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I want to measure the current draw of a strip of LEDs. This depends on the intensity of the pattern the strips are running so I need to do it in "real time" to get an accurate idea, rather than just do maths from the spec sheet.

I made this cable that should not be in order to insert my multimeter in series with the circuit:

DC barrel socket to barrel-plug cable with positive wire cut

After I plug this in between the (earthed, double-insulated) 12V power supply and LED controller box and clamp the two red wire ends together, everything works fine.

But when I set my multimeter like this:

Multimeter set to Amp range

... and complete the circuit with the multimeter's probes (the red probe on the wire closest to the power supply, black probe closest to the LED's controller box), I get a reading around 3.5A on the multimeter but the controller does not seem to be receiving power and the LEDs don't light.

Any idea what I could be doing wrong?

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    \$\begingroup\$ I can't make out the text around the banana jacks... but are you sure you have the wires plugged in to the correct jacks for measuring current. They will be different from those when measuring voltage. \$\endgroup\$ – Jason Jul 24 '12 at 22:59
  • \$\begingroup\$ The unused middle jack is Common, left is 10A jack, right is for everything else. So your connections on the picture don't look right. Use COM and 10A for current measurement. \$\endgroup\$ – Juancho Jul 24 '12 at 23:27
  • \$\begingroup\$ @Juancho you're exactly right. If you re-do this comment as an answer I'll accept it. \$\endgroup\$ – Robert Atkins Jul 24 '12 at 23:44
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    \$\begingroup\$ It's handy to put banana plugs on your cable. Then you can plug those into the meter. \$\endgroup\$ – John La Rooy Jul 25 '12 at 0:32
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    \$\begingroup\$ ... also, make sure your meter is set to DC A, not AC A. At work, we have multimeters that always default to AC A after being switched on, and I always make a fool out of myself because I forget to push the little button that will toggle it to DC. \$\endgroup\$ – zebonaut Jul 25 '12 at 6:46
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The 3 jacks on the multimeter are (from left to right):

  • 10 A
  • Common
  • Volt, Ohm, mA, uA, etc.

The correct connection for (high) current measurements is via the Common and 10A jacks.

The setup pictured is wrong, with no lead connected to Common.

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  • \$\begingroup\$ In my defence, the 10A socket is a dark green which was indistinguishable from black in the incandescent light I was using at the time. I thought I had the choice between a red socket and two black sockets :-). \$\endgroup\$ – Robert Atkins Jul 25 '12 at 16:12

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