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I'm hoping somebody can shine a light on this crazy scenario that I can not get passed right now.

I have a device that needs 3.6v to be powered on.

When I put a 3.6 battery straight to the device, it powers on as expected.

With my meter, I'm trying to measure the mA and uA usage of the device. The device's typical ON state current use is 100mA.

When I put the battery through the meter to test the current use, the device no longer powers on and the mA spikes I see on the meter reach 150mA.

However, when I use an adjustable power supply and set it to 3.6v and wire it exactly the same as I would the battery, the device powers on and I see almost exactly 100mA use.

I have replaced the batteries on my multimeter, replaced the croc leads from meter onto battery etc, but nothing makes a diff. What am I missing?

Why does the device power on when straight to battery, but not when wired through meter with battery. But meter works fine and device powers on when an adjustable power supply is connected?

Thanks in advance!!

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  • \$\begingroup\$ do you have a 2nd meter that you could test the impedance across your current meter's leads (in current sense mode) with? \$\endgroup\$ – Robherc KV5ROB Apr 25 '17 at 0:47
  • \$\begingroup\$ OK just did that. While I had the current reading meter in uA, it was showing 145uA and the other meter is showing 102ohm. Then I put the current reading meter in mA mode and it showed 15mA and the other meter reading 2.3 ohm. Does this look right? All I did is connect com from one meter to the com of the other meter and + to +. \$\endgroup\$ – Simon Dexter Apr 25 '17 at 1:10
  • \$\begingroup\$ 2.3ohm*150mA=0.345V voltage-drop across the meter when reading milliamps from the battery. Thus, if your device is made to shut down if battery voltage drops (over-discharge protection, needed in LiPO/LiFePO/Li-ion powered circuits), this 0.345V drop could be tripping that protection & shutting down your circuit. \$\endgroup\$ – Robherc KV5ROB Apr 25 '17 at 1:19
  • \$\begingroup\$ Switch your meter to 2A or 20A range. Does it still fail? \$\endgroup\$ – Bruce Abbott Apr 25 '17 at 1:56
  • \$\begingroup\$ Excellent - thank you very much for the comments, I shall take the advice and experiment some more! \$\endgroup\$ – Simon Dexter Apr 25 '17 at 3:41
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It sounds like your mA meter is dropping enough voltage that the load is not getting the 3.6V it needs from the battery. Your power supply is being adjusted to overcome this drop?

One way around this is to use a current probe (clip-on type) with a scope. They use the magnetic field from the wire to measure current through the wire, and avoid voltage drops experienced with meters.

It is an expensive solution, but it should solve your current measurement problems.

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  • \$\begingroup\$ Aren't clip-on ammeters AC-only devices? \$\endgroup\$ – Robherc KV5ROB Apr 25 '17 at 2:26
  • \$\begingroup\$ Thank you for the comment, I will research and look into a current probe also if AC only it wont work, but Google will quickly confirm that.. \$\endgroup\$ – Simon Dexter Apr 25 '17 at 3:42
  • \$\begingroup\$ keysight.com/en/pc-1659326/… This one integrates hall effect sensors to cover DC measurements. \$\endgroup\$ – Dan_LXI Apr 26 '17 at 3:26

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