Mosfet Based Output Protection

[Rerouter]

Currently resuming a long shelved project, and have thought up and simulated an idea that appear to work, but i figure i may as well run it by here first. as it feels too simple,

I have to output a 5V supply (+-0.5V) of at least 1mA, into the world, the internal supply its fed from is 5V +- 0.02V, and the plug it connects to may be connected between hard + or -32V DC.
(by hard i mean a direct unfused connection to a supply that can supply many amps, and cannot be crowbar'd)

so far i have dreamed up the following, it looses about 0.35V across the schottkey, and 0.1V across the resistor, it's within my specs, limiting current when negative, and the diode blocking when positive, and as a bonus, disabling the output when the 5V internal supply is turned off,

But i cant shake the feeling i have missed something obvious.

[T3sl4co1l]

Why not use a single (0.5V) / (1mA) = 0.5k resistor?

Tim

[Rerouter]

the negative extreme is 5V to -32V, so 37V, over 500 ohm would be 2.7W on the resistor, so likely a through hole 3.5W one for reliability, ($0.736 on digikey)

with the limiting its 0.05W on the resistor and 0.7W on the mosfet, which is within the ballpark of the small package smd mosfets. (24c for mosfet + 3c for resistors + 44c for diode = $0.71 on digikey)

It ends up with almost the same cost, just less heat lost, and (I believe) the perk of disconnecting when input power is lost.

edit: (just rechecked my math, and the circuit limits at 20.5mA)

[rx8pilot]

Why not use a single (0.5V) / (1mA) = 0.5k resistor?

Tim

I am trying to figure that out too.

Sent from my horrible mobile....