How do I trigger a fans function after a setpoint temperature is reached? I want to avoid using a PID Controller, and my heating element will be external of the circuit. Once the fan cools the thermister back to regulation, it should turn off. I have a fan that operates at 24VDC(full speed). Thanks a lot

  • \$\begingroup\$ Thanks - that's better, but more is needed. What are you trying to achieve. What is being heated. How big in volume, just air being heated or ??? More ...??? \$\endgroup\$
    – Russell McMahon
    Nov 18, 2011 at 4:36

2 Answers 2


This circuit looks like it would do exactly what you want. BUT it may not, as we don't yet know exactly what you want. This shows a 12V fan but 24V would work equally well. This use a relay to turn the fan on and off but you could connect it in the transistor collector if the transistor was suitable. As you have not told us the fan power or current rating we can't be sure about the transistor. That circuit is from here but they stole it from somewhere else to get people to look at heir ads so ignore them.

enter image description here

Here is a direct fan drive circuit that is otherwise similar.
IF you use the 24V for op amp supply and FET you'd need a zener on the FET gate to limit gate drive. A say 12V zener so Vgate - ground ~=12 V would be OK. Change R2 to say 10k. P1, R1, P2 could all be larger with increased voltage. They are non critical as long as you understand how they work and can adjust as required. Circuit and OK writeup here. Note that P2 operation is important. It provides "hysteresis" which controls how much diffetrence there is between fan stop and fan start temperatures.

enter image description here



But THIS may be what you REALLY want.
You said you wanted to avoid using a PID controller.
You did not say why - and you MAY have meant that you did not want to pay the usual price for one - ie a cheap PIC controller or similar may be OK.

This simple but clever circuit is a P controller :-) (P for Proportional).
You can easily make it a sort-of PI controller

It's clever in several ways - read the referenced text to find out why.

The MOSFET is directly controlled by the NTC thermistor. As the thermistor cools is resistance goes up, the FET gets less drive, the fan slows and the cooling rate slows. Slow the fan too much and the fan can't keep the temperature down, the temperature rises, the thermistor heats, its resistance drops, the MOSFET gets driven harder the fan speeds up the emvironment cools, we are all happy. It will probably "hunt up and down". It will be fun.

The circuit is from here which gives a good writeup. You will probably have to play with R1 - make it a say 10K pot. Put a 12v zener gate to ground on the FET as above or it will die (if using 24V.)

Note that FET will heat when used like this in its linear mode. Max FET power is a bit complex due to non linear motor power_in / voltage / load relationship BUT PFET max is probably about PFAN max when fan is run on supply by itself. eg if this is a 2A 24VDC fan = 2 x 24 = 48 Watt (quite some fan!) then PFET =~~~ 48/4 = 12 Watt. YMMV. Use a heatsink. Take due care. Put FET on heatsink on exhaust side of cooled area if possible. Doesn't hurt your cooling and uses the air flow.

I said you can make it a PI controller of sorts.
Thus: Mount the thermistor on a block of thermally massive material.
To het the thermistor the system has to heat the block.
Once heated it takes a while to cool.
The longer it is at stable temperature the more it settles down.
This may be a strop of aluminum or Al plate or ... .
You can put it in the air flow to change it's cooling "I" value. Or not. Very rough. more fun. A cap from gate to ground also adds "I" but it needs to be large as the gate resistor is small.

enter image description here

You can make this a "bang bang" controller than switches on an off with the on / off ratio being controlled by thermistor resistance. Then the FET does not get hot and needs no resistance. Usually you'd you end up back with an opamp or comparator but it can be done with just discrete parts. Ask ... .


Ask questions ...

