# DC motor speed controlled by temperature

#### rahulrox1991

Joined Oct 31, 2011
16
I am required to make a circuit that controls the speed of a 12v DC motor with change in temperature. I'd be really grateful if anyone could help me with a simple circuit to achieve this objective.

The other day I came across this circuit on the net. If some one could also explain the working of this one. (Check the link that follows)
http://www.circuitstoday.com/tempera...trolled-dc-fan

Also, I found this which implements the design for an AC motor. Could I modify this and use it for a DC supply. I can only use a 12V motor though. Also could anyone please guide me as to why the optocoupler and triac is ised here. If a triac is required for a 12v DC motor as well, what should be the rating? I mean which model number? (Check the link that follows)
http://pecworld.zxq.net/Assets/SOURC...LLED FAN.pdf

Any new design ideas would also be really appreciable. Thanks in advance
(Please explain the working of the circuit whatever you suggest since I'll be required to answer all sorts of cross questions in the viva as well )

#### gerty

Joined Aug 30, 2007
1,291

#### ajinkya24

Joined Mar 18, 2012
4
I am required to make a circuit that controls the speed of a 12v DC motor with change in temperature. I'd be really grateful if anyone could help me with a simple circuit to achieve this objective.

The other day I came across this circuit on the net. If some one could also explain the working of this one. (Check the link that follows)
http://www.circuitstoday.com/tempera...trolled-dc-fan

Also, I found this which implements the design for an AC motor. Could I modify this and use it for a DC supply. I can only use a 12V motor though. Also could anyone please guide me as to why the optocoupler and triac is ised here. If a triac is required for a 12v DC motor as well, what should be the rating? I mean which model number? (Check the link that follows)
http://pecworld.zxq.net/Assets/SOURC...LLED FAN.pdf

Any new design ideas would also be really appreciable. Thanks in advance
(Please explain the working of the circuit whatever you suggest since I'll be required to answer all sorts of cross questions in the viva as well )
Dear Rahulrox 1991,

It is possible to design this type of circuit/project. I tried opening your links, but unfortunately they don't. However i can suggest you something which i think you may find useful.
For controlling the speed of DC motor with temperature as a feedback, you will need following components.
1)A temperature sensor- Will monitor the temperature continuously and feedback to the drive
2)Drive-A DC drive which will control the speed of motor on the basis of feedbac it will get from temperature sensor
3)A DC motor

But honestly speaking, this project would get too tedious & there would be lot more technical difficulties involved. Instead if u want any new idea, let me know. However if u are interested in the same project i am ready to help..

#### rahulrox1991

Joined Oct 31, 2011
16
I sincerely apologise for the inconvenience caused to everyone across the forums. my intention wasn't to spam. I ain't a regular visitor here and just hastily copied my thread to all the forums. I will make sure this doesn't happen in future

http://pecworld.zxq.net/Assets/SOURCE/PDF_fun_circuit/All_pdf_files_manual/166_AUTOMATIC TEMPERATURE CONTROLLED FAN.pdf

I intend to modify this by removing the transformer, rectifier, optocoupler and all since my requirement is only to make this circuit work on a simple 12V DC supply. I couldn't understand the exact purpose of the triac here. Also, if i need to use the triac, which one should it be? (I have also attached the relevant file(s) in case the link doesn't work this time)

In this second circuit, I tried running the simulation on MULTISIM, but didn't really work since my version didn't have preset models for motor and thermistor. But since this circuit appears on a lot of websites, I hope it does work. I have been trying to make this run but have encountered some issues. the speed doesn't change even if i increase or decrease the resistance of the thermistor by adding resistances in series or parallel. I'd be in a better position if someone could help me with the working of the circuit.

This is the circuit I was talking about
http://www.redcircuits.com/Page109.htm

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#### wayneh

Joined Sep 9, 2010
16,399
The AC circuit is overkill, and I recommend going simple from the start, not trying to adapt that circuit to DC. The simpler DC circuit you posted seems reasonable. Is there a reason you're not pursuing it? Basically, it varies current to the motor based on the temperature detected by the thermistor. The pots allow adjusting the response.

A better but more sophisticated approach would be to vary the duty cycle of a PWM signal to the fan, depending on temperature. The problem with the simpler circuit above is that the fan may not start at low speed because the torque is too low. PWM solves this and allows smoother speed control especially at the low end. I think there are several references on this site that can explain simple 555-based PWM motor controls. Of course there are tons of temp-controlled-PC-fan projects out there.

