Tuesday, June 16, 2009

MilliOhm DVM Adapter


My recent works with SMPS entails winding of coils and, sometimes, current sense resistor which normally is very low in value. Decided to make the above adapter to measure low resistance which cannot be done using standard DVM. Yet to make it but will update on the usability later. Would be helpful also in tracing for PCB or winding shorts, etc...

2009.06.18 Update: The adapter worked as expected. The only issue is that the current limit resistors need to be 'adjusted' based on the LM317 used. LM317 reference voltage is guaranteed to be between 1.2V to 1.3V. If the resistors are calculated based on 1.25V, maximum possible error is around 4%. It's best to tune the resistor using a digital ammeter. Also note that circuit also measure probes resistance. As such, you need to deduct the voltage drop when the probes are shorted in order to get the actual resistance... If you set the current to 10mA then resistance is 1 milliOhm per 1 milliVolt.

Maybe I program a PIC for that and LED displays later... :-)

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