Re: [mowbot] [OT] Driving 6v DC motor with 12v power supply

david steinke (dsteink nospam at
Tue, 18 May 1999 03:34:50 +0100

At 08:13 AM 5/18/99 +1000, you wrote:
>At 10:29 17-05-99 -0500, adamdb nospam at wrote:
>>Can I drive a 6v DC motor with a 12v battery by using PWM and a 50% (or
>>less) duty cycle? How will this affect the life of the motor?
>You can, but only if you reduce the frequency and put a low pass filter on
>the output. This would effectively turn the PWM signal into a variable
>analog voltage.
>Having said that, DON'T DO IT! You'd need a substantial power resistor and
>would waste energy in heat.

Whoa! Look at any speed controller for RC cars. They are typically 4kz
switching frequency. Lower frequencies are worse, higher better. The problem
with higher frequencies is the lead inductance, transistor switching times,
and flyback damping. A good book is Bob Boucher's (??) Electric Motor
Handbook from Astroflight. It discusses at all this. Running a 6 volt motor
off a 12 volt source with the appropriate duty cycle is fine. Actually, the
6 volt motor will run fine on 12 volts if the brushes and bearings can
handle it. Assuming the mechanics can handle the higher rpm then it is
current which determines the load limitations.

>The best choice is to use a 6v battery.
>>Is there a rule of thumb for the frequency to use? I was going to use a
>>PIC for control and the timer overflow interrupt, thus giving a frequency
>>of 4 KHz. Does this seem reasonable or should the frequency be higher or
>>lower (in general).
>The freq should be much higher. Each motor will have an optimum frequency
>and you should try to find it. It will certainly be above audio (4khz is in
>the audio range).

Like I mentioned earlier, RC car speed controllers typically use 4 khz.

Go for the 12 volts if you need to. The 6 volt motors running at 50% duty
cycle off a good speed controller might actually be more efficient than the
same motors running off a 6 volt battery.

David S.