One thing you'll need to think about is what you really want, in terms of rpm versus temperature.

#### mcasale

Joined Jul 18, 2011
210
I agree. The AC diagram is WAY overkill.

You did not say how precise your temperature control needs to be.

If you can tolerate a few degrees ripple, a bang-bang control scheme might be sufficient. You will need a comparator circuit hooked to your temperature sensor. When the difference gets above a certain threshold, the fan is turned on FULL. When the difference is below a certain amount, the fan is turned OFF.

A simple MOSFET to drive the fan should be adequate.

#### mateenyasin1

Joined Mar 20, 2012
6
Well the post you uploaded was quite complex.
1. LM 35 Temperature Sensor
3. 74LS85 (4 bit) Comparator
4. A relay
5. Fan

The temperature sensor connected to the ADC converter which sends data to the 4 bit comparator, this compares the values with a user provided binary input.
The output from the comparator can be connected to a relay which powers a fan!

#### wayneh

Joined Sep 9, 2010
16,399
Or, in the analog world, an LM35 and a LM339 comparator (I recommend the quad because it's available everywhere, of course you could use the dual) driving the MOSFET and fan. I agree that on-off control may be just fine, instead of a proportional control. If the fan is too loud, you could try a current limiting resistor. That'd be fine as long as it can still keep up.

Using the comparator , you need to establish a voltage reference on one pin to compare to the output from the LM35 sensor. You use an adjustable reference to set the on-off point, just like a household thermostat.

#### rahulrox1991

Joined Oct 31, 2011
16
The AC circuit is overkill, and I recommend going simple from the start, not trying to adapt that circuit to DC. The simpler DC circuit you posted seems reasonable. Is there a reason you're not pursuing it? Basically, it varies current to the motor based on the temperature detected by the thermistor. The pots allow adjusting the response.

A better but more sophisticated approach would be to vary the duty cycle of a PWM signal to the fan, depending on temperature. The problem with the simpler circuit above is that the fan may not start at low speed because the torque is too low. PWM solves this and allows smoother speed control especially at the low end. I think there are several references on this site that can explain simple 555-based PWM motor controls. Of course there are tons of temp-controlled-PC-fan projects out there.

One thing you'll need to think about is what you really want, in terms of rpm versus temperature.
@wayneh: the reason i am not going for the simpler DC circuit is because I am facing trouble finding these transistors in particular. I found these model numbers but with plastic heat sinks which aren;t very sturdy and didn't really work. Would any conventional ones work in their place? (Assuming my motor is a 12v 700mA one)
Also, I had an alternate strategy where i could map the desired range of temperatures to 0-12V using a wheatstone bridge and an instrumentation amplifier. Since I can easily control a motor speed by varying the voltage across its terminals, this strategy should ideally work. But due to loading effect and insufficient current supplied by an instrumentation amplifier, its nearly impossible to drive a motor. I'd need at least 100 times current amplification without altering the output voltage. Any suggestions?

#### rahulrox1991

Joined Oct 31, 2011
16
I agree. The AC diagram is WAY overkill.

You did not say how precise your temperature control needs to be.

If you can tolerate a few degrees ripple, a bang-bang control scheme might be sufficient. You will need a comparator circuit hooked to your temperature sensor. When the difference gets above a certain threshold, the fan is turned on FULL. When the difference is below a certain amount, the fan is turned OFF.

A simple MOSFET to drive the fan should be adequate.
I wish to control the speed of the fan as against the thermostat action as u just assumed.

#### wayneh

Joined Sep 9, 2010
16,399
I wish to control the speed of the fan as against the thermostat action as u just assumed.
Well, so far I don't think anyone has assumed you want to control the fan SPEED or the just the ON/OFF state of the fan. Both are possible, but the latter is simpler. In either case you'll need a power MOSFET rated for ~4X the current draw of the motor. The "4" is just a rule of thumb minimum, and it's fine to get an even larger one. It won't add much to the cost.

To turn the fan on and off, you'll just change the voltage at the gate of the MOSFET from 0 (off) to 12v (on).

You can do this slowly, as with a thermostat, or quickly, ie. using PWM. Generating a changing PWM duty cycle based on temperature isn't hard, but it's more than a thermostat. You need to choose the strategy you want to pursue, then research that strategy using the resources on this site.