|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? Use of a fixed duty cycle will generally not give you good results. Nor will a series resistor. If you want to control a motor effectively, you'll have to use some sort of feedback regulation scheme. To understand how DC motors work, you should first consider a motor whose leads are not externally connected, but which is mechanically spun. It will generate voltage directly propor- tional to the rate of rotation. If this motor is fed a continuous voltage but prevented from rotating, it will act like a resistor: the current will be proportional to the voltage, and the torque (how hard it tries to turn) will be proportional to the current. If instead the motor is allowed to turn, the current will be proportional to the difference between the voltage fed in and the voltage the motor would produce if used as a generator at the rate it's spinning. For example, say a particular motor would produce one volt if spun at 10 revolutions per second and it has a resistance of 10 ohms. If you fed 10 volts into this motor and it was stall- ed, it would consume one amp and dissipate 10 watts. If it was instead rotating at 50 revs/second, it would only consume half an amp and dissipate 2.5 watts; it would also do 2.5 watts of mechanical work. If it were spinning at 90 revs/second, it would consume 1/10 amp, dissipate 0.1 watt, and do 0.9 watts of mechanical work. When a particular motor is described as an n-volt motor, there are several factors that determine 'n': -1- Motor brushes and commutators are designed for a certain maximum voltage; if this is exceeded the brushes can arc over and bad things can happen. -2- The power dissipated in the motor windings of a stalled motor will increase with the square of voltage. Some motors will survive being stalled indefinitely at their rated voltage, but at a higher voltage will quickly burn out and overheat. -3- A motor's maximum current draw will be determined by its resistance and the input voltage; its maximum torque is determined by this current. Driving a motor with a higher- than-rated voltage may cause it to over-torque its bearings. -4- A motor's top speed is determined by the input voltage. If the rated voltage is exceeded, the motor may spin faster than its bearings can handle, resulting in accelerated wear or catastrophic failure. If you know what the limitting factors are by which a motor's voltage rating was assigned, you may be able to safely drive the motor at higher voltages if you can ensure that those factors are not exceeded. For example, if the concern is power diss- ipation when stalled, it may be possible to drive the motor with a higher voltage if a PTC is used to cut power if it stalls for too long. If the concern is top speed, it may be possible to ensure in the application that the motor cannot physically rot- ate too fast. In any event, PWM'ing a motor without some sort of feedback is apt to be a recipe for disappointment, since if your power sup- ply voltage is twice the motor's rated voltage you'd have to limit your PWM duty cycle to 25% to limit the maximum power dis- sipation to that produced by the motor's rated voltage. By do- ing that, you'd then be limitting the motor's torque to half of what would be available at the design voltage. In some situa- tions this may be acceptable, but it's certainly not pretty. By contrast, if you use feedback in the PWM controller you can get the full range of torque and speed available at the motor's rat- ed voltage; as a bonus, if motor limitations allow it, you can go beyond the torque and speed the motor would allow, and even do pretty good speed regulation. The key is to know what the actual system constraints are, and regulate to stay just within them. Cool stuff.