# Do I need to use a fuse?

Discussion in 'General Electronics Discussion' started by Madsalts, Nov 22, 2014.

73
2
Sep 25, 2014
I'm building an automatic feeder for an aquarium. A timer will be plugged into the wall. An Ericsson Model 420AS44001 power supply (DC6V, 700 mA) will be plugged into this. This will energize two motors, each rated for 6V. This feeder will run unattended. I want to avoid a fire. I'm wondering if I should use one fuse for each motor after the power supply, or if it is unnecessary. Thanks.

2. ### davennModerator

13,808
1,943
Sep 5, 2009
always a safe way

73
2
Sep 25, 2014
yeah, I guess it's easy enough. I might as well. Thanks.

73
2
Sep 25, 2014
I have no idea how to select a proper fuse value. I see some guides online that deal with large DC motors (1/2, 1/3 HP, etc.). But these motors are about the size of my thumb. Any ideas? Thanks.

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,489
2,830
Jan 21, 2010
Measure the resistance of the motor. Then use ohms law to determine the stall current. You want the fuse to blow if this current flows.

Measure the current drawn when the motor is operating normally and under its designed load. The fuse needs to be rated for significantly more than this.

Let's say it's a 12V motor and it measures 3 ohms. That means the stall current is 4A

Under load it draws (say) 500mA.

I would want a fuse that is rates at least twice the normal load (minimum rating), and at most half the stall current (maximum rating). That tells me it needs to be between 1A and 2A. I would recommend you use a 2A fuse, and I'd also make it a slow blow fuse because the motor starts "stalled" an will draw a lot of current as it starts up -- especially if under load.

If you do this calculation and your minimum value is greater than your maximum value then it may be difficult to get a suitably rated fuse that will blow if the motor is stalled but not fail in normal operation. In this case you might elect not to provide protection for a stalled motor and choose a larger value fuse that will protect you from a short in your wiring.