simple current limiters

That's about 15 more parts than I had in mind.

And those have a lot of voltage drop.

I was thinking the (fig)1.

True. I still have to try the switching variant. If it works, it could be usefull at driving Leds and so on.

John, you outlined a need for a 5V source that pegs at 5V, despite currents to 100mA, or more, and can withstand abuse and shorts, etc., and is small and simple. There's only one true answer to that: an 120mR Ron MOSFET, with an active current-sense limit shutoff:

switches, in sot-23-5 packages, $0.75 each. FPF2125. A single 0603 resistor programs shutoff current, 0.15 to 1.5 amps. Just leave it on all the time, and use it as a protected low-voltage-drop voltage source.

The 2125 looks good: it current limits and then thermal cycles. It should be able to pull up a pretty big capacitive load.

So, connect the base of a PNP to the +5, and the emitter through a 130 ohm half watt resistor to +12V. The customer has a few dozen milliamps of +5 to play with, and probably won't burn anything up. For a bit better efficiency, a 1k base resistor might save a bit of heat when the customer isn't drawing power.

Fir higher current, there's gizmos intended for LED ballast (AMC7135) with current regulation, thermal limiting, and the wrong polarity for a positive-ground power supply.

resitor goes from adj to out, output is taken from adj.

You'll probably find that circuit in the datasheet for those parts

mark

I don't have +12V. If I did, it would be trivial to make a current-limited +5.

Your thing makes about +5.7 and wastes a lot of power.

That's nice in an all-5V system; needs just the Iset resistor, like the FPF2125.

I'm using its big sibling, TPS26600, in a 48-volt application, but it needs a lot of passives around it.

A PNP with emitter to +5 and resistor from base to GND can sort-of-work, beta-limited output current.

The base bias can be moved, either to generate 5.0V or 8.0V (in the latter case, the customer can regulate to +5 at his end). The current is intended to be low, and the power donation is for a worthy cause. Heck, everything electric that isn't a heater, 'wastes' its power input, eventually.

The old NIM power system was wonderful to work with: well-regulated accurate +/- 24V, +/- 12V, and high current available at 6V. There was never any need to wish for better power support. When you make +5-only designs, it's like painting yourself into a corner.

we use some FPF2702 to drive solenoid valves in a 24V system, though the current limit isn't terrible accurate

National once sold a controlled-beta transistor to do just that. One would have to consider power dissipation, so a tiny part wouldn't work.

I have done low-dropout beta-limited current sources to drive bandgap references, where a 4:1 current spread wouldn't matter. The BCX71 is sold in pretty tight beta bins.

How much voltage drop is this device allowed?

Are you trying to control a load fault, or a failure of the source which perhaps goes over-voltage?

If the latter... how much "pop-up" voltage tolerance? ...Jim Thompson

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Understanding is a fountain of life to one who has it, But the instruction of fools is folly. Proverbs 16:22

We are sending some of our +5 out to a customer circuit. I'd say that we can lose a couple of tenths of volt.

A customer short to ground.

Zero. A series switch shouldn't overshoot if the customer load un-shorts.

OK. Any set/reset method preferences? ...Jim Thompson

w=1

It should recover whenever it can. A current limit with thermal cycling looks good. That pulls up loads, including capacitive loads.