# How can I limit the RPM of an electric motor by using resistors or trimpots? (cheap/simple way)?

I've done it with a small trimpot that almost ignited by heating. What the problem? The current is to high? To lower the RPM must i lower the current or the voltage?

And what exactly do the resistance lower? voltage or current?

And what exactly do the resistance lower? voltage or current?

active| newest | oldestWhatever, you'll still need to dissipate a lot of power in the regulator, whether its a fixed resistor or a semiconductor.

Mathew's idea is conceptually sound, but he has omitted any power handling circuits.

A REALLY easy way to build a PWM motor supply is with the Micrel Mic502

Steve

Increasing the resistance of the circuit will reduce the current flow, but in adding a resistor the voltage

across the motorwill also be reduced.L

Attach the wiper pin of the potentiometer to the output of the Op-Amp, with a diode pointing in each direction, (as shown, hopefully).

This will give the full range of mark/space ratios from 1:0 to 0:1 (ish).

The potentiometer inline with a load is likely part of a voltage divider.

Assuming a DC source for simplicity

let R

_{1}= instantaneous resistance of the potlet R

_{2}=load resistance presented by the motor (maybe not exacty correct,but for illustration purposes)The voltage seen by the load is

V

_{R2}= V_{in}x (R_{2}/(R_{1}+R_{2}))I forgot to include it in my last post, but the power can now be calculated (from the above) as

P = V x I, but I = V/R, so

P

_{R2}= (V_{R2})^{2}/ R_{2}