I am building a basic Raspberry Pi robot: the Pi controls the enable pins on a dual-bridge motor controller bridge via PWM and sets the input pins for direction. I have a variety of battery options available (a LiOn power pack with a USB out and a 12V out, a double USB out power pack, battery holder for 2x 18650 LiOn, battery holder for 2x AA, etc). My motors say they are 3-6V and have a no load draw of 70mA and a max draw of 250mA.

My motor controller board documentation suggests a 10A fuse on the motor lines and a 15A to/from the battery. Are these fuses going to be necessary? Does that max draw mean that is the max the DC motors would draw at any one time, or is that the max safe draw for those motors? Are motors that small even going to be able to draw more than 10A? Obviously having fuses burn out would be safer than letting the motors burn out... but it seems like it might be acceptable with motors this small/cheap to take the chance.

Will the risks be different based on what kind of batteries I hook up? For example, the 18650 batteries I have are high drain and can deliver 25A.

  • \$\begingroup\$ The fuses are there to protect the wiring and the batteries too. Consider what happens if a motor or the wiring fails short-circuit. \$\endgroup\$ – Transistor Jan 5 '19 at 22:53

A good bridge is < 0.1 Ohm and good batteries @ 50 mOhms while the motor DCR will be 10% of rated V/I = 10% * 8/0.25= 3.2 Ohms thus Isc= 6V/(0.1 + 0.05 + 3.2 )= 1.8A per motor pole surge current.

But if the motor port is shorted, Isc= 6V/(0.1+0.5) = 40A

So a 10A fuse makes sense.

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