Excerpted and expanded from another post as this is a trap that's 
easily fallen into.

When measuring current on ANY cheapo meter (and some not so cheap 
ones) measure the voltage drop across the meter when operating to get 
an idea of the meter's operating resistance . this may vary somewhat 
with current. The result may surprise you. Resistances of 10 to 20 
ohms on eg 200 mA range are not unknown and can cause major problems 
in some cases if not accounted for. Ten ohms at 100 mA causes a 1 volt 
drop. More current and higher resistance can more than double this. 
Such a drop may give an entirely inaccurate indication of what is 
really happening in a circuit or cause problems directly. eg a 10 ohm 
undecoupled resistance in a circuit may cause oscillations or drop 
voltages well below their expected value. A circuit that runs properly 
on batteries may malfunction when a current meter is added to the 
circuit. (Ask me how I know :-) ). (Still easy to get caught out after 
many years experience).

When current measurement at low voltage drops is required this may be 
achieved using a selected series resistor and a meter set to a low 
voltage range. For example, imagine that a circuit draws up to 100 mA 
and that a drop of up to 0.1 volt is acceptable. This is the 
equivalent of R = V/I = 0.1/0.1= 1 ohm. Placing a 1 ohm resistor in 
circuit, decoupling it's downstream side with a suitable capacitor* 
and measuring the voltage drop across it will yield an acceptable 
ammeter. Set to the 2 volt range a multimeter will read 0.100 at 100 
mA. ie volts = amps. Substituting a 0.1 ohm resistor and switching to 
the 200 mV range will give a reading of 10.0 at 100 mA.

If you want to keep your brain young you can use resistor values which 
are not multiples of 10 :-)

* If this is measuring current from eg a power supply rail then 
downstream decoupling is desirable. In some cases the circuit may be 
better not decoupled. In such cases the effect of the series resistor 
must be allowed for.

Use of an opamp based solution allows far smaller sense resistor 
values but in most cases the above will suffice.




                RM 

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