Best way to use PTC thermistor in fan controller ?

[lordvader88]

I'm making something basically like this PWM circuit for a 12V 4-pin CPU fan. It doesn't have to be super accurate and I might just make it without the thermisitor for now.
http://www.paulvdiyblogs.net/2015/08/controlling-temperature-driven-fan-with.html

I'm wondering , what's the difference between a circuit that sends a precise constant current through the PTC therm. like from TL431 and transistor,,, and a circuit that allows the PTC current to change with it's resistance ?

[cur8xgo]

I'm making something basically like this PWM circuit for a 12V 4-pin CPU fan. It doesn't have to be super accurate and I might just make it without the thermisitor for now.
http://www.paulvdiyblogs.net/2015/08/controlling-temperature-driven-fan-with.html

I'm wondering , what's the difference between a circuit that sends a precise constant current through the PTC therm. like from TL431 and transistor,,, and a circuit that allows the PTC current to change with it's resistance ?

Well so analyze like this:

PTC resistance changes in both cases.

In constant current case, as resistance changes, voltage will change, so voltage is signal.

In constant voltage case, as resistance changes, current changes, so current is signal. If you put another resistor in series with it, you turn that current signal into a voltage signal.

[lordvader88]

Somewhere I've mapped out the PTC therm PWM circuits on quality computer PSUs, but that's OTT for this, and there's not much space left on the PerfCB. So I might use only a 555 circuit. Soon I need an excuse to copy that good computer PSU circuit tho. They use the TL431 in a type 2 or 3, something or other setup icr, such that the Bode plot and transfer functions act better.

[mariush]

You could use the PTC to limit the voltage to the fan... for example you can use a linear regulator to adjust between 7v and 12v depending on temperature.  Just insert the PTC in the adjustment loop
You could use a npn transistor in linear region to limit the current based on the PTC value.
You could also just use the PTC in a voltage divider and read the voltage with the ADC of a microcontroller, then PWM accordingly - a 6-8 PIC10 micro (or an attiny) would do it.

No need for a TL431 voltage reference, when you have an internal reference or you can just use 5v as reference  ... and pic programmers are now 15$ or less.
For example a PIC10F320 will do the job... in DIP package like a 555 : https://www.digikey.com/product-detail/en/microchip-technology/PIC10F320-I-P/PIC10F320-I-P-ND/2764510