DC motor current issue

jamesedward44 · May 11, 2011

I was hoping someone could help me out with a problem. I have a 4.5 volt 100 ma max DC motor and wanted to use my 9 volt DC 1700 ma power supply to power it. I thought that I could use a resistor to bring the voltage from 9 down to 4.5 volts DC. I've read there might be an issue with the current and it could burn out the motor, so a current limiter is needed. When I use this motor, I'll be using it 10 seconds at a time so it won't be constantly running. Do I actually need a limiter? If I do I don't know which component values to use for my specific set up.

ericgibbs · May 11, 2011

jamesedward44 said:
I was hoping someone could help me out with a problem. I have a 4.5 volt 100 ma max DC motor and wanted to use my 9 volt DC 1700 ma power supply to power it. I thought that I could use a resistor to bring the voltage from 9 down to 4.5 volts DC. I've read there might be an issue with the current and it could burn out the motor, so a current limiter is needed. When I use this motor, I'll be using it 10 seconds at a time so it won't be constantly running. Do I actually need a limiter? If I do I don't know which component values to use for my specific set up.

hi,
The easy way would be to use a 1Amp 7805 +5V Voltage regulator, I know this will give 5V, but for a 10secs burst all should be OK.

jkp · May 11, 2011

Instead of a 7805 giving 5V, You could go for the LM317 which can be configured to give 4.5V. Search the net for datasheets and examples on how to calculate passive components for a 4.5V configuration.

jamesedward44 · May 11, 2011

Thanks a lot for your help. I'll give it a shot.

Jaguarjoe · May 11, 2011

If you chose the elcheapo resistor method, you'll need an R = E/I = 4.5/0.1 = 4.5 ohm resistor. 4.5 is a ******* value, so use 4.7. Your wattage = P = E*I = 4.5*0.1 = 0.45 w. At least double that for a good safety margin so get a 1 watt one.
If you can't find that resistor, you can use the parallel resistance formula Rt = R1*R2/(R1 + R2) backward to come with two resistors in parallel to get your ohmage.

colin55 · May 11, 2011

A resistor is not the answer.
You can use some1A power diodes in series.

crutschow · May 11, 2011

As collin55 stated diodes would work. Depending upon the voltage drop of the actual diodes you use, about 5 or 6 in series should give you the drop you need.

DC motor current issue

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