# How to measure noisy AC

Discussion in 'Electronic Basics' started by Gary Atkins, May 10, 2004.

1. ### Gary AtkinsGuest

My son has a high school project where he has built a simple AC generator
where the magnets spin, and the coils are static. It will be driven by
running water from a tank.
Problem is, he cannot measure the current with his multimeter as the values
jump all over the place!
Is there a simple trick to smooth out the power so it can be measured?
eg a small resistor in the circuit ? Rectify to DC with a diode?
Any links to generator projects on the web?
Appreciate any help,
Gary

2. ### Bill VajkGuest

1) Gear drive the alternator so the frequency is driven above/into
the dampening range of the meter

2) Put a full wave bridge rectifier on the output and measure
dc current. However is the frequency (rpm) is very low
you'll still see severe fluctuatuions. If that's the case, you
could add enough capacitance to smooth out the meter jumping.

3) Add more magnetic poles to increase the frequency resulting
in the same solution as #1, above.

There may be others, but these spring to mind.

3. ### Rich GriseGuest

Is it a digital multimeter? Go ahead and spring \$20.00 or so for an analog
meter - it'll be much more instructive, as you can see the needle move
in response to the movement of the armature. And you do understand that
this generator produces AC?

Have Fun!
Rich

4. ### Watson A.Name \Watt Sun - the Dark Remover\Guest

One could pull the old bolometer trick. Use the power to heat some
indicator, such as a small 6V lamp. Compare the power by switching the
lamp to a power supply to give the same brightness, then use the voltage
and current of the power supply to give a good estimation of power.

Ya know, it always bumfuzzles me why Edison insisted on having those
cumbersome commutators and brushes on his generators.