Fastest AC Electronic Fuse

This is probably an oldie question on here...

I'd like to break the AC "hot" if there's too much AC current. This would be due to a short or ground fault. Or.... Shunting the load to burn a fuse on the AC line would be ok too..

I'm afraid that a fuse maybe too slow to protect the electronics from burn out.. Let's say the electronics I'm protecting cannot withstand 1.5x overcurrent.

What's faster than a fuse on a AC line?

Details Load: The load on the line is not a transformer and is not inductive. Current draw: 2Amps Line voltage: 240Vrms, 60Hz Surge rate: guessing 45Amps in 1uS

Rule: 1/4watt current sense resistor.

The load circuit has a power bridge rectifier. Maybe I could modify the bridge with transistors to interrupt the surge current? D from BC

Reply to
D from BC
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Years ago I used an SCR to blow fuses (on the load side). I think it was in the microsecond range. Warning, traces need to be fat, otherwise you blow traces instead of fuses... I had ~100A circulating momentarily to blow a 1 Amp fuse ;-)

...Jim Thompson

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

One of our customers recently asked us to do that. He needs, basically, an SSR that also acts as a programmable circuit breaker

*and* a current limiter. He needs to simulate cable faults and switch power circuits without exploding anything. We've been scribbling circuits for days, and it's non-trivial. We just a few minutes ago finished whiteboarding the design of a breadboard pcb so we can test the dynamics.

John

Reply to
John Larkin

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How fast?
Reply to
John Fields

There's some work already out there on IGBTs re shorts into hundreds of volts. IIRC, it's bad to try to turn it off too fast (or too slow, obviously), plus there are some other subtleties.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Spehro Pefhany

I drew a circuit here

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which someone built, and evidently works quite nicely. Maybe you could change the MOSFET to two back-to-back so they'll handle AC. Which I also had to draw for someone it seems:
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Alternately, you could put a single FET across the DC terminals of a FWB.

Obviously, the +12V supply needs to be floating. It could possibly be provided by a resistor+capacitor and diode from the AC terminals of the thing. Interfacing external control/feedback would probably be best done with optoisolators...

Tim

-- Deep Fryer: A very philosophical monk. Website @

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Reply to
Tim Williams

Wow!...peaking to 100A before blow out..... Scary....

I guess it takes time for the fuse element temp to rise to the melting/vaporize point. D from BC

Reply to
D from BC

To break a DC circuit, a pass transistor can be used.. Easy...

But breaking 240V AC...huhhh.. :(

I might look at using a little transformer to sense a current surge and have that fire a shunt circuit until a fuse blows..

D from BC

Reply to
D from BC

I might have to blow up some stuff to find out. :)

I'm just interested in rough ideas for various levels of cut-out speed.. Doesn't matter what cost, complexity or parts count.

At the moment, the faster ...the better..

D from BC

Reply to
D from BC

mmm...makes sense to see transistors to shut down the mosfet.. Op amp current sensing might be considered generally slower.. The cct looks like it has a ~6A trip level.... It's easy when a sense resistor has enough V to bias a transistor.

Using a floating circuit to control a push pull pair of mosfets looking interesting..

Thanks.. D from BC

Reply to
D from BC

I'm wondering what's more explosive... Slowly sneaking up the current through the fuse until it blows... Or..... suddenly connecting the fuse across a 300V charged 100uF low ESR capacitor?

Should I worry about fuse shrapnel and BANG! when it comes to fuse selection? D from BC

Reply to
D from BC

This was a conventional fast-blow fuse. It flashed like a flash-bulb (but not as much light) and made a tiny "click" ;-)

The filament was completely plated out onto the glass.

After we cracked a few, we went to the ceramic tube type.

...Jim Thompson

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

Definitely the second. In the first case, the alloy just quietly melts. In the second, it vaporizes.

If you exceed the fault current rating of the fuse, all bets for containment are off.

Reply to
John Popelish

D from BC a écrit :

How about an AC current limiter?

.--------------------------------. | | | | | .---+---. | | | | | | .---+ | .-. .-. | | | | | | | | | | +-|| .----+ | | | | | - ->|| | | '-' '-' - ^ +-||--' >| | | ^ | | |-+ +-|

Reply to
Fred Bartoli

Arrrrghh... At first I couldn't figure this cct out..

The only way for current to pass is if the N Ch mos turns on. Otherwise the diodes block.. I see the bridge rectifier in this circuit.. Cool...

I see the negative feedback loop from the Rsense through the NPN-PNP compound and back to the MOS that limits the AC current..

Is the AZ a zener?

The circuit looks fast..

The only thing I don't like about this cct is that the current sensing resistor will be dissipating a nominal 1.2Watts in my app..

0.6V*2Amps= 1.2Watts.. 0.6V~=Vbe To use a smaller current sense resistor, I could use an op amp, but I'll be trading off speed... However...I might be able to come up with a transistor arrangement that uses a smaller (cooler) sense resistor..

Maybe I might try an SCR to shut down the mosfet... Or CMOS logic... It wouldn't be a limiter anymore but a breaker..

Thanks...

Another circuit I'm looking at is

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Triac blowing line fuse. I think this also has a hot current sense resistor (If I=2amps and Itrip = 3Amps). D from BC

Reply to
D from BC

D from BC a écrit :

The positive FB loop between the NPN/PNP bjts make the 'limiter' trip. With the right time constants, the *small* bypass cap voltage gets down to the point where the 'SCR' self resets. This obviously has to happen within one line period (before injecting a new current pulse through the series RC).

Yep. Limits the gate voltage and provides a path for the RC branch 'negative half period' current.

Should be.

Can be fixed with just one more resistor.

It is already. Well a periodic, self resetting one.

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Thanks,
Fred.
Reply to
Fred Bartoli

"Fred Bartoli" wrote in message news:46497209$0$18017$ snipped-for-privacy@news.free.fr...

The power dissipated in the resistor is less than that, and as Fred says can be fixed easily (it isn't steady state, more like a sawtooth).

I used something very similar as an inrush limiter on a SMPS a many years ago, and got bitten by ** changing the fab that supplied the Mosfet just after we went into production. I forget the details, but it was to do with wimpy metallisation to the Mosfet gate - to turn the Mosfet off fast you have to pull the gate charge back out, which takes current.

Regards Ian

Reply to
Ian

The fastest fuse in the world is an expensive microwave transistor. Works a treat to protect line fuses.

Regards Ian

Reply to
Ian

Ian a écrit :

But you'll have to call your power company so that they speed up their generators quite a bit.

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Thanks,
Fred.
Reply to
Fred Bartoli

I did tests, for example, on 5A 250V rated fuses. The 20mm glass variety would literally explode (a VERY loud bang) sending shrapnel everywhere, just on 240VAC/60Hz with an industrial source capable of enough source current.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Spehro Pefhany

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