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Output protection

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abbtech:
Hey guys,

I have a project that will have 16 12 volt outputs which is expected to deliver up to 250mA each.

The circuitry is capable of providing about 600mA so it could pop a fuse but I would like to keep the cost down to the bare minimum. How is everyone else providing short circuit protection? I was thinking of going with a bunch of PTC devices but even at 25 cents each this would add $4.00 to each unit.

DJPhil:
I think it would depend a lot on how your circuit is set up and what degree of protection you're after.
(I don't know exactly what you've got, so I'm thinking of something like a computer power supply.)

If speed isn't really a factor and you're only concerned about preventing a hard short you might consider protecting the primary 12V rail further back in your circuit. A hard short on any of a bunch of parallel outputs would drive up the total current draw on the supply rail well above nominal output and trigger any protection you choose: fold back limiting, fuse, whatever. You can implement a much more thorough protection scheme on one rail for the money than attempting to protect each separately. The only downside is that a fault drawing >250mA and less than your protection scheme (4A plus margin) isn't handled, and your power supply will happily incinerate whatever's attached if it can't take the load.

Adding circuitry to protect each output would make the supply more robust, but more costly (as you mention). At some point it may be cheaper to regulate each output separately with a three terminal regulator and take advantage of the inbuilt protections they offer. Of course, this doesn't really qualify as cheap, but I don't think there's an elegant method to have it both ways.  :-\

Hope that helps. :)

NiHaoMike:
How about use PTCs to protect a group of outputs?

abbtech:
Thanks for the feedback DJ Phil (I thought it was DR Phil at first  :) ) and Mike

The outputs will be used to power LED lighting, in our existing product we have biased the output transistor to power the expected 20 to 40mA LED load and on a short circuit it draws something like 80mA, no damage to the circuit results from a short circuit.

On the new design there is a need to drive much more current, I am thinking that it will still be a simple transistor driven output but since it will be biased to drive hard I am sure that it will be damaged if the output is shorted. That being said I am not sure if a fuse or a PCT would act quick enough for the 2N4403 transistor to be saved anyway.

The power supply will be a 12V 5A supply with an on board 5 volt regulator for the low voltage electronics.  

I was thinking of having a current sense resistor that could bias a transistor to kill the circuit when desired. This circuit looks promising.
http://www.vidisonic.com/2008/07/10/current-limiting-circuit/

I would still need to flip the circuit since I will be using a low voltage MCU to turn on and off the output.

Any thoughts?

NiHaoMike:
Don't include overload protection and warn that overloading the unit will damage it.

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