Really stupid multimeter question

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hey - there must be a really simple answer for this question, so i will make sure i kick myself when i get the answer.

i have one of those cheap digital multimeters with 3 connectors for the probes - one is GND, the other is to measure AMPS (up to 10A) and the third is positive terminal for V, mA, Ohms.

to measure AMPS - no problem, just connect multimeter in series and it works.

to measure mA - i connect it in series and no current flows (ie the device i want to measure is off) i have also checked the fuse and it is fine.

What am i doing wrong??
 
Yep, that's a problem. Easiest way to get around it is to put a switch in parallel with the meter. Close the switch and power up the device. Now the caps are charged and the current should be low, hopefully lower than the fuse rating. Open the switch and measure the current.

Back in the good old days (before my time) when many things were done with sensitive galvanometers, one got very good with switches and limit resistors- failure to do so would result in blown galvanometers and a slip-up took about 1 mS. Later d'Arsonval meters were better, but there were lots of bent pointers as the pointer slammed into the stop from too much current. You just don't know how good you've got it with a modern DVM!
 
There is already a switch in parallel with the leads. Put the meter in the 10 Amp position, wait for circuit to stabilize, switch to mA range. Of course while it is in the 10 Amp position you will verify that the steady state current is less than the next more sensitive meter range.

Yes, there will be a momentary interruption during the switchover. Switch it quickly and it should be fine.

Buy plenty of spare fuses, you are not alone with this problem. The real problem is that most multimeters have too large a step from 10 Amps to 200mA.
 
or you could measure the current by the indirect method. put a 1 ohm resistor in series with the circuit and set your multimeter to read millivolts. measure the voltage across the resistor. 1ma=1mv. just select the proper wattage resistor, because wattage increases quickly 1A=1V=1W, but 3A=3V=9W. with a 1 ohm resistor the formula for wattage simplifies to P=I^2 or P=E^2. so 10 watts would be 3.16 A. most 5 and 10W ceramic resistors will handle much higher inrush currents than this, but not continuous currents.

i always use the indirect method, and have yet to blow a meter fuse. in fact i'm so used to measuring current that way, that the one time recently that i did use the ammeter function on my meter, i forgot to switch the leads back to the Volts/Ohms plug. i then tried to measure the line voltage from an outlet. several trips to the breaker box later, i finally looked at the meter and realized what i forgot to do...... (BTW, that particular meter's 10 amp range is unfused and the shunt is a piece of what looks to be 12ga wire, which is why it tripped the breaker rather than a meter fuse).
 
unclejed613 said:
or you could measure the current by the indirect method. put a 1 ohm resistor in series with the circuit and set your multimeter to read millivolts. measure the voltage across the resistor. 1ma=1mv. <snip>
(BTW, that particular meter's 10 amp range is unfused and the shunt is a piece of what looks to be 12ga wire, which is why it tripped the breaker rather than a meter fuse).

While your practice is correct, 1 Ohm as a current sensing resistor is kind of a lot, it may influence the circuit under test. Of course it depends on what it is you are measuring.

A 0.1 Ohm still produces 0.1mV for 1 mA and is generally less likely to interact with the circuit you are trying to measure. For 0.1 Ohms, a 5 watt wire wound would allow up to 7 Amps before overheating.
 
thanks guys for the info and ideas, i actually like the parallel switch idea.

Well i am trying to measure the standby current of a car radio, and as soon as you connect it up it draws about 350mA and then shows nothing (on the 10A scale)

I will get some fuses soon.
 
the reason i recommended 1 ohm is that the OP was trying to measure low milliamps. some multimeters don't have a 200mV range, but have a 2V range. for anything above an average current above 1A i would agree with using 0.1 ohm. it's the electronic corollary of Heisemanns uncertainty principle (or at least the simplified understanding of it that "the smaller something is, the more that your measuring of it will disturb that being measured. the actual principle is that you can know the velocity of a particle, or the location of a particle, but not both)


i used to calibrate multimeters when i was in the army, and i've seen far worse than what happened to you. it seems that the guys in the motor pool didn't have much training on multimeters. one particular model had a 10A fuse in the input plug, and a 50A cartridge fuse inside the meter in series with it. it seems they regularly tried to directly measure the full output current of a truck battery, because the meters would regularly come in for repair with the 50A fuse blown. this particular multimeter had no separate AMPS plug.....
 
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