How to measure microampere current

[raco_rage]

I have a circuit based on Low power PIC microcontroller. For the application, a battery will be used. To estimate the battery life, I want to measure the current consumed by the PIC and its peripheral circuit.

I already tried with a multimeter with mA range, but the PIC was not receiving power from the supply due to this. I think the internal resistance of the multimeter may be hindering the input current to the PIC, which should be consuming current in the range of a few uA to 100 uA.

I was thinking of charging a capacitor and then looking at its discharge pattern to determine the current consumed by PIC, but the capacitor is not able to drive the circuit.

Any suggestions from anyone on what possible techniques can be used??

 

[IanP]

You can always use an external shunt resistor and your multimeter to measure mV/uV across that resistor or smaller shunt resistor followed by an instrumentation amplifier and then your multimeter ..

IanP
:wink:

 

[enjunear]

I'm in the same boat as Ian on this one. Measure the voltage across current shunt between battery and Vcc on the PIC. Since you expect 1-100 uA, I'd start with a 1k resistor. The voltage dropped across the resistor will be 1mV to 100mV. If that's too much, then reduce the value, but make sure you are within the measurement range of the voltmeter... especially at the low-end.

 

[raco_rage]

yea, seems feasable to me.

But can you tell me why the PIC couldn't draw any current from power supply when the multimeter was configured as ammeter and range of 5mA?

I tried to understand it but I belive that the current being in few hundred uA range, the voltage for the multimeter was not enough to be measured. But still dont understand why the circuit would not be powered at all.

 

[enjunear]

yea, seems feasable to me.

But can you tell me why the PIC couldn't draw any current from power supply when the multimeter was configured as ammeter and range of 5mA?

I tried to understand it but I belive that the current being in few hundred uA range, the voltage for the multimeter was not enough to be measured. But still dont understand why the circuit would not be powered at all.

It's possible that the internal resistance of the meter was high enough to create a significant voltage drop across the meter (which kept the PIC from fully turning on), or the internal fuse was blown (that's a regular occurance with the handheld meters in the labs). I'd use a power supply with a leaded resistor to check the multimeter's operation on the 5mA scale. Use 5V and a 1K resistor... that should give you ~5mA of test current (not counting the internal resistance of the meter, which can be significant).