ratios and stuff

[JWilliams354]

Im sure this has already been talked about.  But  how do you figure the relation between amp, watts, and volts.  Like when i test out a test coil.  I get close to 5 volts@ a certain rpm.  How do I figure the amp wattages etc.

[henjulfox]

Volts X Amps = Watts

[Kevin L]

To measure watts Pass the magnet over the coil and set your meter to amps.  One side of coil to battery Positive, other side of coil to positive side of meter negative side of meter to battery ground.  

Watts = Volts * Amps

Volts = Amps * Ohms

Amps = Volts / Ohms

Ohms = Volts / Amps

[Kevin L]

Opps, you'll need to include a diode if you connect this to a battery or you'll smoke your meter, as this would create a dead short.

[JWilliams354]

thanks you guys are great.

[RP]

By the way:  Remember that watts from a genny are volts X amps at the same time.  In other words you can't multiply the unloaded volts X the dead short amps or you'll get a false high wattage.

[ghurd]

Why not?  The surplus place is selling solar panels by the watt that way.

Just kidding.

Open circuit volts times short circuit amps means just about nothing by itself.

Or I have a 17W (4.2Vopen X 4.1Ashort) generator that really only makes 1W connected to anything (rated at 75w in its previous life as a stepper motor).

The false high watts can get far more extreme than that.

[finnsawyer]

But the ratio of open circuit voltage over short circuit current gives you the internal resistance (or impedance if ac and the voltage and current are in phase) of the generator, which is very useful to know.  Technically, this would be true only for the given value of rpm.  

[OPEC BUSTER]

Using Kevin's example to calculate the watts, couldn't you use a resistor as the load in place of the battery, then plug the value of the resistor into the formula

to get the correct wattage?

Good luck,

Otis

[ghurd]

The resistor would need to be sized to the RPMs of the generator.

You would have to use a resistor sized to build a voltage quite near the battery voltage.

The watts will change at 6V, 12V and 24V.

W = I X    (V)

W = I X (I X R)

[RP]

Has anyone ever made a "dummy battery" load using a zener and a handful of big transistors on a heatsink?  This way you could simulate a battery that holds itself at 12 volts for example.

Commanda, you could probably explain this better than I.

[ghurd]

I just use a 12V 7AH battery.