I have been wondering about using the good old LM317/338 as a basis for a fan controller - CPEmma covers it well on his site - but the lack of 12v is a majr drawback - SO I wondered about a POT with a switch and Farnell do the perfect thing - a rotary pot with a SPDT switch on And the switch is push toggle - so run ya fans at the set chosen voltage and to give em a quick blast of 12v just press the knob.. Only thing is they are only available as 10k But my maths says a 7.5K ohm resistor in series should adapt that to a 1k pot.. Can any Electronics guy confirm that this will work ok? Have ordered some pot/switches and will build one in the coming free time if I can manage it
The 317T doesn't like high-value resistors in the control chain, IIRC the datasheet suggests 1k max for the resistor between output and adjust pins. However, my maths says you want about 1k or 1k2 with a 10k pot: Max Vout = 1.25(1 + 10,000/1000) = 13.75v or 1.25(1 + 10,000/1200) = 11.7v so should be OK. I assume the switch will bypass the 317, so only question is if it will handle the fan current.
4a switch so should be Ok for most fans And ya probably right about the resistor values - was mainly interested in doing it your way frank ie 1k + a 1K pot - but all the 1K post with switches are DPST and I like the idea of the push-on&off idea..
