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power supply cooling Answered

normally I'm pretty good with electronics, but I need some help with this problem. I'm building a power supply for a cnc machine and I need help designing a circuit to power fans when it's over a certain temperature. Here's some things it needs: 1) 12 volts supply (easy with a transformer and filtering it) 2) power fans when it reaches a certain temperature. 3) still keeps the fans on even though the rest of the system is off. 4) after the system if off and it's down to temperature then the transformer turns off, to save electricity and to not make it hot after a long time. If possible can someone draw up a schematic? Thanks in advance!

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the resistor controls at what temp the unit powers on

ok, I solved this problem yesterday. Here's what I'm going to do: 1) hook up my homemade 12volt power supply to a timer (those things that you use for chirstmas lights, etc). I'm going to set it so it's off from 11PM to 10AM (I'd never use it during that time) so the transformer is tottally disconnect during those times. When it's outside of that time slot it turns on and monitors the temperature. 2) Hook up a potentiometer and a thermistor (possiably a couple of thermistors in parallel) into a voltage divider circuit. 3) Hook up the voltage divieder circuit to a 5 volt relay, so when the 12 volts drops to 5 volts the relay turns on. 4) when the relay turns on it turns on all of the fans. Quick question about this power supply I'm making. In addition to the 12 volt supply there's also a 36 volt, 8 amps power supply to power the stepper motors, do I vneed to regulate this? The real output of the transformer is around 39 volts, but my stepper motors can handle between 12 and 48 volts. So do I need to hook up 8 1A regulators in parallel? Nice new avatar goodhart

i have heard of circuit using a trimmer pot and a MOSFET to control temperature. i believe the link was somewhere on overclockers.com some thermistors offer less resistance as they heat up, eliminating the need for the relay.

yeah, I was thinking about that as I wrote this (the relayless part) but I was too lazy to go back and edit all of the stuff I wrote.