Constant 5V Output from DC Generator

The voltage output and current vs the speed the shaft depends on the design of the motor you are using and can not easily be changed without using a different motor.

To get a constant 5 volts you have three choices but it depends on what you are using it for.

Your first choice is to make sure you spin the motor at a constant speed, whatever speed gives you 5 volts. This is your most efficient option but has some downfalls since if your load changes so will the force required to drive it, which may or may not change the speed and or voltage.

Your second option is to drive the motor to a higher voltage than you need and use a voltage regulator such as a 7805. You will lose some power with the regulator but it will make sure you have a nice steady 5 volts.

Your last option is to use a dc to dc converter, the converter should be capable of delivering an output of 5 volts if the motor output is either below or above 5 volts, how far above and below depends on the converter. This also suffers from efficiency losses but will give you the most stable output independent of the motor speed.

Which you choose really depends on what you are using the power for and how efficient and stable the system needs to be.

Frank,

the second option is a less expensive way to get a regulated supply but you waste power - the higher your motor voltage is compared with the 5V you need.

I would prefer the third option and use a DCDC-controller which is capable of stepping up or down depending on the input voltage.

These devices have 3 modes:

1. Uin < 5V => stepup-mode
2. Uin nearly = 5V => transparent mode
3. Uin >5V => stepdown-mode.

If the stability requirements especially with noise and spikes are not so high, thats it (= switch on, be lucky).

If you need a highly stable and noise/spike-poor output you should set the DCDC-output slightly higher than 5V (5.5V p.example) and add a lo-drop linear regulator cutting the 0.5V above 5V (wasting not so much power as the good old 7805) and you have a high quality supply even for sensible analog applications.

Regards Uwe

Most don't understand that a good dc motor does not make a good generator or alternator. The dynamics are totally different. I have a hand size pm alternator that will produce in excess of 500 watts. Total capture of flux at the correct induction angle.

John

A brushed, permanent magnet (PM) motor will have very nearly the same speed and torque constants when used as a generator or motor. It sounds like your motor is a PM type, with a speed constant of 1300 rpm/volt. Therefore, if you turn it at a constant 6500 rpm, you will produce a constant 5 volts. If the load varies on the generator, the load will likewise vary on whatever is turning it, so the prime mover would need to be regulated by a governor, or something similar.

If your motor had a wound field, then you could vary field strength to vary voltage, as is done in automotive alternators and generators.

Both hand cranked flashlights and bicycle generators are geared to turn faster than the input speed, but the other technique for higher voltage is more turns of wire passing through the magnetic field.

Yep: Run it at exactly 6,500 RPM constantly, and don't put any electrical load on it.

OR you can run it at somewhere above 8,000 RPM and put a 5 volt regulator such as an LM109 on it. Less speed will cause the voltage regulator to "drop out (it needs about 1.5 volts extra)," but more speed will continue to put out 5 V with higher power dissipation.