Fan control circuit for cooling a LM317

[bogdanfirst]

I have to build a power suply using a LM317 for someboy. the problem is that the case of the suply is not that big. this means that i cannot use a big heatsink. from my calculations, the heatsink i will use will be about 85-90% of the required. but there is another problem. the case is small, so the air inside it will heat fast. the transformer will heat as well. so i want to add a fan wich will be mounted on the heatsink, and cool it, by removing the air form the case, and so fresh air comes in trough some holes.
so what I want to do is to design the circuit that will sense the temperature of the heatsink and start the fan when it reaches a certain value and stop it when it reaches a lower temperature.
I would like to use some NTC's. i have a couple of 33K ones, wich I would preffer to use.
now, the question is: at what temperature should the fan start, and at what temperature should it stop? i was thinking at something like 60-70 deg C for start and 40 deg C for stop.
are these values ok?
and ....the second question, what kind of circuit should i use? couldn't find the right one on the net, maybe you have some suggestions?

 

[andrew2022]

u wud need the start temp higher than the stop temp

 

[bogdanfirst]

oh yeah. thanks for the tip!

 

[bryan]

Some thermal control schematics for fans

http://www.cpemma.co.uk/thermal.html

 

[niq_ro]

another schematic is at **broken link removed**

 

[bogdanfirst]

Thanks!

 

[Klaus]

You can make this complicated or simple, depending how much 'engineering' you wish to put into the project.
If I was doing that I'd choose the simplest control available, a heat sensing Bi-metal switch. These are available from 40 degrees C upwards and have a nice hysterisis to prevent constant on/off cycling.
Klaus

 

[Crofty]

As the case is small, I would in this situation forget temp 'Control' as such, just fit a fan in the case rear permanently running aka pc psu style. all the guts would run much cooler. Note.. as long as the air input intake is equivalent to the fan exhaust area. Blowing on the heatsink alone is not the solution, as the removed hot air still has to get out of the case, extracting 'all' the hot air from the case includes heat generated by the transformer and other components etc..

Steve

 

[bogdanfirst]

well, the simplest solution is the constantly on fan. but, i don't like the noise, and as I said before, the suply is not used at full power all the time, so a temperature controlled fan is better. i will give this a thought and see wich is better.
as for the air, i was thinink that the air will be blowed out of the case, trough the heatsink heating it. air comes on some holes inside the case, somewhere near the transformer to cool this as well.