Ladies and Gents,

Here is an alternative to big power resistor voltage regulators when you are trying to step down 12V to 6V to drive a high current device. When I was going through my 38 truck I noticed that there were two 6V electric motors driving the wind shield wipers. I wanted to keep the motors 'cause I though they were cool. Problem was I had already converted the truck over to 12V. Each motor draws about 4 Amps at 6V. With two motors that would mean I'd need to dump about 50 Watts (2 * 4A *6V = 48W). I was looking for something more elegant.

I knew that the fan adjustment in your car typically uses a pulse width modulated signal to give you various fan speeds.

It is possible to make a motor or bulb think it is getting 6V by giving it a 50% duty cycle 12 Volt signal. On average the energy that is being put in is what it would get if it was a 6V DC signal. The following plot is what this circuit does.

Now it isn't exactly like a DC 6V signal because during each pulse there is twice the current running through your device. I suspect that if you drove a 6 V device this way, it might wear it out a little faster. But I will hardly ever use my wipers so I don't really care. If you don't mind dumping a lot of power I would go the power voltage regulator route



The duty cycle on this circuit can be adjusted by the 50k pot off pin #13. If you replace that with a two switched resistors you can have multiple speed fans.

Some important facts are:

Make sure to put a diode across your motor. MOSFETs are very sensitive to static and highly inductive loads like motors create a lot of back EMF that will fry the MOSFET.

 If you need your circuit to run in cold weather make sure to pick a 556 chip with appropriate temperature specs.

 I put a heat sink on the MOSFET, they are cheap

The MOSFET below is supposed to handle 18A (I would use a heat sink for sure at that load)

This circuit has a pulse width train of 240 Hz which ought to be fast enough for any motor.
I didn't go faster because I wanted to minimize switching through the FET and thus control heating.

I am not sure you need the wire wrapped 10 Ohm resistor, it is just good practice to use one.

 I am not sure you need the big caps on the input to condition the input signal, again it is just good practice.

This MOSFET is great because it turns on with 15 mA current and has 0.18 Ohm internal resistance.
Just about any chip can drive it without a buffer.

 You can use a volt meter to figure out the output. The output frequency is fast enough that a voltmeter will average the output signal.
Just adjust the pot until you get the output voltage you want.




a.k.a Jim Padget


Return to Home Index.