I need a current limiter, so I looked at poly fuses. They're pretty sad. And regular fuses are awfully crude--make one mistake and you have to desolder a fuse? That's no fun.
So I was thinking about the Larksonian current limiter...
Fig. 1 (View in Courier font) ====== Q1 >-------. .------->
V / ------- | | | .-. | |R1 | | '-' | | ===
...which was offered here as kind of a half-serious lark by a certain John Larkin.
As John pointed out, Q1 can be a 2:1 Hfe-binned part, giving a 2:1 limit tolerance, which isn't too bad for a non-critical application.
i.limit ~= Hfe * i.b
So, I'm supplying a little +5V to the real world via a small connector, to communicate with and power a small external device. I'm not sure what the draw is, maybe 50-100mA, max., but I do want to supply the full +5V and not a lot less--the voltage drop should be low.
John's circuit's tolerances are fine--I'd set it to 200mA or so, and get 150-300mA. That's great. But if the output's shorted, that's possibly 5v x 300mA--too much dissipation for Q1.
So I added foldback--
Fig. 2 ====== Q2 >--+-----. .-------+---->
| V / | | -------- | | | | '>|Q3 | R2 | |------|----/\/\/--' /| | | | '--------+ | .-. | |R3 | | '-' | | ===
There, that's better. If Q2 ever desats, Q3 comes on, robs Q2's base drive, and the output collapses. Once shorted, an idling current flows through Q3 e-b and R2 to the load, so the thing restarts once the short is removed.
(The classic foldback uses a divider from Q2(c) to GND, tap goes to Q3(b). I didn't do that here--we don't need that much of a hair- trigger.)
Hmmmm.
It's pretty fast. Maybe a cap to slow down Q3, so we can tolerate a brief spike without triggering...
Fig. 3 ====== Q4 >--+-----+---. .--------+---->
| | V / | | C1 --- -------- | | --- | | '>| | | R2 | Q5 |---+-------|---/\/\/---' /| | | | '-------------+ | .-. | |R3 | | '-' | ===
Hmmmmmm.
--James Arthur