Electronic – How to measure current with a multimeter? Currently getting inconsistent measurements

currentmultimeter

I have a current controlled power supply giving out 8 A (at least that's what I'm setting), plugged into a cable (call it #1) which is then connected via screws&crimps to a copper coil (call it #2, 70 turns, 1.25 mm diameter, 25.4 m length) to generate a magnetic field.

At the crimps, with a multimeter with a 10 A max fuse, I measure 3.8 A. I am using croc clips on the screws.

If I connect the ground of the multimeter to the '-' terminal of the power supply, the reading becomes 5.5 A.

Why?

Is it normal that the multimeter does not read exactly the correct current? Should I trust that current or the one displayed on the power supply?

P.S. = cable #1 is probably not rated for 8 A. But that should not affect the current right?

schematic

simulate this circuit – Schematic created using CircuitLab

Image of test setup:

annotated image with power supply, cable, crimp+screw connection, multimeter, and copper coil

Best Answer

Simplest answer: you are using the multimeter incorrectly and can cause damage to your equipment that way. From the looks of your photo, your multimeter is shorting out the the coil in the present configuration. You should connect the multimeter like this:

schematic

simulate this circuit – Schematic created using CircuitLab

The small cable will not change the current recorded by the power supply, but will add resistance to your system. This may be what is currently saving your multimeter's 10A fuse.

PLEASE do some more research on how to use your instruments. This will help avoid damaging them or, worse, hurting yourself/others.

For using the multimeter, remember these things:

  • An ideal voltmeter has infinite resistance and is connected in parallel to the voltage being measured
  • An ideal ammeter has zero resistance and is connected in series with the current being measured
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