But if you have a current-limited supply, and all you're trying to do
is turn it off for undervoltage (i.e. short circuit), you could do
worse than getting a 5VDC coil relay (possibly with a small ohmic
value series resistor).  Here's how it works (view in fixed font or M$
Notepad):

|
|         Start
|           T
|  +4V     ---           V+
|   o---o--o o-o-----o---o
|       |      |     |
|       |      |     |
|       |      |     |
|       |  ||  | RY1 C|
|       '--||--'     C|
|          ||        C|
|         CRY1       |
|                    |
|  COM               |  COM
|   o----------------o---o
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Most all 5V relays will pull in with 4V.  Press the Start button, and
the relay in energized.  The contact of the relay will keep the
circuit closed, and allow current to flow to the load.  If you get a
short circuit, and the output voltage drops to less than 1.2V, the
relay will open up, disconnecting the circuit until you press the
start button again.  If you've got a double pole relay, you can use
the second pole to turn on a fault light.

The problem, of course, is that lowering the voltage decreases the
current in many fault conditions.  As the current limit voltage drops,
the current drops too, so the fault voltage might be only 2.5V or 3V,
not low enough to reliably open the relay.  Relays will usually pull
shut at about 75% of rated voltage, and will guarantee to open once
they've closed at about 20% of rated coil voltage.

There are a number of power supply monitor ICs which can supply a
logic signal in the event of undervoltage, and work reliably down to
1V or so.

But from a s.e.b. perspective, another way to do this would be to use
a double pole momentary contact switch, and also switch a resistor in
series with the relay coil, like this:

|
|        ||
|   .----||---.
|   |    ||   |
|   |   CRY1  |
|   |         |
|   |    T    |
|+4V|   ---   |              V+
| o-o---o o---o------o--------o
|                    |
|                    |
|        T       RY1 C|
|       ---          C|
|    .--o o---.      C|
|    |        |      |
|    |        '------o
|    |               |
|    |              .-.
|    |            R | |
|    |              | |
|    |              '-'
|    |               |
|    '---------------o
|COM                 |      COM
| o------------------o--------o
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

You will have to fiddle with this if you're picky about the relay open
voltage.  If you choose R at the same resistance as the coil, it
should open if your power supply dips below 2.5V or so.  But try
experimenting with different values of R if you want to tweak it in.
Make sure the resistor can handle the wattage, if you're using a 5V
relay with a low ohm coil.

Cheers
Chris

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