measure a voltage across of a dc motor

[mariomoski]

hello,i am trying to measure a square wave(pwm) with amplitude 3V, so when i try to measure it with a multimeter across my motor,should i watch the next:

Vmultimeter=3*sqrt(duty cyle)

is it correct? because sometimes i am watching more than 3V on my multimeter across the motor about 3.7V but i think that it shouldn´t be higher than 3V.
i think that it can be caused about the inductance of the motor but i try to put a zener diode of 3V at the output of the AO but the problem is still happening.

somebody could help me with that? the Vcc=6V and the multimeter is set between the output of the power operational amplifier and ground

thanks

http://imageshack.us/photo/my-images/401/dudaq.jpg/

[Hero999]

Firstly, why are you using a power op-amp to simply drive a motor?

It seems like a bizarre thing to do. A single transistor would be much cheaper and more efficient.

Are you even sure the op-amp can supply enough current to power the motor?

You're right about the formula to calculate the RMS value of a square wave, what you've missed is your cheap meter probably gives the average value, rather than the true RMS value. If you used a higher quality true RMS meter, you'll discover it reads the correct voltage.