Current-Sense Resistors: Mind the Power

The bench supplies I use have current limiting, but the 10 mA meter resolution leaves a lot to be desired, so I conjured up a simpleminded 200 mA meter from a panel-mount meter and a 1 Ω sense resistor. That means it’s good for only 200 mA, so I insert it in series with the supply only when it’s needed. Lately it’s been reading more than a little bit high and I took it apart to find this obvious evidence of abuse:

Homebrew millammeter with burned sense resistor
Homebrew millammeter with burned sense resistor

The loose resistor sitting atop the chip shows what the burned resistor soldered in the circuit should look like.

The power supply has a 3 A current limit. No surprise: 9 W is more than the unfortunate 5 W resistor can handle.

It’s all better now …