# How much torque required to get 5V 500mA

i am trying to build a hand cranked battery that will generate 5v 500ma output.
i will be using 1000:1 gear ratio with 6 gears
the question is
what will be the torque required to drive the whole gear train+the generator and what will be the size of the gears.

Doug Huffman
Gold Member
The only thing that you have told us is that your desired output is 2.5 Watts.

russ_watters
Mentor
We need the rpm you plan to spin it at....and the generator's specs would be useful too.

sophiecentaur
Gold Member
I reckon that 1000:1 gear ratio could be very inefficient due to friction. Where did you get the value of rotation speed which gave you the 1000:1 requirement? Seems to me that you could do better by choosing a more suitable generator in the first place. A generator with more poles could be the answer.

guys i am not an engineer and i dont know much about i am just trying a new thing that came to my mind, excuse my lack of knowledge.
so here i will rephrase the question:
if i want to generate 2.5Watt using a hand cracked system, what will be the best gear ratio and generator to use?
and will appreciate if you tell the amount of torqure required to turn the generator as well

russ_watters
Mentor
Is this a school project? If not, have you considered just buying one to play with?

This isn't a trivial project, but it isn't too difficult either. We can walk you through it, but we're going to make you do most of the work.

Steps:
1. Select/calculate output requirement.
2. Calculate/select input requirement.
3. Search for and select generator.
4. Calculate required gear ratio.

First, using the electrical data you provided (the USB charging spec?), calculate the power you need. Don't know how? Google for "electric power equation". Let us know what you get.

Next, recognize that rotational power is comprised of both rotation rate and torque. Read the wiki on "torque", particularly, the relationship between power, torque and rotation. Do the calculation.

Steps:
1. Select/calculate output requirement. same as usb output (5v 500ma = 2.5watt)
2. Calculate/select input requirement. 500rpm
3. Search for and select generator. this is my problem, i dont know where to look, since whenever i search i end up with big generators of 200v or something, i need a small and affordable generator that can get this output with the least RPM and torque, if you have any links lt me know
4. Calculate required gear ratio. after i fins the generator i will

russ_watters
Mentor
Steps:
2. Calculate/select input requirement. 500rpm
That's too high. Typical bicycle pedaling is in the 80-100 rpm range. You might go a little faster, but not that fast. Do you have something you can crank by hand now to test, such as spinning a bicycle pedal with your hands. Count/time a few seconds of it.

In any case, that's a sample you can plug into the equation I points you to to calculate torque...

CWatters