# Adjusting thermistor output to fit circuit requirements...

Discussion in 'General Electronics Chat' started by MikeA, Jan 25, 2016.

1. ### MikeA Thread Starter Member

Jan 20, 2013
125
17
I'm trying to use an LM2596 chip to step down about 35V DC into the 5V to 12V range to control the speed of a fan that will be cooling an electronic device. And I want to use a thermistor to keep the device at a target temperature of say 50C degrees.

I might use one of the cheapo LM2596 boards that sell on Ebay and come with a 330 ohm resistor in place of R1, and a 10K ohm trimpot for R2.

And according to the formula provided with the LM2596 I would need a thermistor with a resistance range of 1K to 3K ohms to get the desired voltage range. So at 1K ohms the output voltage will be 5V, and 12V at 3K ohms. I obviously do not want to stall the fan either, and not go below 5V no matter how low the temperature is.

I have some 10K ohm thermistors that will read about 1K ohms at 50C degrees.

What would be an elegant solution/circuit to bring the thermistor output into the range LM2596 needs in this scenario?

2. ### crutschow Expert

Mar 14, 2008
13,508
3,385
You could look at the curve of the thermistor resistor versus temperature and then determine what fixed resistor in parallel and or series with it will give the voltage range you want.

3. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,159
772
I take it as the temperature increases you want the fan to go faster, and when the temperature decreases to slow the fan down, if so your thermistor needs to go in place of R1, you need to find out what voltages the fan runs fast and slow, then you can start to work out the values of R2 and the parallel resistor to trim the output voltage.