# Testing a mosfet h-bridge (24v) with a 3v motor

#### us202000

Joined Jul 5, 2011
25
Hi, sorry if this is a dummy question. I am experimenting with a breadboard mosfet h-bridge for a simple 24V DC motor but due to potential high current, I wanted to test it first with a small toy fan motor (1.5-3V). Is this possible? Can I just put a series resistor with the motor? If so what the resistor value should be? I would like to test both motor speed and direction. -Thanks.

#### iONic

Joined Nov 16, 2007
1,654
Do you know what the current requirements are of this motor?
Did you think of using a PC Fan, even two of them in series?
With a 12V CPU Fan there is usually a current listing on it such as .8A

For example with a 12V fan and 800mA max you want to use a resistor that will regulate the current to 800mA and drop 12V across it.

With Ohms Law R = V/I, so 12V/.8A = 15 ohms.
Now what is the power rating of the resistor? Ohms Law again. P = VI, P = 12V(.8A) = 9.6W. Do you have a 10 ohm, 10W resistor? Or two 20 ohm 5W resistors in parallel?

If you do the calculations for the 3V motor you will find that you would need a much larger power resistor.

#### us202000

Joined Jul 5, 2011
25
iONic,

Thanks for the thorough explanation which is very helpful. Apparently this would require a resistor with high wattage which I don't have. The Radioshack mini motor is rated about 0.3A, 3V, which would still require a power resistor for a 24v power supply.

Instead, now I am thinking to use a couple of LEDs connected parallel in opposite directions (probably don't need diodes for reverse voltage) in series with a 1K resistor, assuming the LED current is typically about 20mA at 3.6v.

Not as exciting as running a small motor but at least it allows some functional testing.