how to make an active fuse

Latching relay? Use NC (normally closed) contacts, when it trips on overcurrent the contacts open.

Solid state is less power efficient.

f

Tim

On a sunny day (Thu, 16 Apr 2009 14:35:28 -0700 (PDT)) it happened Tim Williams wrote in :

Yes, that works, you can get a bit better efficiency by using the voltage drop over the MOSFET to sense the current, and use some RC to allow it to start. Here q1 senses the voltage across the MOSFET, this is actually a regulator, but the same can be used in the circuit you referred to: ftp://panteltje.com/pub/MOSFET_low_dropout_regulator_with_short_circuit_protection2.gif

For an AC type fuse you would likely use a MOSFET in a bridge, and perhaps need some small transformer to supply the control logic... You would need to blow a lot of fuses to break even...

I put together a much simpler one, to stick in series with my screwdriver antenna motor, after I pulled one of those "expensively stupid" stunts.

Regretfully, a few months back I bumped the (non-spring-loaded) switch with my knee when getting into the car; it set the motor on "down", I didn't notice it, the motor cranked the antenna down and then stalled and ground away for an hour or so. The motor overheated and cooked itself... current draw increased and it eventually blew the fuse, but the damage was already done.

What I ended up with was a very simple little circuit: one TO-220 NPN Darlington (pass element), two small-signal transistors, one cap, four resistors. It'll pass the screwdriver motor's running current (100-150 mA) indefinitely, with an acceptably low voltage drop across the Darlington and the sense resistor. When the motor stalls, the current jumps up to around 500 mA, and the "fuse" opens in a fraction of a second. It latches nicely, and resets automatically if the load is disconnected for a second or so.

You can see the basic circuit at

All sorts of embellishments are possible... I added a zener around the whole thing to deal with automobile load-dump spikes and accidental reverse-polarity hookup, and one could hang an LED or beeper in the shutoff path to indicate "fuse open".

It's not suitable for all applications, due to the significant voltage drop (1V or so at the trip point) but it might give you inspiration for your own design.

--
Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
     boycott any company which has the gall to send me such ads!

I use switch duty magnetic circuit breakers on my metered variac supply.

Bob

"panfilero" = universal f****it

** Shame how a SS relay will NOT turn off in a circuit carrying DC current.

Better get a few real fuses and use them - see as you have zero design ability.

...... Phil

Semiconductors don't tend to make very good fuses- they tend to fail "on" when subjected to overloads. That includes the thyristors or MOSFETs in your proposed SSRs. They also fail with rather small overloads compared to fuses. Protecting an SSR against a short circuit is a rather difficult job for a fuse!

Unless the available fault current in your circuit is quite modest, and safety is NOT* the reason for fusing (you'd likely have to add real safety agency approved fuses to protect the wiring from causing a fire), this is not usually the best (or even a feasible) approach.

• NOT

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

some will.

snipped-for-privacy@att.net Inscribed thus:

"Polyfuse" any help ? Raychem make them.

--
Best Reagrds:
                        Baron.

A bit lossy with that sense R. A comparator would operate at a much lower threshold but you'd have to have latch after it, plus ensure that power-on resets the latch except in a fault condition.

Graham

start.

ftp://panteltje.com/pub/MOSFET_low_dropout_regulator_with_short_circuit_protection2.gif

How about a filament wire in a tube of gunpowder? That would be pretty "active".

I'll throw out an idea. View in fixed font.

p-channel mosfet

+in ---+-----, ,------+---load | | | | | _____ | | _ | | | | | | < | | ,-| | | | \\ | | | | | | | | > | | | |-----|----|---+ / | | | | | | R1 | | | | +-------+ '---+ | | | R3 reset R2 | button | | | | '-------+--------' | gnd

It will trip when the voltage across the mosfet equals the voltage across R1 (more or less). The R1 R2 voltage divider sets the trip voltage. You might end up using a pot. Obviously, current setpoint equals voltage setpoint divided by the on-resistance of the mosfet. I'm not taking time to work through specific numbers, but keeping the collector currents about equal by making R3 equal to R2 (actually R1+R2) seems like a good idea. Otherwise you're adding in an error.

This circuit won't start up by itself, you have to turn it on with a momentary button either as shown or that connects source to drain. If it trips, you use the same button as a reset button.

