I'm looking for a method of protecting a circuit against shorting.

I would rather not use a regular fuse as I expect I will short the circuit (unintentionally) fairly often (probably once or twice each time I use it) and so I would end up getting through a lot of fuses.

I've looked online but all I seem to be able to find are breakers for din rails which are very high current.

I've seen some power supplies which have an LED which lights up if you short them and this is something I'd quite like to go for but I don't know how it was done.

What sort of thing should I be searching for?


Sorry, I was a bit short on details last night when I posted this.

The application is a model railway system.

My intention is to run 16V AC along all the rails and then have a controller board on each train engine with an mcu so that they can have sensors and be programmed.

The reason it could be shorted is because tools and such may be (accidentally) placed across the rails or whatever.

The under normal operation current is going to be anything up to maybe 5 amps.

  • 1
    \$\begingroup\$ How much current, and what sort of reaction time do you need? \$\endgroup\$
    – Dave Tweed
    Mar 15, 2014 at 0:48
  • 1
    \$\begingroup\$ Polyfuses might work, but it depends on the requirements for holding current and interrupting current, both of which are currently unspecified. \$\endgroup\$ Mar 15, 2014 at 1:25
  • \$\begingroup\$ I've added more details to the question. \$\endgroup\$
    – m3z
    Mar 15, 2014 at 15:55

1 Answer 1


If the short-circuit current of your transformer is less than 40A you could use a Polyfuse such as the Littlefuse 30R500UMR. They're cheap (under a dollar in singles), and automatically reset when the fault is removed.

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It needs to have a DC voltage rating of more than 22V, so this model is okay (30V rating).

Here is the trip time curve for this series of parts:

enter image description here

If the fault current can exceed 40A, it might not shut off reliably (and could damage the Polyfuse). You might want to put a somewhat higher current rating 5x20mm 250VAC fuse in series if you're not sure about this detail. It's possible to estimate the maximum fault current if you have the ability to measure the transformer secondary resistance accurately, but that's another question.

Another option, probably the one I would choose, would be to use a thermal circuit breaker such as the TE Connetivity W51-A122B1-5 or the W58-XB1A4A-5. Those items require manual reset, but have interrupting capacity in the 1000A range, which is not going to be a problem unless you are using a transformer sitting outside on a concrete pad for your train set.

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