Hand-crank generator... How to get more amps?


Flashlight Enthusiast
Feb 6, 2001
Perth, Western Australia
The other post about the handcrank flashlight reminded me that I had a question about the generator. I started a new thread so as not to hijack the other one.

Basically, the question is this:
Supercapacitors need to have their input voltage controlled so as not to detroy them. But they can accept very high currents, which allow them to be charged very quickly. Given a standard hand-cranked generator, how can it be modified to produced increased amounts of current?

Increase number of windings? Increase crank speed?

I sort of recall a physics class that demonstrated the voltage is related to the speed at which the windings move through the magnetic field, while the current is related to the strength of the magnetic field or the number of windings going through the field.

Am I right? I couldn't find my high school physics text to check it out.

I just wanted a generator that will charge up my supercaps quickly (minimum number of cranks). If I can get a generator to produce 5V 500mA, I could probably fully charge a supercap in 10seconds.

Doug Owen

Flashlight Enthusiast
Jan 30, 2003
A five Volt, half Amp generator? Easy. I'd start with D.C. motors in a about twice that current, same sort of voltage.

But I'm nor sure we're talking lots of useful power here, are we? We can 'draw down' the one Farad cap you describe (charged to 5 Volts by half an Amp in 10 seconds?) even faster with a Luxeon.. Four seconds from fully charged to Vf, right? We'll need to get real fancy to get any more of our charge back out. Chemical systems address this issue. You could, of course, use a smaller LED, but even then we're still talking under a minute. Most folks don't want to crank that often I'm bettin'.

Anyway, try a motor to start. Any DC motor is also a generator at the same time. Try to drive the shaft faster and it draws less and less current, eventually going the other way and producing current (more and more as you drive harder and harder)). That is my electric car charged it's batteries going down a hill without any changes at all. Say 200 amps on the flats at 35, 400 going up at 25, over the top to a couple hundred *charging* at 45. Same gear, same configuration, same field drive.

Field drive brings up a good point, you might want to look a bit and find a 'shunt' motor, one where the field windings come out separately and are hooked across the source as is the brush assembly and therefore the moving coils. Shunt being another name for parallel. In such motors you can change the field independent of speed, thus 'loading' the generator to whatever speed you want (within reason). More field (at the same speed), and it 'wants to run faster still (and can't), more energy coupled into the load (output).. Without this feature, speed is a big issue.

Oh, yeah, don't forget the diode, or it will be a motor (like when the cutout on old timey auto generators would fail 'to short'...).


Doug Owen