I found this circuit that below to use control my cooling fan. When the temparature exceed 90 degrees, the mosfet must conduct. I am not sure it will work or not. Can anybody help me analyze this circuit?

 

Yes, we can help you analyze the circuit. It will probably work but you might want to use a pot instead of R23 and R62 so you can adjust the threshold, that is because it is unlikely that you will have the same thermistor as was used in the original design.

What do you want to know about it?

 

You're using a non-standard symbol for the fan switch, but should be an N MOSFET for low side switching and looks like P.

The bottom left opamp is unnecessary.

 

The entire lower left branch circuit can be replaced with 2 R's and a C.

The circuit might chatter and misbehave when the temp hovers near the threshold, I would add hysteresis to make it 'snap' on and off.

 

You guys do realize this is a PWM temp control, right? The lower op amp creates a pseudo-triangular waveform that is compared to the voltage from the thermistor to create a variable duty cycle signal feeding the gate of the MOSFET. A little hysteresis in the upper right op amp might be a good idea

The fan is not just on and off, it is speed controlled by temperature.

 

You guys do realize this is a PWM temp control, right?The lower op amp creates a pseudo-triangular waveform

Yes.
It's a relaxation oscillator generating an exponential sawtooth for PWM.

Below is the LTspice simulation.

 

You're using a non-standard symbol for the fan switch, but should be an N MOSFET for low side switching and looks like P.

The bottom left opamp is unnecessary.

You mean buffer is unnecessary?

 

There are two ways to control a small cooling fan. These are the basic descriptions, not complete in every detail.

1. On/Off : Above 90 degrees, fan runs full speed. Below 90 degrees, fan is completely stopped.

2. Proportional: Above a minimum temperature (for example, 80 degrees), fan starts running at a slow speed. As the temperature increases, the fan speed increases. At some higher temperature (for example, 100 degrees) the fan is running at full speed.

Which way describes what you want to do?

ak

 

You guys do realize this is a PWM temp control, right? The lower op amp creates a pseudo-triangular waveform that is compared to the voltage from the thermistor to create a variable duty cycle signal feeding the gate of the MOSFET. A little hysteresis in the upper right op amp might be a good idea

The fan is not just on and off, it is speed controlled by temperature.

I stand corrected. It's funny how the way schematics are drawn can lead you astray.

 

There are two ways to control a small cooling fan. These are the basic descriptions, not complete in every detail.

1. On/Off : Above 90 degrees, fan runs full speed. Below 90 degrees, fan is completely stopped.

2. Proportional: Above a minimum temperature (for example, 80 degrees), fan starts running at a slow speed. As the temperature increases, the fan speed increases. At some higher temperature (for example, 100 degrees) the fan is running at full speed.

Which way describes what you want to do?

ak

There are two ways to control a small cooling fan. These are the basic descriptions, not complete in every detail.

1. On/Off : Above 90 degrees, fan runs full speed. Below 90 degrees, fan is completely stopped.

2. Proportional: Above a minimum temperature (for example, 80 degrees), fan starts running at a slow speed. As the temperature increases, the fan speed increases. At some higher temperature (for example, 100 degrees) the fan is running at full speed.

Which way describes what you want to do?

ak

Actually, my target is that achive proportional control. But my circuit it is only work when the temparature higher than 90 degrees. I have idea about right comparator opamp will control mosfet just conduct or not. But in this circuit I do not know how to run it in full and slow speed. In addition mosfet is ZXMN6A09DN8 in this circuits.

 

A couple of projects for fan control -

http://www.cypress.com/documentation/application-notes/an89346-psoc-4-intelligent-fan-controller

Attached the low end version example.

http://www.cypress.com/documentatio...soc-3-and-psoc-5lp-intelligent-fan-controller


Regards, Dana.

 

Actually, my target is that achive proportional control. But my circuit it is only work when the temparature higher than 90 degrees.

From my simulation of the sawtooth generator, the output from the comparator to the fan MOSFET is off until the voltage from the thermistor circuit is about 2.5V.
Thus you want to select the value of resistor from the thermistor to ground (presently 22k) to give 2.5V @ 90 degrees.
What is the resistance of the thermistor at 90 degrees?