They're not as bad as I thought. I mistook the max resistance number to mean when tripped, but that's not the case.
With that clarified the polyfuse is slow, but not horrible for short- circuit protection.
You could've just said "Use a PIC."
-- Cheers, James Arthur
Yes, there will be lots of shoot through. Hopefully under a = microsecond, which will abuse the supply, a good reason to have it = connected on long wires. Poor enough wire is self snubbing. :-p It's = only one edge (assuming no bounce, which if the supply drops below 5V, = the 4001 could reset; but, it won't go less than 2*Vgs(th), so it can't = be too terrible), so the average dissipation is small.
Tim
--=20 Deep Friar: a very philosophical monk. Website:
"John Larkin" wrote in = message news: snipped-for-privacy@4ax.com...
The bottom one would be particularly good with a TLV431. Well, maybe = with a schottky to keep supply current into it.
Tim
--=20 Deep Friar: a very philosophical monk. Website:
schottky to keep supply current into it.
Sure, it you're one of those people who fling 13 cents around as if it didn't matter.
That's the upper circuit here
ftp://jjlarkin.lmi.net/Goofy_lims_2.JPG
which suggests the lower one. Add a cap somewhere for surges/spikes maybe. 5 parts, 9 cents total, including a cap. Interesting little circuit, actually, a flipflop of sorts.
John
schottky to keep supply current into it.
Newbies are invited to LTspice it, testing it with various element values. ...Jim Thompson
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Obama: A reincarnation of Nixon, narcissistically posing in
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agenews: snipped-for-privacy@4ax.com...
with a schottky to keep supply current into it.
The lower one is my Fig. 2. (first post in this thread) Add a cap and you've got my Fig. 3.
I like it, obviously.
Grins,
James
"Wherever you go, whatever new trail you blaze, eventually you'll find...footprints."
sagenews: snipped-for-privacy@4ax.com...
with a schottky to keep supply current into it.
The 2nd one behaves pretty well as far as startup and resetting itself, and protecting the switch against excessive dissipation. The main drawback is it lacks accurate "settability," mostly because hfe changes so radically with temperature.
If you don't mind a worst-case current that's 4-6x your trip point, it's fine. (Get a transistor binned to a 2:1 gain range, then multiply by how much hfe changes over your operating temperature range.) Might be able to tune it closer than that even.
There's a benefit from this arrangement. Here, as Q2's gain increases, raising the trip point, Q3's gain increases (and Vbe drops), lowering the trip point. The effects offset. It's still very sloppy, but that makes this circuit a lot more stable than just a pass transistor with a base resistor to GND.
A low-sat transistor for Q2 keeps the output pretty flat right up to the trip point.
A small sense resistor in Q2's collector goes a long way to taming (but not curing) the worst of the pass-transistor dependencies. It's sloppy, but remember what we're replacing: a skinny strand of metal. It's better than that.
Fig. 2 =3D=3D=3D=3D=3D=3D Q2 >--+-----. .-------+---->
| V / | | -------- | | | | '>|Q3 | R2 | |------|----/\/\/--' /| | | | '--------+ | .-. | |R3 | | '-' | | =3D=3D=3D
-- Cheers, James Arthur
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