Electronic – Multimeter experiment with 10A setting — what happened

multimeter

I have a little experience with a multimeter, but I had never used the 10A setting.

I was messing around with an old stereo tuner that has four fuse-type lamps used to the light the dial. They are each 6.3V, 250mA. So I switched the cable and selector to 10A. Then I put the probes on either side of one lamp, and all four of them went out! Took the probes off and the lamps came back on. Suddenly got a sick feeling, thinking I had fried it, but everything appears to work fine.

What happened? Did I steal current from the lamps or short them? The second-highest setting on my MM is 200mA, so I thought that 250mA would blow the fuse on the meter. Hence my thinking that I should go to a higher setting.

Best Answer

In order to measure current, your measurement device must have that current flowing through it. In order to do this without disrupting the behavior of the circuit, the resistance of the measurement device must be very low.

Therefore, consider what happens when you put a multimeter set to current measurement mode in parallel with a device hooked up to a voltage source. Because the multimeter has a very small resistance (much less than 1 Ohm), whatever current the voltage source can supply will go through your multimeter, while none will go through the lamp. Note that this current is not typically the normal operating current of the lamp, and is usually much higher, and potentially out of spec for both your meter and the power supply. Therefore, you run the risk of destroying your power supply or destroying your multimeter (or at least blowing a fuse in either).

The proper way to measure current is always in series with your load. For example, in the following circuit, you must break the existing connection to the load to properly insert your multimeter and find the current labeled 'i?'.

schematic

simulate this circuit – Schematic created using CircuitLab