# Slowing Down a 12 volt pump with a potentiometer?

I have a small pump hooked up to a 12 volt battery. But as is, it is working too fast and I would like to slow it down. The pump draws 300mA (though it's start up inrush is 3x that).

I am a little confused as to what kind of potentiometer I should get. If my rudimentary skills are correct, at 12 volts I need a minimum of a 40$\Omega$ potentiometer capable of handling 3.6 watts (not including the inrush).

But most of the potentiometers I am finding are only rated for 1/2 a watt, and many are ratted for 1,000+$\Omega$...... so can anyone recommend a good potentiometer for my needs?

That's because potmeters are made to control signals, not power. There are power variable resistors, called rheostats, like in silico says, but you don't want that.

What you need is a PWM controller, for Pulse Width Modulation, which switches the pump's motor on and off at high frequency, so that the average voltage is lower than what you have now. You can control the average voltage by varying the pulse width, hence "PWM".

Least components solution is a microcontroller. Connect a potmeter to a ATTiny5 controller and let the controller create a PWM signal based on the potmeter's position. Use that signal to control a MOSFET, which works as a power switch. Three components: potmeter, controller, FET.

Many people would suggest the LM555 as an alternative, but it controls not so nicely: the frequency varies with the duty cycle, and control isn't linear. Much nicer is a triangle wave which you feed into a comparator, setting the comparator's threshold with the potmeter.

That gives you perfectly linear control at a fixed frequency.

I'm sorry if this would sound complicated to you, but for someone who's a bit familiar with electronics it's very simple, and it's the best way. The potmeter in series with the pump is a bad idea.

• stevenvh, a voltage follower with a linear potmeter is also silly idea, but I think it's what extract OP ask. – Standard Sandun May 26 '12 at 6:36