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I’m looking for advice on how close to nominal current draw a fuse should be dimensioned. On the internet I have seen numbers from 125% and to 800% in different applications, which is a rather wide range.

In my system, I have a motor which draws around 1,3 A of DC current and I’m looking to protect the system against short circuit by adding a slow-blow fuse of around 2A (This value can't be changed in my design). Is this reasonable in a motor application? In my case repeatability is not important and other current limiting measures are also acceptable.

Are there any rules of thumb when it comes to choosing fuse size?

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A design for protection must 1st define what needs to be protected then match the protection to suit the spec. for peak current * t, avg current.

Fuses have an I*t profile for fast, normal and slow blow, each different in the speed of protection of I> rated holding current vs t duration.

Where as cap loads and light bulbs have a very short peak current >> Iavg, motors can be much longer in duration with Ipk=800%*I_rated in many cases.

This time duration depends on the inertial mass ratio/torque ratio and can result in very long times. Peak current will drop as speed increases and is limited by the DCR [Ω] of the coil and switch, but It (Amp-sec) was common rating for fuses but more accurate is I²t used now due to time to melt fuse link with I²Rt energy (Wattsec)

What do you want to protect ?

  • the transistor?, then use an active current limiter with 75mV currents sensor.
  • the tracks from vaporizing?
  • the battery current from a short? ... a Polyfuse can work or a fast blow 2~5x rated current depending on time of full motor acceleration.

A rule of thumb design requires a spec for t vs I and a SOA (safe operating area) spec for the fragile part needing protection.

With either missing, one can only hazard a guess.

My guess

use a Slow-blow 0.5W 1206 SMD fuse rated 20% above your motor worse case I²t value that you compute or measure.

1/2W is the power dissipated in the fuse at max rated current which blows in >=4hrs. Allow some 10%~20% degradation for aging depending on frequency of motor surges at full voltage.

enter image description here ref https://www.digikey.com/product-detail/en/bel-fuse-inc/C1S-2/507-1189-2-ND/809333

e.g. if Imax=1.3 and Ipk=8*Imax=10.4A and Ipk²= 108[A²] which decays to 1.7[A²] at full load and full speed. Then determine time of acceleration which depends on friction and mass or change in kinetic energy.

opinion

In a well designed system the fuse loses about <=1% of the load for holding current. Depending on voltage yours may be worse such as 5% which affects peak torque, so an active 75mV shunt current limiter can be faster and more efficient if done correctly with complex filter.

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The first thing to ask yourself is what exactly you mean by "protect the system against short circuit". Presumably you must protect the wiring, and prevent it from causing a hazardous condition if there is a short. If you intend to protect the motor from overload (eg. stalling) then you will have a somewhat different requirement. Fuses (even non-slo-blo types) can take quite a while to open up with a moderate overload. Or they can open early if they get too hot. They are not precision devices.

If you are using a solid state device to switch the motor, chances are good that it will fail if a short circuit occurs well before the fuse opens. A brief short circuit could result in a continuously conducting transistor (or a welded relay) and the motor never shutting off. Protecting a semiconductor with a fuse is usually an expensive and often troublesome task.

Motors draw a surge when starting up and that can blow the fuse immediately, or just weaken it gradually so it fails after some number of cycles. Test for this!

Finally, fuses work by dissipating power, so you will lose a bit of voltage in the fuse in normal operation. Make sure it is not too much for your circuit to work well. This is particularly important with low DC voltages.

Your 2A slo-blo sounds reasonable, devoid of any other information, but it may be too low. You don't necessarily derive any benefit from calling it close. If the wiring can take 10A and the transistor will be dead anyway, a 5A fuse may be just as effective.

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Pick a fuse based on how much overcurrent your device can handle in the fuse's fusing time. If your motor can handle a sustained 1.9A, then the 2A fuse will be fine--but it also has to be able to handle 2A for however long it takes the fuse to melt. Since it's a motor, and not sensitive electronics, it will probably be fine with a little overcurrent.

On the other hand, don't pick too low a fuse. Motors have a large inrush current, up to several times the normal operating current, for a few moments right after they are turned on. This can be mitigated by slowly ramping up the applied voltage or by not connecting power until the motor is already up to speed, if that is an option in your design. So if you use a 1.5A fuse for a 1.3A motor, it's likely the fuse will blow when you switch power on.

In general, if you're concerned about having too much current for even a millisecond, a fuse alone is not the way to go. There are other devices, like the current-limit diode, or a voltage regulator in current-limit configuration for example, that can act much quicker. These devices will drop any excess voltage across itself to limit the current to its rating, leading to rapid overheating in the case of a short circuit. However, a properly-chosen one can limit the current to a safe level for long enough to give your fuse a chance to blow.

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