A question for @gunrunnerjohn and the other electronics gurus:
I'm rigging this Pullman panel:
to run a 6 volt fan for a friend. My question is: At high, the fan would get 6v, medium 4v and low 2v. What value resistors should I use to get the desired results?
Thanks!
Mitch
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Need more info on the fan. Is it AC or DC? It would help if you power it with 6 volts and measure the current. That will indicate what resistors are needed. Actually better of you used series diodes to lower the voltage but the diode configuration will depend on whether is AC or DC.
Pete
6VDC fan, runs off of either USB or 4 AA batteries.
Mitch
In that case current draw is likely small, still to use resistors you need to know current draw when running. Diodes will drop the voltage by approximately 1.5 volts. So instead of 4v one diode would give 4.5v and two diodes in series 3v. 3 diodes of course would give 1.5 volts. I guess it depends how close you want to be or just see the effect of slowing the the fan.
Pete
Okay. Assume current draw is equal to output of 4 AA batteries in series. I have zero idea of what that might be, however.
(also waiting for Amazon to ship the fan.) 
Mitch
Mitch, not trying to be difficult. Just saying how it works. You have to know the current draw of the load, not the source. I bet if you shorted out one AAA battery it could supply over 1 amp (same for 4 in series BTW). If you have a meter with a 10 amp current range you could do this yourself. Simply put the probes across the battery. Your fan likely draws less than 200 ma.
Pete
Fair enow. Let's assume a draw of 200 ma, then, and we don't need precise end voltage, just low, medium and full. So, what's a ballpark number, resistorwise?
Mitch
Ok, assuming 200ma and 6 volts DC, the fan has an equivalent resistance of 30 ohms. Add a 10 ohm resistor in the 4 volt terminal and a 20 ohm resistor in the 2 volt terminal.
Pete
Thankee kindly! Much obliged.
I'd use diodes for 200ma, that way you don't actually have to know what the precise current draw is, and you'll still get consistent operation. Also, it's not carved in stone that the voltage increments of 2,4, and 6, are going to be the ideal speed settings, so you may want to do a small experiment with a variable supply to see where you'd actually like the set points to be.
Fair enow. What value diodes would you recommend? And, yeah, I do intend to experiment with the components once I get them in to get the desired settings. What is needed is a jump off point, as it were.
Thanks!
Mitch
The common 1N4003 would be fine in this application. You'll get about a .6-.7 volt drop for each series diode.
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