Question about selecting propper fuse

Thread Starter


Joined Oct 11, 2016
I am a novice and tinker. I made a ball lifter for a pachinko machine that feeds 6mm balls to a feed tray.

The circuit is a simple 12 Dc 5a powersupply , feeding a buck converter that outputs 5vdc that powers the sensor and relay board. The sensor triggers the 5v that trips the relay powering the full 12v to a small dc motor. That motor can draw max of 1.8A.

I figured power supply is 5A sobi used a 5a fast blow fuse.

It's working but is this correct?

Should it be closer to 1.8 A of what the motor draws?

I am using a small 20mm glass fuse holder. The fuse is inline from the + side from the power supply feeding everything. I want to protect it just incase the motor jams up it does not burn up the wires.

Video here of it working



Generally, fuses are rated for the current they will carry forever. A 5x20mm 5A fuse curve is like (pics) and may not blow before the motor burns up or the 12VDC PSU shuts down or hiccups. Car blade fuses are extremely slow. A polyfuse could work depending on the ratio of motor run/jam current.
I would intentionally jam the mechanism/motor and see what happens to motor current, measure it. I don't know your project but would have a S/W timer for the maximum motor on-time. If the 12VDC PSU collapses, then you would instead get stuck in a boot loop.

Fuse curves from:



Joined Jan 23, 2018
The purpose of a fuse is normally to protect the wires and the power supply in the event of a short circuit. So if the power supply is rated at up tp 5 amps, any fuse below that current should protect the supply. Depending on the size of the wire you might need a lower rated fuse. But since there are really 2 power supplies, the 12 volt one and then the 5 volt one, a fuse in the 12 volt line may not protect the 5 volt supply from a short circuit on it's output line. Some switcher supplies have overload protection and some do not.
One caution is that most motors draw more than their rated current for a very short time when they start. So a 4 amp fuse should avoid popping the fuse when there is no problem with the motor. So 5 amps is OK as well.