Ok .. so I got my parts from Maplin (didn't cost too much to ship to the States) and now I'm looking to buy 4 potentiometers to build my rheobus. The one I'm making was from the article titled "A Rheobus You Can't Overload" or something like that. I don't remember the article # or who wrote it, but does anyone know the specs on the potentiometer I'll need? Thanks, Vert
Hehe, I can't look up the parts list right now But if you tell me what the resistor values you got are, I'll tell you that about 10 times that is suitable for the pot value. (For example if you got 1k resistors, you want 10k pots.) Power rating is not critical for this design.
180 & 240 were the resistors used in the article. (Maplin parts M180R & M240R). I believe the pot used was a 2K or 2K5 .. does that make sense? Thanks, Vert
Sounds right on the money. The pot just needs to be about 10x the resistor, so if you can't get the 2k or 2.5k (US notation) pot, get a 10k (should be a slam dunk) and use a 1k resistor. Vout = 1.25V * (1 + R2/R1) so about ten to one is the magic ratio.
Thanks for the inpho .. those 2k5 pots weren't all that expensive so I think I'll order a few =) Thanks again, Vert
With 317 fan regulators I can see little point in being able to control down to 1.25v, it just wastes pot travel & sensitivity, plus the danger that a fan will stop and you'll not realise until something nasty happens. Optimum values IMHO are R1=240R, R2 = 1k fixed + 1k pot, which gives a range of 6.5v up to the maximum (usually about 10.3v from 12v input). If you want a touch lower, R1=270R, R2 = 1k fixed + 1k pot gives a theoretical 5.9-10.6v range, or R1=300R, R2=1k2 fixed + 1k pot gives a 6.3-10.5v range. I've included the small but positive error term in my calculations, making: Vout = 1.25 * (1 + R2/R1) + 50 * 10^-6 * R2
