- #1

- 3

- 0

i know i need to use ohm's law somehow but it's been a while since i've had anything to do with this and cant remember just how to do it....

thanks

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter beefertoo
- Start date

- #1

- 3

- 0

i know i need to use ohm's law somehow but it's been a while since i've had anything to do with this and cant remember just how to do it....

thanks

- #2

- 649

- 2

I don't think the speed of an electrical motor is necessarily proportional to the voltage. It depends heavily on the characteristics of the load you are putting on the motor. I think you need to experiment to find the correct values.

Say that a 12V motor can barely lift a given weight, running at some non-zero RPM. On 6V it will then not be able to lift the weight, and the RPM will be zero. So the relasionship will be complicated, and I think it is even for an "unloaded" motor (i.e when the only load is the ball bearing friction and air resistance within the motor itself).

EDIT: This seems like a good source of info:

http://www.4p8.com/eric.brasseur/emamem.html [Broken]

If you know the power needed to drive the load for each value of RPM, then you might be able to use those formulas to derive the approximate relationship between voltage and RPM for your motor. I didn't look thoroughly, though.

Torquil

Say that a 12V motor can barely lift a given weight, running at some non-zero RPM. On 6V it will then not be able to lift the weight, and the RPM will be zero. So the relasionship will be complicated, and I think it is even for an "unloaded" motor (i.e when the only load is the ball bearing friction and air resistance within the motor itself).

EDIT: This seems like a good source of info:

http://www.4p8.com/eric.brasseur/emamem.html [Broken]

If you know the power needed to drive the load for each value of RPM, then you might be able to use those formulas to derive the approximate relationship between voltage and RPM for your motor. I didn't look thoroughly, though.

Torquil

Last edited by a moderator:

- #3

- 86

- 0

- #4

- 1

- 0

Use diodes instead as they are voltage limiting not current.

The forward voltage drop on a diode is 0.75 volt so if you put eight diodes in series you drop from 12v to 6v.

The diodes used would have to be able to carry a bit more than your maximum load of 10 amps.

I have used this to run the windscreen wiper motor of an old car that I upgraded from 6v to 12v.

The load on the motor depended on how wet (or dry) the windscreen was. If I had used resistors the wipers would have stalled.

Experiment by adding or removing a diode to obtain the speed that you require.

- #5

- 649

- 2

The forward voltage drop on a diode is 0.75 volt so if you put eight diodes in series you drop from 12v to 6v.

The diodes used would have to be able to carry a bit more than your maximum load of 10 amps.

I have used this to run the windscreen wiper motor of an old car that I upgraded from 6v to 12v.

The load on the motor depended on how wet (or dry) the windscreen was. If I had used resistors the wipers would have stalled.

Experiment by adding or removing a diode to obtain the speed that you require.

Yes, alternatively use a Zener diode if you don't want a string of diodes. You can find them for different voltages.

Torquil

Share:

- Replies
- 11

- Views
- 9K