<div dir="ltr">On 21 July 2013 11:14, 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:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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Hello<br>
<br>
Can anyone suggest how to limit the voltage and/or current coming
out of a dynamo, before audio amp? <br>
<br>
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. <br></div></blockquote><div><br></div><div>If it is a DC motor, it will output DC with a bit of noise. This is what you're measuring.</div><div>If this is an AC motor (typically, a dynamo would do that), then you get AC output.</div>
<div>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.</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<br>
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. <br></div></blockquote><div><br></div><div>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)).</div>
<div><br></div><div>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.</div>
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<br>
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. </div></blockquote><div><br></div><div>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.</div>
<div><br></div><div>'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.<br>
</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000">Also, the speaker
driver heaves, which suggests to me either an impedance issue,
grounding issue, or low-frequency content. <br>
<br>
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. <br></div></blockquote><div><br></div><div>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 filter".</div>
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<br>
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. <br></div></blockquote><div><br></div><div>It does sound you operated the motor way out of its specs.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<br>
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. <br></div></blockquote><div><br></div><div>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.</div>
<div>If it is an AC motor, then you get a sine wave.</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000">
<br>
How can I limit current (or is it voltage?), without squaring the
original sinewave?</div></blockquote><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000">As always, seeking a lowest-parts-count solution,
passive if possible. Any help is appreciated!<br>
<br>
<a href="http://islandcastaway.com/alternator-secrets/" target="_blank">http://islandcastaway.com/alternator-secrets/</a><br>
<a href="http://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/?ALLSTEPS" target="_blank">http://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/?ALLSTEPS</a><br>
<a href="http://goo.gl/MG4mF" target="_blank">http://goo.gl/MG4mF</a><br>
<a href="http://forum.allaboutcircuits.com/showpost.php?p=331463&postcount=52" target="_blank">http://forum.allaboutcircuits.com/showpost.php?p=331463&postcount=52</a><br>
<a href="http://ludens.cl/Electron/dynareg/dynareg.htm" target="_blank">http://ludens.cl/Electron/dynareg/dynareg.htm</a><span><font color="#888888"><br>
<br>
<div>-- <br>
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Johny Radio<br>
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Stick It In Your Ear!<br>
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