Subject: DIY Temerature Control - Problem
By: Robin (IP: 63.84.4.*)
Written on: 21-04-2005 16:59

I built the Temerature contorl circuit from the instructions on this site and it worked ok for the first 30 seconds but after that the Thermistor burned out.

I am not using this to cool a CPU but to try and cool a cabinet that holds my TIVO and HT Receiver. So i'm using a larger 12v fan that's rated at 0.32A (3.8w). and powering it from a 12v 200mA transformer.

Am I just putting to much power through the circuit with the larger fan or was the thermistor faulty ?

By: Tillmann (IP: 84.167.156.*)
Written on: 28-04-2005 12:38


it's very strange that the thermistor burned out. That really shouldn't happen. It is normal for the transistor to get rather hot and in case of overload, it may burn out, but in no case that should happen to the thermistor. Are you sure you got the right pice? 10K (10000 Ohms) - not 10 Ohms or 100 Ohms or something?

Try again using the correct piece. Even with a 3.8W fan, nothing should burn out; the circuit can handle that kind of power. If the transistor gets very hot, you can add a heatsink it it, but I don't think it will be required.

By: tim (IP: 192.250.34.*)
Written on: 06-03-2006 14:36

I had the exact same problem when I attempted to use a radio shack 12v 1000mA power supply. I was only running 2 .08A fans. Thermistor went red hot and burnt up in about 1 min.
By: Tillmann (IP: 146.140.7.*)
Written on: 06-03-2006 15:19


with proper choice of components, it is not technically possible for the thermistor to overheat. At 12V, the maximum theoretical power dissipation of the thermistor is around 0.015W, and this will not cause any noticable heat.

If you accidentally used a 10 Ohm thermistor (as opposed to 10K Ohm), max power dissipation is around 15W, and of course the thermistor will burn down within seconds.

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