Voltage drop across DMM when measuring current?

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penneyj

Joined: 02 Jul 2009
Posts: 9

02 Jul 2009 16:23 voltage drop across a fuse

I was trying to measure the DC current of a circuit by inserting a DMM in series with the positive 3.3v supply terminal so that the circuit's positive supply was being fed from the DMM lead instead of the bench power supply.

I noticed the voltage was only 2.9 volts coming out of the meter while measuring the 220mA of current, so my circuit was not receiving the full 3.3 volts.

I duplicated the observation by trying another different type of DMM as well as replacing my circuit with just a load resistor to draw 220mA and there is always a loss of up to half a volt when I insert the DMM.

Is this normal and if so, is this current measurement even valid because my circuit is only getting 2.9 volts instead of 3.3 and its operation is impacted by supply voltage (RF transmitter).

How do I properly measure the current?

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LordAli

Joined: 22 May 2009
Posts: 40
Helped: 2
Location: Czech

02 Jul 2009 18:03 measure voltage drop

Use resistor with known resistance value and measure voltage across this resistor with milivolt meter. In your case I would use resistor 0.1R - 0.4R and mV.

If you measure current with fused amp meter input you can have some voltage drop across a fuse and across measuring resistance. If you know resistance exactly you can calculate also current going through accurately. Wink