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I just got my first multimeter (TekPower TP8268) and I've been try to test it out, but I have been getting some strange readings with the current measurement. I have an AC adaptor to DC 12v and 5A. Here are the results I got across various resistor readings.

10Mohm -> 164mA -> 12.3V 10Mohm -> 173mA -> 12.3V

Here's how I hooked it up:

DC pos --- RES --- Mult-In --- Mult-Out --- DC neg /ground

I can't seem to figure out how I could have gotten these readings. If my calculations are right with Ohm's Law, 12V / 10Mohm = 12 x 10-7A (or .12 uA). I made most of these readings on a breadboard, but I have a hard time thinking it would throw the readings off this much.

I did, temporarily, switch the fuses in the multimeter, so these readings are using the u/mA range, but the 20A fuse. I don't know if that could cause the problem, but that's a possibility.

So, does anyone have any idea what might be wrong? Are there any additional tests I could do to help me figure it out?

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    \$\begingroup\$ Get another DMM. Measure the voltage drop across the ammeter. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Aug 5, 2015 at 2:59
  • \$\begingroup\$ Is that something I could do with a really cheap DMM? I would rather not spend and extra $50, just to check this one's voltage drop. \$\endgroup\$
    – caffein
    Commented Aug 5, 2015 at 3:01
  • \$\begingroup\$ As long as it has a mV range it should be fine. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Aug 5, 2015 at 3:03
  • \$\begingroup\$ Thanks! I'll give that a try and hopefully, that will clear the whole thing up. \$\endgroup\$
    – caffein
    Commented Aug 5, 2015 at 3:04
  • \$\begingroup\$ Also, remember that current is measured in series, but voltage is measured in parallel. And it would help to have an exact diagram of how you connected them; press the schematic button above the text entry to bring up a schematic editor. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Aug 5, 2015 at 3:05

1 Answer 1

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First, do not ever operate that with the 20A/250V fuse in the 400mA/250V position. Any momentary mistake and you will severely damage your meter. You may have already done that, unfortunately, and damaged the shunt so that it reads very high. Unless the test leads are in the wrong position.

When you get the proper fuse to replace the one you blew, always remember to connect the meter in series with something that limits the current, and always remember to switch the positive test lead to the appropriate current position and always return it to volts/ohms when done.

Never leave the test lead in either current position when you are finished, always in volts/ohms.

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