I'll throw out an idea. View in fixed font.

p-channel mosfet

+in ---+-----, ,------+---load | | | | | _____ | | _ | | | | | | < | | ,-| | | | \\ | | | | | | | | > | | | |-----|----|---+ / | | | | | | R1 | | | | +-------+ '---+ | | | R3 reset R2 | button | | | | '-------+--------' | gnd

It will trip when the voltage across the mosfet equals the voltage across R1 (more or less). The R1 R2 voltage divider sets the trip voltage. You might end up using a pot. Obviously, current setpoint equals voltage setpoint divided by the on-resistance of the mosfet. I'm not taking time to work through specific numbers, but keeping the collector currents about equal by making R3 equal to R2 (actually R1+R2) seems like a good idea. Otherwise you're adding in an error.

This circuit won't start up by itself, you have to turn it on with a momentary button either as shown or that connects source to drain. If it trips, you use the same button as a reset button.

I'll throw out an idea. View in fixed font.

p-channel mosfet

+in ---+-----, ,------+---load | | | | | _____ | | _ | | | | | | < | | ,-| | | | \\ | | | | | | | | > | | | |-----|----|---+ / | | | | | | R1 | | | | +-------+ '---+ | | | R3 reset R2 | button | | | | '-------+--------' | gnd

It will trip when the voltage across the mosfet equals the voltage across R1 (more or less). The R1 R2 voltage divider sets the trip voltage. You might end up using a pot. Obviously, current setpoint equals voltage setpoint divided by the on- resistance of the mosfet. I'm not taking time to work through specific numbers, but keeping the collector currents about equal by making R3 equal to R2 (actually R1+R2) seems like a good idea. Otherwise you're adding in an error.

This circuit won't start up by itself, you have to turn it on with a momentary button either as shown or that connects source to drain. If it trips, you use the same button as a reset button.

I would suggest not reinventing the wheel.

There are solid state devices, that are known as Poly Switches PPTC (Polymeric Positive Temperature Coefficient) Resettable Fuse Capacitors. They are direct fuse replacements in DC Circuits and are auto resetable (when power is cycled).

These devices have many applications such as energy-saving lamps, ballasts, loudspeakesr, power supplies, motors, fans, compressors, and other power related equipment. They are also used in automotive applications, LAN equipment, mobile phones, DSL modems and other Telcom & Network equipment.

They come in a wide variety of voltage ratings, and trip currents.

More information can be found at

and search for Poly Switch Fuse Capacitor.

Hope that helps you.

Jim

"jimwest" panfilero

I would suggest not reinventing the wheel.

There are solid state devices, that are known as Poly Switches PPTC (Polymeric Positive Temperature Coefficient) Resettable Fuse Capacitors. They are direct fuse replacements in DC Circuits and are auto resetable (when power is cycled).

** Unfortunately PolySwitches are VERY slow acting and far from precise in their tripping current - plus do not remove all current nor work at AC or DC voltages greater than about 60.

OTOH, a low cost domestic thermal / magnetic breaker has none of the above defects, works at any voltage up to 250 volts AC or 50 volts DC, can tolerate massive current overloads and be reset hundreds of times.

...... Phil

Simple DC electronic fuse

Rload ___ Vin -------|___|---o-----o-------------. | | | .-. .-. | 10k | | | | 10k | | | | | | '-' '-' ||-+ | | ||

