Electronic fuse

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

Reply to
dagmargoodboat
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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

--=20 Deep Friar: a very philosophical monk. Website:

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Reply to
Tim Williams

base-emitter cap for surges.

on the FET gate. Good enough.

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

Reply to
dagmargoodboat

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Reply to
dagmargoodboat

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.

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

Reply to
John Larkin

Why bother? You're beyond hope. Where's the product you used it in? With details... no pimpy hand-waving :-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

That's hilarious! Yes, geezer seizure for sure!

Meanwhile I was going exactly the opposite way, trying to minimize the transistor dependencies. I was just looking at some BJT datasheets. The main problem is d(Hfe)/dt.

Here's a compromise:

Fig. 6 =3D=3D=3D=3D=3D=3D Q60 R60 >--+------+---. .--/\/\/---+--->

| | V / | | C60 --- -------- | | --- | | '>| | | R61 | Q61 |----+-------|----/\/\/----' /| | | | '--------------+ | .-. | | R62 | | '-' | =3D=3D=3D

It's simple, self-resetting, drifty and crude, but R60 tames the transistor dependencies considerably.

To keep droop reasonable I'd set the limit waayyy high, so the 0.6v trigger point doesn't happen until the load is already majorly pathological.

-- Cheers, James Arthur

Reply to
dagmargoodboat

base-emitter cap for surges.

Here's my own version:

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It's shown in-circuit in its intended application - hooked up to a mobile "screwdriver" antenna's motor, to act as an automatic shutoff when the antenna reaches the top or bottom of its travel and starts to clutch-stall. This circuit's behavior is a bit like a polyfuse - it's tolerant to short overcurrent surges, and it's self-resetting - but it's somewhat easier to adjust the current set-point, and it's far less temperature-sensitive than a polyfuse.

Unlike the Circuits_2008 version, mine can be treated as a two-terminal device - it doesn't need a separate ground connection. It does have a significant voltage drop (between 2 Vbe and 3 Vbe) which may make it unsuitable for certain applications.

--
Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
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Reply to
Dave Platt

Ahh -- I drew one of those for an automatic window drive. I never = tested it, but apparently the subject built it and it worked first time. = I like it when that happens.

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Again, could be simpler, blah blah. Rise time isn't a big deal for = driving motors on and off, so the 4001 is fine, or you could replace = each flip-flop with a pair of transistors (or even an SCR!). The pin = count is about the same, and the 4001 saves on resistors. If you don't = mind the Vbe tempco (for this, you don't), the LM311 can be replaced = easily with a transistor, which would save a few pins. If you're = partial to relays instead of FETs (or need to drive a higher voltage = motor), you can do the same thing with relay logic, saving a few parts = thanks to the DPDT contacts (as with your switch).

Tim

--=20 Deep Friar: a very philosophical monk. Website:

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Reply to
Tim Williams

If you're willing to put a sense resistor in the current path, well, you're cheating. You may as well do a vanilla 2-transistor foldback current limiter. Adding a schottky diode can reduce the voltage loss to a few tenths.

John

Reply to
John Larkin

Or do it right...

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...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

By "vanilla" do you mean an emitter-follower with a sense transistor that robs its base-drive?

James

Reply to
dagmargoodboat

That logic-output current-sensor's pretty close technique-wise to my electronic circuit-breaker of Fig. 5 above, except for the method of generating the less-than-Vbe offset voltage for the threshold. That's cute. I recognize it as an old analog IC trick, but I've never done it. I like the hysteresis too.

Thanks.

-- Cheers, James Arthur

Reply to
dagmargoodboat

Right. Add a schottky to take away some of the Vbe requirement, and a couple more resistors to add foldback.

Or go with an opamp and a small-value sense resistor, and a p-channel fet.

Or maybe one of these...

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

The polyfuse is looking better and better.

Foldbacks can have strange behavior with some non-resistive loads. That's why a shutdown with periodic retry is good.

Hey, have a uP port pin turn on a series PNP or pfet, and use a spare ADC channel to digitize the load voltage. Then do the math. I'm doing some fairly complex shutdowns in software on the 4-20 mA gadget we're doing... software lowpass filtering, trip thresholds, automatic retry, stuff like that. Another thing I like to do is measure power fet voltage and current and heatsink temp, run a realtime simulation of thermal dynamics, and shut down based on junction temperature.

John

Reply to
John Larkin

It just doesn't do the electronic fuse function.

John

Reply to
John Larkin

Gaw-w-w-wd Larkin, you are a nit-picking old hen. Anyone with half their brain tied behind their back can add the pass device... to something I posted here in 2005, then again in 2006. Wait-a-minute, I guess _you_ can't ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Except they take many seconds to reset? But my experience is just the one for rotary tool. Certainly better than letting the smoke out when one stalls the drill or some heavier work tool.

The polyfuse is foldback too (less than half the trip current to reset), might have similar problems you see with foldback?

Well, all that is tempting to add, once you have the smarts onboard :)

Grant.

Reply to
Grant

s

Here's the fuse implementation (below). It's identical to my Fig. 5, except that I got the threshold with a two-resistor divider on Q71(e), whereas you saved a resistor with the aforementioned trick on Q72 to get a drop for Q71(b) that's less than Vbe. Same result.

You had a hysteresis resistor but that doesn't make any sense in this usage, so I killed it.

I also had to update yours to straddle the switch, like mine, so that it can stay folded-back when the output's hard-shorted, and added the cap so it can tolerate spikes.

I think the new guy has the potential to latch on start-up; the fix is the same as before.

Fig. 7 =3D=3D=3D=3D=3D=3D Q70 R71 >--+---------+-----. .---/\/\/---+---->

| | V / | | | -------- | | C71 --- | | | --- | '>| | | | Q72 |--. | | | /| | '>| | | | | Q71 |-------+---------|--------------+ | /| | | | | | .-. | | | R72 | | | | | | | | | | '-' | | | | | '-------------------+ +----' | | .-. .-. | |R73 | | R74 | | | | '-' '-' | | =3D=3D=3D =3D=3D=3D

So, scratch one resistor off the parts list, performance is the same, and 253 green jobs were created or saved!

-- Cheers, James Arthur

Reply to
dagmargoodboat

ase-emitter cap for surges.

That's cute. It trips & latches to a lower current, which it holds. How does it reset itself? The holding current to the latch looks robust--just glancing it looks to me like it'd stay latched until the power's interrupted.

-- Cheers, James Arthur

Reply to
dagmargoodboat

dagast.org...

ed it, but apparently the subject built it and it worked first time. =A0I l= ike it when that happens.http://webpages.charter.net/dawill/tmoranwms/Circu= its_2010/Motor_Thin...

Spike-zilla! (How do you avoid big shoot-through currents? High Vth FETs?)

-- Cheers, James Arthur

Reply to
dagmargoodboat

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