Voltage regulator for cooling fan

[jbord39]

Hey all,

I am working on a 0-40V variable power supply right now, and while the power supply is working I am wondering if it is in danger of overheating. I am using a LM317 with a 0-5k potentiometer between adjust and ground, and a 165ohm resistor between output and adjust. Input is regulated from the wall to 40V.

My question is, that since the voltage across the LM317 varies from about 1.25-38.75 inversely with the output voltage, is a fan necessary (I am using a heatsink) to prevent overheating? I know it features internal overheat protection but I would rather avoid having to trust it.

My next question is, if a fan is required, would a 12V fan I pulled from an old computer work?
Which of these two circuits would waste the least power?

Circuit 1: Input voltage of 40V, with zeners in series up to 28V. Cooling fan in series takes the other 12V.

Circuit 2: LM7812 +12V regulator. Seems like the best idea, but the input voltage is +40V and so it seems like it might waste a lot of power.

Thanks, and sorry for long post.

Johnedit:
Possibly a combination of both circuits? Two 12V zeners between the +40V and the LM7812 input could lower the voltage difference on the 7812.

 

[DrMik]

Yes
I use a couple of 12V pc fans to cool both a laser and the LM317 attached to a CPU heat sink. I drilled a hole in it and mounted the laser and then attached the LM317 metal tab to it with a self tapping screw. The two 12V fans were mounted either end so the air is blown through the heat sink. Works very well. If you can get hold of a CPU heat-sink then you will have a stable platform to mount the fans on to. Good luck. Opps nearly forgot, remember the circuit depends on the current you will be drawing, first sounds more straight forward.