noise: [Noisebridge-discuss] Dynamo Regulator Help Needed

Henner Zeller h.zeller at
Sun Jul 21 22:07:55 UTC 2013

On 21 July 2013 11:14, Johny Radio <johnyradio at> wrote:

>  Hello
> 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.

If it is a DC motor, it will output DC with a bit of noise. This is what
you're measuring.
If this is an AC motor (typically, a dynamo would do that), then you get AC
You should just measure it with a common multimeter; you can set it to DC
and AC and see what setting gives you a substantial reading.

> 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.

The DC output voltage would be roughly the same amount you'd need to apply
to the motor to make it turn that fast. If this is 6V, say, then you output
6V DC. If you're spinning it with 1000 cycles per _second_, then this is
like 60000 RPM, which is way higher than what typical small motors usually
do (they might be between 6000 and 20000). That means that you're
generating a voltage that is way higher (your voltage is roughly
(nominal_voltage * 60000 RPM / nominal_RPM)).

6V DC or AC and more can likely kill the input stage of an amplifier. You
should use a circuit that removes the DC part. Search for "RC high pass
filter". The important part is the capacitor. If you need to minimize the
voltage,  for your input, you should use a voltage divider to be in the
right range (search for "voltage divider"). Before connecting to the
amplifier, use the multimeter to measure that the AC voltage is in the
order of 1V, which is a typical input.

> 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.

It is the voltage or the DC part of the voltage that is killing your amp.
The 'Amps' are the result of the input impedance together with the voltage.
If you voltage is _way_ to high (which I suspect is happening here), then
you can blow the input stage, because you send too much current through
some poor transistor-base. But to fix your problem, you need to make sure
that the input voltage is in the proper range.

'Fat resistors' don't have a meaning if you don't know the input impedance
- the resistor you have 'on the outside' forms a voltage divider with the
input impedance. If they are in about the same order of magnitude, you get
half the input voltage.

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.

That sounds way to complicated. All you need is a multimeter, Ohm's law, a
capacitor and a resistor. Search for "voltage divider" and "RC high pass

> 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.

It does sound you operated the motor way out of its specs.

> 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.

You keep using these words :) a DC motor does _not_ generate a sine wave,
at best it returns a wave that you would get as the result of rectifying a
sine wave; that is what the commutator is for.
If it is an AC motor, then you get a sine wave.

> 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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> Noisebridge-discuss at

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