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 …

  1. #1 by ewf on 2012-09-13 - 08:31

    I did a tour of duty at Circuit Power Inc. (now defunct) and they used the appropriate gauge copper wire as a sense resistor, in their larger (>30 Amperes) supplies. It was mounted on the bottom of the printed circuit board, in staked-in turret posts that were open at the top (in this case, the bottom). They were soldered into the post with eutectic solder, which allowed gravity to open the circuit, as a fail safe, if the fold-back current electronics were not working (mostly tampering). Their primary customers were universities and many strange things were done to the supplies. Testing this feature usually drew an audience.

    • #2 by Ed on 2012-09-13 - 09:35

      the appropriate gauge copper wire as a sense resistor

      Copper’s thermal coefficient (something like 0.4%/C) pretty much kills any hope of accuracy for a precision meter, but for typical currents in fat wires under shirt-sleeve conditions it works surprisingly well.

      I’ve seen cheap meters using a 10 AWG copper wire resistor calibrated with a diagonal cutter: squeeze until the meter reads the right current! Of course, that puts all the resistance in the notch, but …

      many strange things were done to the supplies

      Improvements, all of them, I’m sure of it! [grin]

  2. #3 by hexley ball on 2012-09-13 - 11:38

    A couple of shunt diodes in parallel with the sense resistor might keep the new resistor from going all crispy critter, no?

    • #4 by Ed on 2012-09-13 - 11:48

      I have another resistor in the heap, but … yeah, a pair of power diodes looks like a really good idea!

      The 3 ADC “fault current” would require something bigger than 1N4001s. With a bit of luck, there’ll be room inside the box for a pair of junkbox stud diodes.

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