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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.

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Most likely the transformer supplying the lamps could not supply enough current to blow the multimeter fuse (assuming it had one on the 10A range, not all do). So you shorted it out. That's okay for a few seconds on a transformer, usually, but much longer and you could damage the transformer or blow a fuse or thermal fuse somewhere. If you did this one a high current supply (put an ammeter across it) you would have damaged something or could even have gotten hurt (eyes or burned). Anything to do with the mains is particularly dangerous, but even a car battery should be respected. –  Spehro Pefhany Jul 12 at 4:56
    
I checked and the meter does not have a fuse on the 10A range. The meter seems to work fine now. So either the transformer went into OCP mode, I took the probes off soon enough, or both. I think I will not be doing experiments on the 10A range anymore. –  Grid Trekkor Jul 12 at 5:14
    
There's nothing wrong in short-cutting a transformator for a short moment. You even have to do so to determine the short circuit current (and with that the inner resistance of the supply). You just have to make sure your testing equipment as well as the transformator is fine with that (and you don't connect/disconnect on hot wires). –  Mario Jul 12 at 8:51
    
@Mario If that transformer is a 100kVa pole pig instead of a small electronics power transformer, shorting it out might be... unwise. –  Spehro Pefhany Jul 12 at 12:13
    
@SpehroPefhany Short-circuit tests are usually performed with reduced (or adjustable) primary voltage though. –  ntoskrnl Jul 12 at 12:32
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2 Answers 2

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

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Thank you for the explanation. So I stole current from the lamps; the MM took it all. I suppose if I'd had a Kill-a-watt hooked up, the current on THAT would have skyrocketed. I was looking at your "correct" diagram. Is that essentially removing one of the lamps and placing the meter probes on the terminals? That the power supply kept up with that demand spike is pretty good for a device made in 1979. –  Grid Trekkor Jul 12 at 4:45
    
They had OCP even back in 1979. –  Ignacio Vazquez-Abrams Jul 12 at 4:51
    
You cannot merely replace the lamp with the multimeter, since the multimeter's electrical characteristics won't be the same as the lamp. If you really want to measure current through the lamp, you need to: remove the lamp, use a wire to connect one terminal of the socket to one terminal of the lamp, and put the multimeter between the other terminal of the socket and the other side of the lamp. –  Zuofu Jul 12 at 4:55
    
@IgnacioVazquez-Abrams - are you saying that maybe the power supply sensed the spike and immediately cut the output? –  Grid Trekkor Jul 12 at 5:04
    
Or at least kept the current to a level that would be not more than the electronics would normally be expected to draw. –  Ignacio Vazquez-Abrams Jul 12 at 5:05
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If your meter still works, you are a lucky person, if your transformer still works, you are an even more luckier person because it is easier to find a replacement meter than the required transformer. When you use your meter as an ampere meter, you are turning your meter from the tip of the black probe to the tip of the red probe into a wire with almost no resistance; in other words, a dead short. Understand that you only hoof up the ammeter in series with the circuit where you want to measure the current. I would strongly suggest that you splice in an automotive fuse holder in line with your probe lead, you select whether the red or black. Then insert a 10A fuse when using the 10A selector and a .25A fuse when using the milliampere selector. Buying blown fuses is cheaper than buying cheap multimeters. With over 40 years in the electrical/electronics field I still blow a fair share of fuses in my leads, sometimes when I forget to move the red lead to the voltage hole because in a hurry.

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