# Low value manually resettable adjustable electronic fuse

I might be asking for too much with this one, but I hope it is out there. I would like a fuse that can be reset, but not automatically. I need to be able to know that the fuse is blowing and with a thermal fuse it will just start back up after it opens. I would also like to be able to change the value in case it doesn't open in time or opens too often. I need something low around .25 amps up to 1.5amps, and this is going to happen often so I don't want to use glass fuses because of the cost. Has anyone seen anything like this? Perhaps a circuit? Or has anyone seen some test gear that has these features that I can use on the motor until I find out exactly where it will safely blow. It needs to be small because of where the motor is mounted. Thanks very much, Russ

RM

It is a 12VDC motor. At the moment I don't have the circuit diagram available.
I don't think it matters if the fuse is at the low or high side.

Sometimes the motors stop switch does not work and it cause the motor drive train to jam and grind down some brass gears. When it jams the current goes up from about 10mA to 30 to 40mA and this is why I need some way to stop the motor.

Hopefully there is a fuse or circuit breaker available that will meet this need.

• Do you maybe just want a power supply with a crowbar circuit (look it up)? – The Photon Apr 11 '12 at 3:58
• That extra information is useful. The second circuit in my answer (in grey) would probably meet your need and my electronic circuit - discussed but not yet shown - will do what you want. You may need the ability to provide extra current at start up - my circuit can accomodate this. – Russell McMahon Apr 11 '12 at 12:41
• Questions: Does the motor run continually when started or does it star and stop? If start stop - how long does it run and how long does it stop for typically. Originally you said 0.25 A and 1.5A but in your extra information you say 10 mA and 30-40 mA. Which is correct? (Did you mean 100 mA and 300-400 mA?) | Does the motor HAVE to run on 12V? Can it run on 13 or 11 or 10 or ...? – Russell McMahon Apr 11 '12 at 12:45
• It starts and stops and goes backward and forward. It only runs for for a few seconds in either direction and overall runs for maybe a minute or two going back and forth, then stops for awhile. I would like an adjustable fuse from .25 to 1.5amps and yes, I did mean 100 and 300-400mA. The motor can run on a little less or more it will just slow the speed down which isn't that critical. But, the power supply for the motor is fixed at 12VDC. Thank you very much, Russ – user7684 Apr 12 '12 at 0:44

Comment on any of these if more detail is anted:

Easy enough electronically, and I'd recommend an electronic solution - but a relay with at least either 1 x changeover contact or 1x make and 1 x break contact will do this. Relay is rated at lowest value current that you want. Place a wirewound pot of suitable rating in parallel to desensitise it by shunting some of the current.

The relay's break contact disconnects the supply. The relay's make contact self latches itself.

Electronic: A foldback current limiter will do what you want. When current exceeds a trip point the circuit adds resistance until some lesser value is reached. This can be latching.

Example from here - at bottom of page. Circuit explanation given on that page.

Measured and calculated performance (from their site).

A circuit that powers itself completely off when a set current is exceeded is also "easy enough".
eg Placing a diode in series with R4 in the above circuit so the diode conducts when LM317 output is low, and lowering the value of R4 to around a few hundred ohms would probably make the circuit fully latching. May not then start well - may need a little thought to tidy the result.

Here is an electronic fuse circuit using a relay as the output switch and a flip flop to latch the tripped state. The relay could be replaced by a MOSFET and the latch could be replaced by hysteresis feedback.

I drew up a circuit before I saw the above which does much the same but removes the latch and uses a MOSFET. It uses 1 x opamp section, 1 x MOSFET, 1 a voltage reference (or a zener with less precision), 5 or 6 resistors, a diode and a pot to set trip level. Turnoff is complete and instant on exceeding Itrip and it latches off. Trip delay can be added with one capacitor and operation can be switched to constant current limit by opening the diode circuit. Can draw (slightly) more tidily and post if needed.

Here is their circuit.

3 transistor foldback current limiter:

Second circuit on page here.
This too can be made to latch off.

A few idea starters here Gargoyle "foldback current limit circuit" image search

Gargoyle "electronic fuse circuit" image search

Just using a constant current source may suffice - so that current can not go above some preset limit.

• I would do it like in the gray schematic. If you swap the comparator inputs it can drive the FF reset directly, without the C9013/1k/470$\Omega$. – stevenvh Apr 11 '12 at 13:24
• @stevenvh - yes. MOSFET can replace relay and C9013. Heavy positive feedback on IC1 via a diode to latch it eliminates IC2. Much simpler. Filetring sense resistor allows some attack time for motore start. And you are about at the circuit I mentioned :-). – Russell McMahon Apr 11 '12 at 14:18

# Hot Swap ICs

Hot Swap ICs sometimes implement functionality, which you are describing. For an example, see TPS2421-1 . It has an integrated FET*. $\overline{FLT}$ and $\overline{PG}$ pins can tell about the stasus of the "fuse". $\overline{EN}$ can reset the fuse. $ISET$ can sets the trip current.

* Some Hot Swap ICs work with external FET and current-sense resistor.