[Stoves] Variable Speed Stove Fan

[William Carr]

On Nov 29, 2006, at 12:46 PM, Tom Miles wrote:

> Kyle's comment reminded me of a conversation between Charlie  
> Sellers and
> Paul Anderson at Stoves Camp this summer. Charlie was describing an
> electronic  "chopper circuit" to adjust the speed of a DC fan. Has  
> anyone
> built one? Can anyone provide the circuit, list of components (and  
> sources)
> or a step by step how-to (with pictures) for building one? Or is  
> there an
> internet source?

This sounds like a "type D" amplifier.

It came up in High School, when I was interested in controlling the  
speed of small DC motors.


(Turns out it's also the only way to 'dim' fluorescent lights.

You see, a fluorescent tube won't fire without full voltage.   Lower  
the voltage, no light at all.)



You can control the speed of a DC motor fairly well by dialing down  
the voltage, that trick works somewhat with an AC motor, but it won't  
work at all with a fluorescent tube.



So instead of turning down the voltage, you want to dial down the  
wattage instead without affecting the voltage.



The only way to do that is with a type D amplifier, by converting the  
input signal into a squarewave (AC) or a series of pulses (DC).


If you look at the sinewave of standard 60 cycle AC,  you see a trace  
going up, and down, up, and down.

The circuit is getting peak power at the tops of the wave, of course,  
but it's still getting SOME power on the way up and on the way down.

At any given point you can calculate the amount of power by a process  
called "Root-Mean-Square", a term that still triggers my math  
aversion 24 years later....

By generating a squarewave output, you can provide the full voltage  
at the correct frequency.


But if you cleverly reduce the width of the squarewave, you will be  
still supplying the correct voltage, at the correct frequency, but  
you'll starve the circuit on wattage.

Thus, the motor slows down in a nice linear fashion as you decrease  
the squarewave width, and a fluorescent tube dims gradually.

The method works with DC also, just by making the squarewave a series  
of DC pulses.


Here's a link to controlling your DC motor:

http://www.solorb.com/elect/pwm/index.html

William Carr