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From: dagmargoodboat on 4 Jul 2010 01:07
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
From: Tim Williams on 4 Jul 2010 01:33
http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/Electric_Fuse.gif

Of course, this is a latching type, so you want to slow it down with a base-emitter cap for surges.

Could be simplified plenty of course. Instead of an R-S, you could use an SCR on the FET gate. Good enough.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

<dagmargoodboat(a)yahoo.com> wrote in message news:7bdd24f2-48ec-4d3b-8888-12fc2cab4fa2(a)s9g2000yqd.googlegroups.com...
>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.
>
> ...
From: dagmargoodboat on 4 Jul 2010 09:13
On Jul 4, 1:33 am, "Tim Williams" <tmoran...(a)charter.net> wrote:
> http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/Electric_F...
>
> Of course, this is a latching type, so you want to slow it down with a base-emitter cap for surges.
>
> Could be simplified plenty of course. Instead of an R-S, you could use an SCR on the FET gate. Good enough.
>
> Tim


That's interesting--it looks suspiciously like a protection device for
an induction heater :-).

But, I have a few extra requirements for my use--I need a high-side
"fuse", it has to be self-resetting, and preferably shouldn't sag too
much before tripping.

Here's a further twist or two on mine...

Fig. 4
======
Q6 R4
>--+-----+---. .---/\/\/---+---->
| | V / |
| C2 --- -------- .-.
| --- | | |
| | | R5 | |
| | | '-'
'>| | | |
Q7 |---+-------|--------------+
/| | |
| | .-.
'-------------+ | |
| R7 | |
.-. '-'
| |R6 |
| | |
'-' ===
|
===

R4 senses the current outflow. When the drop across R4 becomes too
large, Q7 turns on and the output collapses as before, with restarting
current supplied through Q7(b-e) and R5, as before.

The R5-R7 divider lets you set the trip point to less than a full
Vbe(Q7) if you want.

This change takes the pass transistor's saturation characteristics out
of the equation--just turn it on hard and let the sensing circuit
handle the thresholding. In fact at this point you could even use a
MOSFET series-pass transistor if you wanted to.

So now the drawbacks left are the trip point drift due to Q7's Vbe
drift, and the fact that the output sags before the circuit trips.

We can minimize the latter by just setting a low sense voltage. If my
output sags <300mV, that's still okay for this application.


Fig. 5
======
Q50 R51
>--+---------+-----. .---/\/\/---+---->
| | V / |
| | -------- |
.-. | | |
R50 | | | | D51 ---
| | C50 --- | \ /
'-' --- | ---
| | | |
+---. | | |
| >| | | |
| |---+---------|--------------+
| /| | |
.-. | Q51 | .-.
R52 | | '---------------+ | |
| | | R54 | |
'-' .-. '-'
| | |R53 |
| | | |
=== '-' ===
|
===

So, D51 compensates Q51's Vbe, and you can set the trip point by the
drop on R50.


--
Cheers,
James Arthur
From: dagmargoodboat on 4 Jul 2010 09:52
On Jul 4, 9:13 am, dagmargoodb...(a)yahoo.com wrote:

> Fig. 5
> ======
>                         Q50       R51
>   >--+---------+-----.      .---/\/\/---+---->
>      |         |      V    /            |
>      |         |     --------           |
>     .-.        |         |              |
> R50 | |        |         |         D51 ---
>     | |   C50 ---        |             \ /
>     '-'       ---        |             ---
>      |         |         |              |
>      +---.     |         |              |
>      |    >|   |         |              |
>      |     |---+---------|--------------+
>      |    /|             |              |
>     .-.  |  Q51          |             .-.
> R52 | |  '---------------+             | |
>     | |                  |         R54 | |
>     '-'                 .-.            '-'
>      |                  | |R53          |
>      |                  | |             |
>     ===                 '-'            ===
>                          |
>                         ===
>
> So, D51 compensates Q51's Vbe, and you can set the trip point by the
> drop on R50.

Oh, diode D51 in this version blocks the idling current, so you need a
resistor across Q50 if you want self-resetting. Might even need it to
avoid latch up on power-up depending on the load and how quickly the
+5v ramps. YMMV.


James
From: John Larkin on 4 Jul 2010 11:50
On Sat, 3 Jul 2010 22:07:56 -0700 (PDT), dagmargoodboat(a)yahoo.com
wrote:

>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.

This circuit is perfectly serious in some situations. As a plus, it
sets off geezers who were taught never to do it, back in the days of
alloy diffused transistors.

We do use polyfuses in situations like yours, but the surface-mount
ones have low voltage ratings and their trip current depends on the
pads and traces, the major heat sink mechanisms. The leaded parts are
generally better.


>
>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

The equivalent has been done with a mosfet as the pass device, in an
IC even as I recall.

But why beta limit once, when you can do it twice?

ftp://jjlarkin.lmi.net/Beta_Twice.JPG

Geezeer-seizure for sure.

John

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