## 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

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.