/O 4 N/C or 8 N/O & 8 N/C and so on.

This has proved to be an interesting problem and I'd welcome any ideas however small.

I will try to test all ideas and post the results on here.

[vvvv]

Try a potential divider (a precision wirewound 1 ohm resistor in series with the relay contact) and then feed the junction to the ADC of a simple PIC. Scale the resulting voltage ( a function of the volts drop across the 1 ohm resistor to an ohmic value ) and then output result via the PIC's UART to the PC using a terminal emulator.

for a supply of 5 volts the following VD will occur for relay contact resistance of:

1 ohm =2.5 volts
5 ohm = 1.0 volts
10 ohm =0.5 volts
20 ohm=0.25 volts