I have a cheap multimeter, which gives me different readings between the two inputs. I would like to set 100 mA using a potentiometer in a circuit, which runs from two AA batteries. If I measure the circuit using the standard slot which works up to 200 mA, it shows about 80-90 mA. In the same circuit, if I plug the cable into the 10 A slot, and switch the multimeter to the 10 A mode, it shows 0.11 A.
Could you tell me why is it happening and how should I know that:
- is my multimeter actually modifying the current, or just showing it?
- if not, then which measurement is the correct?
As a small question: if I buy a new multimeter, what should I look for if I want to buy a better one? Should I buy one from a known brand? Is there any way to identify the ones I should avoid?
Update: I think I should tell you what is my circuit: 2xAA batteries ---- 10 Ohm ----- IR LED (specified at 1.35V/100mA) --- multimeter ---- batteries
Thank you for all the detailed answers! Based on markrages' answer, I think in my case the multimeter is indeed modifying the current, because
- without the multimeter its 10 Ohms
- with the multimeter in 10 A mode its 10.1 Ohms in total.
- with the multimeter in 100 mA mode its 12.5 Ohms in total.
Which would prove the fact why am I seeing 80-90 mA vs. 110 mA in the two modes, as thats the same 25% difference as the difference in resistance between 2. and 3.
Even more extreme was when I tried to run the 1.35V LED from a single AA battery. In that case I used a 1 Ohm resistor, and the two readings in the multimeter were 35 mA vs. 100 mA. Now I understand why, because the multimeter + 1 Ohm acted as a 3.5 Ohm resistor.