I was trying to do a power calibration on a Tasmota smart switch, using this procedure:


I had an 25W incandescent globe (resistive load), wired up in a bayonet holder:

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I have a digital multimeter (Fluke 287), set to A (current), with the probes plugged into COM and A:


I turned on the smart switch, and then tried to measure the current by touching the two terminals on the bottom. (The smart switch was showing roughly 0.1 A flowing through)

There was a flash, and a bang from the probes. I also noticed part of the red probe tip appears to have melted.

It's been a while since my elec education, so I'm assuming I did something silly her. Can somebody explained what happened here?

Is it likely there was damage to the DMM?

What is the correct way of measuring the current here?


You forgot an ammeter is low resistance and shorted something out. Can't connect it to an unbroken circuit because that is parallel. Gotta break the circuit and connect in series. The ammeter would already be wired up before you turned things on if you did this. You would not turn thing on then touch the probes.

Replace the fuse in the DMM.

  • \$\begingroup\$ Measure voltage with the probes in parallel, measure current with the probes in SERIES with load. That little mistake just blew a $30 fuse. \$\endgroup\$ – Kartman Jan 17 at 4:07
  • \$\begingroup\$ @Kartman Damn that's expensive \$\endgroup\$ – DKNguyen Jan 17 at 4:08
  • \$\begingroup\$ Depends where you buy it. They’re cheap from AliExpress, but I doubt they’re legit. Also note I’m talking AUD \$\endgroup\$ – Kartman Jan 17 at 4:14
  • \$\begingroup\$ Aha, forgot that ammeter needs to be in series. Thanks for the advice - and yes, I read the DMM manual on how to test the fuse, and turns out that fuse F2 is blown. I've ordered a replacement - it was only AUD 17 from RS Online. Kinda annoying, but replacing a fuse is cheaper than if I had blown the actual equipment under test, or injured myself. \$\endgroup\$ – user37872 Jan 17 at 5:13

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