<div dir="ltr">On 21 July 2013 17:44, Johny Radio <span dir="ltr"><<a href="mailto:johnyradio@gmail.com" target="_blank">johnyradio@gmail.com</a>></span> wrote:<br><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
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Henner Zeller h.zeller at <a href="http://acm.org" target="_blank">acm.org</a> <br>
Sun Jul 21 22:07:55 UTC 2013 wrote<br>
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>>If it is a DC motor, it will output DC with a bit of noise.
<br>
--it clearly outputs an oscillation. Isn't an oscillation always AC,
even if it's offset above zero, and</div></blockquote><div><br></div><div style>Then it is not really alternating between +/- and -/+.. But you could describe it an AC with a DC offset.</div><div style> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000"> never goes negative?<br>
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>>60000 RPM, which is way higher than what typical small
motors usually<br>
--it takes 30+ volts on the power supply of the OTHER DC motor
(which is used to spin the shaft on the "oscillator" motor)<br></div></blockquote><div><br></div><div style>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 losses).</div>
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>>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.<br>
--cool, will try<br>
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>>It is the voltage or the DC part of the voltage that is
killing your amp.<br>
--i did try a 100uf series cap on one of the terminals. Did not
protect the amp. <br></div></blockquote><div><br></div><div style>That only gets rid of a DC component. You still need to limit the over-voltage.</div><div style> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
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>>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<br>
--um, current? or voltage?<br></div></blockquote><div><br></div><div style>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.</div>
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>> 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.<br>
--then shouldn't the series resistors i used give me significant
overdrive protection? They seemed to have no affect. <br></div></blockquote><div><br></div><div style>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).</div>
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>>It does sound you operated the motor way out of its specs.<br>
--in what way?<br></div></blockquote><div><br></div><div style>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.</div>
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>>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.<br>
--on the scope, it sure looks sine-like. But also weird. <br></div></blockquote><div><br></div><div style>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).</div>
<div style>If you can operate the motor with DC, it is a DC motor.</div><div style><br></div><div style>-h</div><div style><br></div></div></div></div>