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I need to limit the current of a motor to around 1A. I would like to use a PTC fuse so that it trips in the rare occasion the motor is forced by the user to draw too much current because the motor is having to work harder than it should.

I understand the PTC fuse has a hold current, e.g. 750mA (the current at which it is guaranteed to hold) and a trip current e.g. 1.5A (the current where it is guaranteed to trip). Inbetween these currents, the fuse may trip, it may not, though I suppose it is more likely to trip if it is made to allow a current to flow between its hold and trip current for long periods of time.

In the datasheet for RKEF075, on page 2 it states that the max time to trip is 1.5 seconds, but that this time to trip is at 8A, not the trip current of 1.5A. Does this mean the fuse will take a long time to trip if 1.5A flows? Could it possibly allow up to 8A to flow (even for a short period of time)?

Fuse datasheet Electrical characteristics

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  • \$\begingroup\$ yes, probably, but your motor won't catch fire in 1.5 seconds either \$\endgroup\$
    – user253751
    Nov 11 at 9:41
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The datasheet says it trips in less than 1.5 seconds at 8A.

This means that 8 amps is flowing through it, for potentially 1.5 seconds before it trips.

This is standard for any type of fuse: it takes time for it to get hot enough to break. The lower the current, the less heat, the longer it will take to break.

There will be a time somewhere for how long it may take before breaking at 1.5A, I have seen this as 60s and at 120s depending on the fuse.

Other things to note about PTCs is that while they are resettable, they do have a memory effect. Often get significantly higher resistance after the first reset, and then they are less and less reliable the more times they get triggered. I try to avoid them personally.

For your application, I would always go for dual devices to make sure things are safe:

  1. A standard fuse to break during major over current events or short circuit events
  2. Current monitoring triggering a transistor to turn off the motor for the more common motor jam or over load events. Yes this is a lot more complicated than just a PTC, but it is a lot safer and leads to a more reliable product.

If you are using the motor in a controlled environment in your desk/office/workshop then a PTC will do the job. But if you’re letting anyone else use it, always put in the extra safety, it’s good for your motor as well as for the user.

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  • \$\begingroup\$ I already have a low side current sense circuit so with an op amp allowing the microcontroller to monitor how 'hard' the motor is having to work and the idea is it will indicate if the user is doing something they should not. The motor supply can be disconnected as a MOSFET, controlled by the microcontroller, can be switched off. Do you think this is sufficient and is along the lines of your second suggestion? \$\endgroup\$
    – MRB
    Nov 11 at 12:46
  • \$\begingroup\$ @MRB So you've already got currrent sense there with a microcontroller able to turn the motor off? I'd say that would be fine. You could put a standard slow blow fuse in there too to be extra safe. But as it's a motor, and motors can take huge gulps of curent during starting, make sure it's a slow blow or you'll either blow lots of fuses or your fuse will be able to take too much current and not protect blow at all. \$\endgroup\$
    – Puffafish
    Nov 11 at 13:04

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