[Stoves] Variable Speed Stove Fan SIMPLIFIED

[Thomas Reed]

Dear All:

While it is nice to turn a know to adjust the fan or blower speed, the 
simplest and most direct way is a piece of paper (etc.) over the air 
inlet. 

While we're at it, you can measure the output of the blower/fan 
measuring the inflation time of a very light weight newspaper bag of 
know volume.  (I learned that from Fred Hottenroth, originator of the 
Sierra Stove, about 1990.)

TOM REED     The Simplifier



Paul S. Anderson wrote:
>>>> Subject: Re: [Stoves] Variable Speed Stove Fan
>>>> 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?
>>>>>
>>>>>           
>
> Solution:
>
> Charlie told me about "Pulse Width Modulator" (PWM) technology.  I eventually
> found a correct-size small unit from a distributor in Canada.  I bought 2 of
> the kits (unassembled for about US$8 each, I think) and paid for him to
> assemble them ($10 to do both), plus shipping, became almost $40.
>
> But well worth it!!!!!   I turn the knob-stem and the 12-volt blower 
> that I have
> goes from stand-still to full speed.  But it does have a modest humming noise
> all the time that the current is connected.
>
> I learned that the kits are from Thailand, so now that I am in Cambodia I will
> be trying to get assembled units.
>
> Sorry I do not have the purchase details right now, but I am just finishing a
> 3-day holiday at Angkor ruins.  Next week  I get started on stove
> implimentation projects here with GERES (French) and CFSP (Cambodian) 
> agencies.
>
> The explanation by William Carr (below) sounds just right.  Google Pulse width
> modulator (or see Wikipedia?) for diagrams of what William has described.  I
> have not heard it called "type D", so I cannot comment about that name except
> that "pmw" is part of the Internet link that William has provided.
>
> Wish you were here!!!!   :-))
>
> Paul
>
>   
>>>> 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
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
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>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
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>>>>
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>>>>         
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>
>