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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

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    \$\begingroup\$ Can you provide a sketch or schematic of how you are connecting your multimeter to the circuit and what that circuit is? \$\endgroup\$
    – user2943160
    Jul 9, 2016 at 2:44
  • \$\begingroup\$ Sure, I added a diagram. \$\endgroup\$
    – SuperCiocia
    Jul 9, 2016 at 2:52
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    \$\begingroup\$ No, you added a photograph. We need a wiring diagram (schematic) to see how exactly you have connected your ammeter. To correctly measure current, the ammeter must be connected so that ALL the current you want to measure passes through the meter. It appears from the photo that you have connected the meter between two points in the circuit, rather than breaking the circuit to insert the meter (but it is hard to tell from the photo). There is a schematic drawing tool available when editing your question - just hit "Control-M" to open it. \$\endgroup\$
    – Peter Bennett
    Jul 9, 2016 at 3:05
  • \$\begingroup\$ Yes, I see it. I have the ammeter in parallel. Wrong. \$\endgroup\$
    – SuperCiocia
    Jul 9, 2016 at 3:11

1 Answer 1

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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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  • \$\begingroup\$ Thanks. But with 8 A set by the power supply, I can only have at most 8 A through the ammeter right? So how could I blow the 10 A fuse? \$\endgroup\$
    – SuperCiocia
    Jul 9, 2016 at 3:12
  • \$\begingroup\$ @SuperCiocia first, please, disconnect the multimeter. \$\endgroup\$
    – user2943160
    Jul 9, 2016 at 3:13
  • \$\begingroup\$ Everything's been off for an hour. Nothing is connected anymore. \$\endgroup\$
    – SuperCiocia
    Jul 9, 2016 at 3:13
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    \$\begingroup\$ Further, most hand multimeters are only rated at 10A for short cycles: 10 or 30 seconds on, 10 minutes disconnected. Next, the current limit of the power supply is not perfect. It is possible to get more than 10A from a power supply for a short period of time because it has internal output capacitance. The current/voltage limits aren't perfect and can damage devices. \$\endgroup\$
    – user2943160
    Jul 9, 2016 at 3:15
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    \$\begingroup\$ Also, please be aware that your coil is almost a short and the voltage across it will be low. Good constant current power supplies can regulate down to zero volt but not all. If you encounter problems, try adding a ~500 mohm resistor in series. \$\endgroup\$
    – winny
    Jul 9, 2016 at 10:00

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