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Re: Fan control

>  I went to Radio Shack this afternoon.  The first salesman never heard
>  of a reostat, but the next one took me over to a corner where they had
>  several kinds of potentiometers, apparently another name for the same
>  thing.

They are not the same thing.  A Rheostat is for power control.  They are 
usually of relatively low resistance and are built to handle a higher wattage 
application, such as dimming a lamp or lowering input voltage to a 
transformer.  A typical Rheostat may typically measure 250 or 500 ohms, and 
may occasionally be as large a 1,000 ohms.  They also carry wattage ratings.  
A typical small rheostat might be rated for 10 or 25 watts, but most of the 
smaller ones I've used were rated for 50 or 100 watts and up.  You also 
frequently see them rated in amperage, e.g., 1 amp, 5 amps, etc.  Your 
rheostat must be capable of carrying the full amperage of the fan, which I 
suspect will be very low for your fan.  Still, such fans frequently pull .25 
amps (250 milliamps), or .4 amps (400 milliamps).  Your rheostat should be 
conservatively rated for at least twice the maximum draw, or it will run hot 
and give shorter life.  A 1 amp rheostat would be reasonable for most small 
fans.   Potentiometers are for much lower power applications, such as the 
volume control on your radio.  They can handle only a small fraction of 1 
watt.  They are usually of high resistance, anywhere from 10,000 ohms 
(usually called a "10K pot") up to 1,000,000 ohms (1 Megohm, or a "1 Meg 
pot").  Rheostats and potentiometers are not interchangeable, and the one 
would be unsuitable for doing the work of the other.  
>  He picked one out for me ($1.49) and I brought it home, but I don't
>  know how to wire it up, or even if it is the correct thing.

I would suggest taking it back for a refund.

>  It is called a "Linear Taper Potentiometer, 1 megaohm".

That would be "megohm," or 1 million ohms.  The "taper" of a potentiometer 
refers to whether the resistance goes up and down in a straight line 
(linear), or varies at a mathematical rate (log taper).  Linear taper is 
fine, but 1 megohm is far, far too high for power control applications.

 > Its a little
>  disk about 1/4" thick with a long bar coming out of the center.  The bar
>  can be twisted back and forth and is obviously the control.  The disk
>  has three tabs coming out of one side where one should obviously
>  attach wires.
>  The salesman told me that the two outside tabs are the hot leads and
>  the middle lead is the common ground, which makes some sense I
>  guess, but otherwise his description of how to wire it is too
>  garbled to be useful.
>  Does anyone have suggestions on wiring it into the fan circuit?  If I
>  guess wrong, I think I can keep from killing myself (GFCI and all
>  that), but how likely am I to ruin the fan I am experimenting with?
No, you won't ruin the fan, but you won't get the speed control you want.  A 
Rheostat may look exactly like a potentiometer, only it's usually larger or 
bulkier.  The end tab is the "hot" lead.  The middle tab is the slider, the 
piece that slides back and forth over the resistor wire inside, and which 
varies the voltage.  Ordinarily, you would not ground any of the tabs on a 
rheostat because you could burn the thing up from excessive current.  A 250 
ohm rheostat with 115 vac applied to one end terminal and with the other end 
terminal grounded would immediately become a space heater, producing about 63 
watts of heat.  A small rheostat would only last for a few seconds before 
bursting into flames.   You COULD ground one outside end of a potentiometer 
in an audio application, for example, because there is very little power 
involved and resistance is high.  To ground the center tab would mean that 
when you rotate the control toward the hot end, you would create a short 
circuit.  On a 115 VAC circuit, this usually manifests itself as smoke and 
flames, tripped fuses, smoke detectors ringing, colorful language, etc.  

Speed controls for fans are readily available at most electronic supply 
houses, already constructed and ready for you to plug something into them.  I 
haven't been to Radio Shack for awhile, but what you want is a small box 
usually called a "motor speed control" unit, or a light dimmer box, rated for 
about 1 or 2 amps.  Rheostats are occasionally used in such circuits, but 
most of the better ones today are electronically controlled.  They may not be 
at Radio Shack, but you may find something suitable at a lighting store, or 
an electrical supply store.  I suspect you might even find something like 
that at Sears Hardware, or possibly even Walmart.  You COULD build such a 
box, but you would need to know the power draw in amps or milliamps of the 
motor so you could choose the appropriate rheostat, or better yet would be 
some electronic circuitry to control the current, and then wire it up in a 
grounded box for safety's sake.  But since they already make what you want, I 
would do a little more shopping around.  I bet a trip to Sears Hardware would 
give you what you want in 10 minutes.