Question about selecting propper fuse

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samslack

Joined Oct 11, 2016
5
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

 

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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: https://www.littelfuse.com/products/fuses/cartridge-fuses/5x20mm-fuses.aspx
 

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MisterBill2

Joined Jan 23, 2018
6,359
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.
 
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