T24gQXByIDE5LCA0OjU0oHBtLCBCb2IgTW9uc2VuIDxyY21vbi4uLkBnbWFpbC5jb20+IHdyb3Rl Ogo+IKAgoCCgIKAgoCCgIKAgUmxvYWQKPiCgIKAgoCCgIKAgoCCgIKBfX18KPiCgIKBWaW4gLS0t LS0tLXxfX198LS0tby0tLS0tby0tLS0tLS0tLS0tLS0uCj4goCCgIKAgoCCgIKAgoCCgIKAgoCCg IHwgoCCgIHwgoCCgIKAgoCCgIKAgfAo+IKAgoCCgIKAgoCCgIKAgoCCgIKAgoC4tLiCgIC4tLiCg IKAgoCCgIKAgoHwKPiCgIKAgoCCgIKAgoCCgIKAgoDEwayB8IHwgoCB8IHwgMTBrIKAgoCCgIKB8 Cj4goCCgIKAgoCCgIKAgoCCgIKAgoCCgfCB8IKAgfCB8IKAgoCCgIKAgoCCgfAo+IKAgoCCgIKAg oCCgIKAgoCCgIKAgoCctJyCgICctJyCgIKAgoCCgIHx8LSsKPiCgIKAgoCCgIKAgoCCgIKAgoCCg IKAgfCCgIKAgfCCgIKAgoCCgIKB8fDwtIKBOTU9TCj4goCCgIKAgoCCgIKAgoCCgIKAgoCCgIG8t LS0tLSktLS0tLS0tLS0tfHwtKwo+IKAgoCCgIKAgoCCgIKAgoCCgIKAgoCB8IKAgoCB8IKAgoCCg IKAgoCCgIHwKPiCgIKAgoCCgIKAgoCCgIKAgoCCgIKAgoFx8IKAgfCCgIF9fXyCgIKAgoCB8Cj4g oCCgIKAgoCCgIKAgoCCgIKAgoCCgIKAgfC0tLW8tLXxfX198LS0tLS0tbwo+IKAgoCCgIKAgoCCg IKAgoCCgIKAgoCCgPHwgoCCgIKAgMWsgoCCgIKAgoHwKPiCgIKAgoCCgIKAgoCCgIKAgoCCgIKAg fCCgIKAgoCCgIKAgoCCgIKAgoC4tLgo+IKAgoCCgIKAgoCCgIKAgoCCgIKAgoCB8IKAgoCCgIKAg oCCgIKAgoCCgfCB8IDBSMQo+IKAgoCCgIKAgoCCgIKAgoCCgIKAgoCB8IKAgoCCgIKAgoCCgIKAg oCCgfCB8Cj4goCCgIKAgoCCgIKAgoCCgIKAgoCCgIHwgoCCgIKAgoCCgIKAgoCCgIKAnLScKPiCg IKAgoCCgIKAgoCCgIKAgoCCgIKAgfCCgIKAgoCCgIKAgoCCgIKAgoCB8Cj4goCBHTkQgLS0tLS0t LS0tLS0tLS0tLW8tLS0tLS0tLS0tLS0tLS0tLS0tJwo+IChjcmVhdGVkIGJ5IEFBQ2lyY3VpdCB2 MS4yOC42IGJldGEgMDQvMTkvMDV3d3cudGVjaC1jaGF0LmRlKQo+Cj4gVGhpcyBvbmUgbWF5IHdv cmsuIEl0IGhhcyBmb2xkYmFjaywgc28gaXQgd2lsbCB0dXJuIG9mZiB0aGUgY3VycmVudCBhdAo+ IGFib3V0IDVBLCBhbmQga2VlcCBpdCBvZmYgdW50aWwgeW91IGJyaW5nIHRoZSBpbnB1dCB2b2x0 YWdlIGRvd24gbmVhcgo+IHplcm8uIFRoZSAxMGsgYW5kIHRoZSAwUjEgcmVzaXN0b3JzIHNob3Vs ZCBiZSBhdCBsZWFzdCAyVy4gSWYgdGhlIGxvYWQKPiBpcyBhIG1vdG9yLCBwdXQgYSBkaW9kZSBw YXJhbGxlbCB0byBpdCwgcG9pbnRpbmcgYXQgVmluLCB0byBwcm90ZWN0Cj4gdGhlIG1vc2ZldC4g VGhlIG1vc2ZldCBzaG91bGQgaGF2ZSBhIGhlYXRzaW5rLCBzaW5jZSBpdCdsbCBkaXNzaXBhdGUK PiBhcm91bmQgMTBXIGFzIHRoZSBjdXJyZW50IGNvbWVzIHVwLgo+Cj4gUmVnYXJkcywKPiCgQm9i IE1vbnNlbgoKQWJvdXQgdGhlIDEwayByZXNpc3RvciBjb25uZWN0ZWQgdG8gdGhlIGNvbGxlY3Rv ciBhbmQgZ2F0ZToKY29ubmVjdGluZyBpdCB0byB0aGUgbG93IGVuZCBvZiB0aGUgbG9hZCBhaW4n dCBnb25uYSB3b3JrLg==