noise: Dynamo Regulator Help Needed

Johny Radio johnyradio at
Sun Jul 21 18:14:45 UTC 2013


Can anyone suggest how to limit the voltage and/or current coming out of 
a dynamo, before audio amp?

Actually, I'm not sure if this is a dynamo or alternator-- i'm cranking 
the shaft of a DC motor, and tapping the electric terminals. I'm trying 
to amplify the AC sine coming off the terminals through a loudspeaker.

It basically works-- i can hear the motor oscillation from the speaker 
driver, which is my goal. But i've blown two audio amplifiers doing 
this. The motor is not large (about 2" long, 1" diameter), but i'm 
spinning it at thousands of cycles per second. According to one article, 
as frequency increases, so does output current.

I'm not sure if the issue is too much current or too much voltage. i 
believe i'm not getting clipping, so i suspect the problem is too much 
current. A couple fat resistors in series with one of the terminals did 
not prevent the amp from blowing. Also, the speaker driver heaves, which 
suggests to me either an impedance issue, grounding issue, or 
low-frequency content.

I've found a couple ways of possibly handling too much current, using 
circuits mostly designed for charging batteries off a generator. I found 
solutions based on BJT transistors, CMOS, power regulator chips, or 
diodes. Links below.

I'm not charging batteries, so i wonder if i can eliminate the reverse 
current protection? Actually, the motor no longer spins when i apply 
voltage to it's terminals, so i think i blew the motor in some way. 
Strangely, it still outputs a sinewave when i turn it's crank. So maybe 
the reverse current protection IS needed.

One solution employs a pair of diodes, wired back to back, between the 
motor terminals (i think). This is intended to clip the voltage, 
resulting in a square wave. This solution is not desirable, because i 
want to preserve the sine shape coming off the DC motor.

How can I limit current (or is it voltage?), without squaring the 
original sinewave? As always, seeking a lowest-parts-count solution, 
passive if possible. Any help is appreciated! 


Johny Radio

Stick It In Your Ear!

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