You mean buffer is unnecessary?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.
I stand corrected. It's funny how the way schematics are drawn can lead you astray.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.
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.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
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.Actually, my target is that achive proportional control. But my circuit it is only work when the temparature higher than 90 degrees.