Multimeter 101 Question: Low Level AC Current

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JoeTele
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Multimeter 101 Question: Low Level AC Current

Post by JoeTele »

I'm trying to measure some low level AC currents and getting unexpected results. Don't know if this is the measurements themselves or incomplete understanding of ohm's law on my part, at least in terms of passive circuits. 2 examples:

1. I tried to measure heater current in a 12au7, which would be about 150 or 300 milliamps for series and parallel respectively. I tried both, and what I found is that my multimeters when set to AC milliamps or microamps would not even allow current to flow. Is this normal?

2. I set my variac to 20 volts and put it across a 2w 47k resistor, which should have given me around 425 microamps. I measured the voltage and the resistance to be sure they were on, but was only getting something like 2.5 microamps when I put the meter in series with the circuit.

Neither of my meters are Flukes. I've got an Extech 411 true RMS and a pretty nice Radioshack true RMS meter that lists for about $60 on their site. Could both of these be insufficient for these types of measurements? For the record my guess is that it's me and not the meters.

Thanks for any insights!

Joe
Bob S
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Re: Multimeter 101 Question: Low Level AC Current

Post by Bob S »

Admission..
Reading series current (act or D.C.) I usually forget to plug the positive lead into the A socket on the meter.
Might just be me.
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Leo_Gnardo
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Re: Multimeter 101 Question: Low Level AC Current

Post by Leo_Gnardo »

There's often a fuse in the ammeter line. DMM's I have use 200 or 250 mA. If the fuse contacts are corroded you might get only a fraction of the current you expect thru the circuit. Sometimes you can clear the problem just by removing and reinserting the fuse. It's free, worth a try.
down technical blind alleys . . .
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pompeiisneaks
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Re: Multimeter 101 Question: Low Level AC Current

Post by pompeiisneaks »

Stating the painfully obvious part that I didn't realize when I first was trying to measure current, did you desolder the connection and put your ammeter inline? Just another quick 'noob mistake' I made and many may have as well. I don't know you or how familiar you are with DMM's so just checking.
tUber Nerd!
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