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From: dagmargoodboat on 5 Jul 2010 14:54
On Jul 5, 11:10 am, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Mon, 5 Jul 2010 01:14:21 -0500, "Tim Williams"
>
> <tmoran...(a)charter.net> wrote:
> >"John Larkin" <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in messagenews:2m72361nrmtpe7vkuet21mh8bnuqvdld1r(a)4ax.com...
> >> Or maybe one of these...
>
> >>ftp://jjlarkin.lmi.net/Goofy_Ilims.JPG
>
> >> The polyfuse is looking better and better.
>
> >The bottom one would be particularly good with a TLV431.  Well, maybe with a schottky to keep supply current into it.
>
> >Tim
>
> 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

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."
From: dagmargoodboat on 5 Jul 2010 15:12
On Jul 5, 11:13 am, Jim Thompson <To-Email-Use-The-Envelope-I...(a)On-My-
Web-Site.com> wrote:
> On Mon, 05 Jul 2010 09:10:33 -0700, John Larkin
>
>
>
> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> >On Mon, 5 Jul 2010 01:14:21 -0500, "Tim Williams"
> ><tmoran...(a)charter.net> wrote:
>
> >>"John Larkin" <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in messagenews:2m72361nrmtpe7vkuet21mh8bnuqvdld1r(a)4ax.com...
> >>> Or maybe one of these...
>
> >>>ftp://jjlarkin.lmi.net/Goofy_Ilims.JPG
>
> >>> The polyfuse is looking better and better.
>
> >>The bottom one would be particularly good with a TLV431.  Well, maybe with a schottky to keep supply current into it.
>
> >>Tim
>
> >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
>
> Newbies are invited to LTspice it, testing it with various element
> values.


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
======
Q2
>--+-----. .-------+---->
| V / |
| -------- |
| | |
'>|Q3 | R2 |
|------|----/\/\/--'
/| |
| |
'--------+
|
.-.
| |R3
| |
'-'
|
|
===

--
Cheers,
James Arthur
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