  • \$\begingroup\$ Thanks guys- the reason why I was avoiding the PID design is simply because I have no background knowledge of it. Your proportional circuit seems pretty straight forward though Russel \$\endgroup\$
    – maraldz
    Nov 18, 2011 at 15:37
  • \$\begingroup\$ Your other circuits are helpful too. I'm not sure which one would be best suited for what I want though. Just for the record the fan is driven with 100mA, 24VDC @ full speed. I want it to cool the heated thermistor to it's original temperature. It's just a school project so I am definately open for changes \$\endgroup\$
    – maraldz
    Nov 18, 2011 at 15:46
  • \$\begingroup\$ Hey Russel, I've been checking out your first comparator circuit that you posted a few days ago, and I was just confused about a few things. -Which type of relay should I use (it has to be through-hole) -VR1~is this a potentiometer? If so, I thought that the fan speed was determined upon R1 rather than VR1 \$\endgroup\$
    – maraldz
    Nov 19, 2011 at 22:54
  • 1
    \$\begingroup\$ Relay must: 1. work with the supply voltage used 2. be able to be driven with transistor used. 3. be able to operate fan load OK || You should try VERY hard to understand how the circuit works.| VR1 is a pot. that set the voltage on comparator pin2 = in_+. R1 and the thermistor form a divider which sets the voltage on comparator pin3 = in_-. The fan is ON when V from VR1 is greater than V from R1 and thermistor. SO any of VR1, thermistor, R1 are part of making fan start or stop. BUT Fan speed is FULL ON or OFF. An on/off relay will not control fan SPEED. Only circuit 3 controls fan speed. \$\endgroup\$
    – Russell McMahon
    Nov 20, 2011 at 0:11
  • \$\begingroup\$ Thank you @Russell McMahon , understood! You mentioned to use a heat sink for circuit 3. I'm not too sure how to incorporate that, I've never dealt with one. Thank you! \$\endgroup\$
    – maraldz
    Nov 20, 2011 at 20:01

There would be various ways of doing this, depending how accurate you want it to be.

You could use a simple comparator that turns on/off according to a set temperature and add a bit of hysteresis to prevent it cycling too quickly.

I know you say you don't want to use a PID controller, but an option would be to use a simple 8-bit microcontroller with a PID algorithm controlling a MOSFET to control the speed of the fan using PWM. With correct tuning, this would settle at a certain speed and keep the temperature stable and very close to the setpoint.
The implementation of PID for something like this is pretty simple.

As Russell has noted, some more info on what exactly you are trying to do would help to find the best answer. Things like what you are trying to keep cool/environment, maximum temperature swing +/- set point permittable, price range for components, time allocated, etc.

  • \$\begingroup\$ Thanks Oli, I'm not too worried about pricing. I just wanted to figure out the best way to approach this concept. It is a school project I am doing so I am open for advice! :-) \$\endgroup\$
    – maraldz
    Nov 18, 2011 at 16:44
  • \$\begingroup\$ @maraldz - The "best" way is the one that meets your specifications fully whilst not being unnecessarily over-complicated. As mentioned above the comparator (see top schematic posted by Russell) is probably the simplest way. If you let us know how accurately you want it to maintain the setpoint temp (i.e. do you mind if it's swinging around the set point +/- 5 degrees or?) then the best way out of the few suggested can be picked. If you do want to try PI or PID in a micro I can advise and send you some code. Or as Russell mentions you can do PI with an analog circuit quite easily. \$\endgroup\$
    – Oli Glaser
    Nov 18, 2011 at 17:24
  • \$\begingroup\$ My specifications are open for changes. Russel's proportional controller makes the most sense, however the circuit might not be complex enough for my teacher--on the other hand, it also looks good that I can simplify a circuit instead of making it more complicated. I am liking the 1st comparater circuit too, the thing is that I have to order/buy my parts so I have to commit to one. Another question I have; What should I use for an external heat source? Thank you for your time guys :) \$\endgroup\$
    – maraldz
    Nov 19, 2011 at 20:50
  • 1
    \$\begingroup\$ @maraldz - I don't know what your teacher is expecting, but if you are wanting to keep it quite simple but with reasonable performance then I think the 3rd circuit in Russells answer is the one to go for, or something like it. This can be made to perform better than the on/off comparator version, but it will require a little more though about transistor power dissipation. If you can select your heat source then why not use something small (e.g. a small bulb, or maybe even a couple of resistors) and then you can downsize your fan too, making power control easier. \$\endgroup\$
    – Oli Glaser
    Nov 20, 2011 at 8:02

Not the answer you're looking for? Browse other questions tagged or ask your own question.