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Imagine the following world's simplest diagram:

12V Battery -> 5V/3A Voltage Regulator -> Load

My question is, would a 3A fuse best protect the circuit if placed before, or after the voltage regulator? Or elsewhere in the circuit for that matter...

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    \$\begingroup\$ Why imagine it when the site has a schematic editor? Ctrl-M... \$\endgroup\$ – Adam Lawrence Jul 30 '13 at 12:23
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Before the voltage regulator. This way, the fuse will open if you have a short in the voltage regulator itself, as well as downstream of the regulator.

Elsewhere in the circuit depends on the circuit. You can have more than one fuse. If you expect that some part of the circuit is likely to short, you can fuse it separately.

Motors like slow fuses, because motors can draw large instantaneous currents as a part of normal operation.

P.S. Fuses typically protect from fire hazard at large, as opposed to protecting the appliance from damage. You need to consider specific failure modes that you want to protect against. For example, if you're considering short circuits, you need to consider various specific places where they may occur (internal short in a motor, 12V shorted to GND, 5V shorted to GND, B shorted to A, etc).

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After :-).

BUT it deep-ends on what you want to do.

IF your voltage regulator current limits at about 3A then a short of any size at the input will not blow 3A fuse which usually trips at around 6A (a whole engineering subject by itself.)

If you short a 3A regulator output to ground with 12V input you will dissipate ~12V x 3A = 36W in the regulator until it either melts or goes into over temperature shutdown. IF the regulator is thermally protected this MAY be better than a fuse blowing, or not.

An output 3A fuse will only blow if the output capacitor after the regulator supplies enough current to drive the ^A + short long enough.

Monitoring the regulator Vout may be more effective. --> Alarm/trip/stop if Vout < say 1V for longer than say 2s ...

If you want to usefully blow a std fuse you need Imax regulator > to >> Iload max.

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What are you trying to protect? You mention "protect the circuit" but do you mean the load or the regulator or both?

If you are trying to protect the load and you are using a linear regulator then placing it before the regulator is probably better because any volt drops across the fuse won't cause extra loss in the 5V rail.

But you haven't said what type of regulator it is and it could be a switching type and if so then your input current to the regulator will be much smaller than 3A so you'll need to consider what value fuse you need to adequately protect things.

You might also consider that the voltage regulator may have some form of current limiting built in to protect itself - this may also protect your circuit.

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