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How can I control the speed of a 7A 12v motor? Answered

 I have a water pump which works at 12v and is rated 7amps. I want to control the flow of water and I think if I could control the speed of the pump it would do the trick. Any help will be deeply appreciated.


The simplest method is to use a PWM controller. Where are you in the world  for some idea of the best place to get one ? 


Maplin then !


You'll have to beef up that output transistor - don't know what they used, but take it off and put it on a better heatsink for a start .

Unless you are short of energy, re-design has a good idea.

That'll do VERY well.

Like I said, the Maplin one needed the transistor beefing up for 7A


 Do you know any transistor that will do the job?

Thank you! The maplin PWM is rated 3A. Do you think is will still possible to use with my 7A motor?
Input voltage:6 - 15V
Output voltage:modulated relative to input
Loading:3A cont. 5A peak
Dimensions:61 x 41 x 40mm

An arduino would also do the trick, at the same price as the controller that steve posted, you could use it for other projects once this one's done. It has a PWM output and there's loads of documentation online. Like Steve's solution though, you'd also need a beefy transistor as the duemilanove (the arduino that I have and would recommend) can't supply more than 50mA.

 Will the PWM make the battery last longer as well as control the motor speed?
Thank you very much!

Well, the simplest method to control the speed manually would be to put a rather huge potentiometer in between the motor and power supply.  Or you could power it with a variac (a variable power supply).  If you wanted to do something a bit better but more complicated, you could use a PWM circuit of some sort.

I would contol the water flow.  Put a tee in the line.  ON the new line put a valve and line that runs back to where ever the pump is drawing from.  If the valve is open the some of the water is going to go back to the source.  Use the valve to adjust the amount of water you want.

 That's a good idea, but what I'm making it's portable and I want the battery to last a bit longer as well as controlling the flow of water. So if I make the motor go slower I should make the battery last longer.

Then the best way for you would be PWM controller as Steve suggested.  Looses less power to heat.

You need a, "variable resistor" to slow down the speed of the water.

example "Dimmer Switch":

 A variable resistor? To power a 7amp motor? What kind of variable resistor can I get that powerful?

As steveastrouk said, PWM is the best approach. This can easily be done with a op-ap based relaxation oscillator, link. However instead of a single resistor in series with the capacitor feeding the negative input, use a potentiometer and two diodes.

This can be reconfigured to operate on a single rail, using two 10k resistors to reach the midpoint of the voltage supply, and using this instead of the earth in the oscillator. For simplicity, it can use the same supply as the motor.

The output can then be used to trigger a transistor, which is used to drive the motor. The transistor may require a heat sink.