noise: [Noisebridge-discuss] re Dynamo Regulator Help

Henner Zeller h.zeller at
Mon Jul 22 01:20:00 UTC 2013

On 21 July 2013 17:44, Johny Radio <johnyradio at> wrote:

>  Henner Zeller h.zeller at
> Sun Jul 21 22:07:55 UTC 2013 wrote
> >>If it is a DC motor, it will output DC with a bit of noise.
> --it clearly outputs an oscillation. Isn't an oscillation always AC, even
> if it's offset above zero, and

Then it is not really alternating between +/- and -/+.. But you could
describe it an AC with a DC offset.

> never goes negative?
> >>60000 RPM, which is way higher than what typical small motors usually
> --it takes 30+ volts on the power supply of the OTHER DC motor (which is
> used to spin the shaft on the "oscillator" motor)

that doesn't say much; are the motors identical ? If so, then the motor
probaly generates a voltage in a similar order of magnitude (minus the

> >>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.
> --cool, will try
> >>It is the voltage or the DC part of the voltage that is killing your amp.
> --i did try a 100uf series cap on one of the terminals. Did not protect
> the amp.

That only gets rid of a DC component. You still need to limit the

> >>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
> --um, current? or voltage?

The voltage on that input impedance creates a current. If you limit the
voltage, you limit the current. They are related. But if you focus on
getting the voltage down to ~1V at the input, the current is ok.

> >> 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.
> --then shouldn't the series resistors i used give me significant overdrive
> protection? They seemed to have no affect.

What were the resistor values ? Input impedance of typical amplifiers might
be between 1k and 100k. Typically more on the higher side, so 40k+. The
series resistance needs to be the same to half the voltage that arrives at
the input. Essentially voltage_at_input_of_amp = motor_voltage *
((your-resistor) / (your_resistor + input_impedance)). You aim for a
voltage_at_input_of_amp for not much more than ~1V ( ~0.77 volt RMS is
usually the 'full input' for amps).

> >>It does sound you operated the motor way out of its specs.
> --in what way?

If the motor doesn't work anymore, something blew inside. Maybe one winding
is gone ? Or there are overvoltage protection diodes that fried. Or some
mechanical failure in the commutator. 60000 RPM is pretty high.

> >>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.
> --on the scope, it sure looks sine-like. But also weird.

If it looks like a sine with one side folded over to the top (so more like
camel humps), then it is the commutated output (so: a DC motor).
If you can operate the motor with DC, it is a DC motor.


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