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11 August 2009
Author: Giorgos Lazaridis
Two Speed Temperature Fan PWM Controller

The circuit in operation. The black 'thing' on the left of the thermometer is the NTC

In many cases, we need a fan to cool a device or an area only when it is hot. In this case, a temperature controller is needed. There are some cheap temperature controllers in the market. But the circuit that i present here is more than this. It is also a PWM fan speed controller, suitable for almost any kind of DC fan. The fan is turned on at low speed when the temperature has reach the first level, and then it runs at full speed when it goes above the higher level. Both levels are controlled by independent potentiometer for easier adjustment. The low speed of the fan is also controlled by an independent potentiometer.

In Action!

The circuit

Following i present you the circuit.

Although i tried to keep it as simple as possible, the requirements i had on the first place increased the difficulty a little bit. I did not want just a simple temperature controller. The circuit has two temperature set-points. The first set-point will turn on the fan but not in full speed. The fan will be controlled through a PWM generator performed by the 555. This set point is chosen from the R2 potentiometer. The fan speed can be set by changing the duty cycle of the PWM generator using the R5 potentiometer. As the temperature rises, there is another set-point that will set the fan in full speed. This set point is selected using the potentiometer R6.

Circuit Operation:

From left to right, first we see the NTC. I chose a 4K NTC performing a voltage divider along with the R1. From their common point, two signals are leaving. The first one will drive the T2 through the R2 potentiometer. The transistor T2 will turn on or off the next transistor T1. When T1 is turned on, the 555 will get enough power to operate. The 555 is connected as a PWM generator. The frequency of the 555 is above acoustic frequencies to avoid any kicking sound from the fan. The output of 555 is driven to the T3-T4 pair that perform an OR logic gate. The PWM pulses are driven as-is directly to the gate of the mosfet.

On the other hand, there is another signal leaving from the R1-NTC pair. This signal is driven to the 741 inverting input. The 741 performs a comparator. The non-inverting input comes from the potentiometer R6, that gives a DC level for comparison. The output of the 741 is driven directly to the other port of the T3-T4 transistor pair. When the output of the 741 is driven high, then the gate of the mosfet is also driven high, and the PWM pulses have no effect on it. Therefore, the fan will run at full speed.

Before start adjusting the circuit, we need to make sure two things:

• 1. The 'full speed' temperature set point is above the 'low speed' temperature set point
• 2. The PWM pulses have a duty cycle able to rotate the fan

You can adjust the circuit under real conditions or using another heat source like a thermo-gun for heat shrinks.

I have the regulator of the thermo-gun set to 1, the lowest scale possible.

First, we need to make sure that the 'full speed' temperature set point is above the 'low speed' temperature set point. To do so, we will set the 'full speed' at a very high temperature, the highest possible. Put all potentiometers near the center. Then power on the circuit. Turn the R6 potentiometer completely to one side. If the fan start to rotate, then turn it on the other side. The side that the fan does not rotate is the one that we want.

Then, we need to make sure that the PWM duty cycle is able to rotate the fan. To do so, turn the R2 completely to one side. Then, start rotating the R5 from side to side until the fan start rotating. If the fan does not rotate in either sides, then go back to R2 and turn it completely to the other side. The R5 potentiometer can be used to set the low speed of the fan. Set the speed that you desire and proceed to next step.

Now you need to set the temperature level for the 'low speed' operation. I use my electronic-controlled thermo gun for my heat shrinks and a thermometer. I turn on the gun and i put the thermistor and the thermometer side to side across the gun. The gun is set to it's lowest temperature possible and i keep it about 20cm away from the thermometer. At this point, i get about 40 ï¿½C. When the temperature is steady for about one minute, start turning the R2 until the fan is turned off. Then turn it the other way. you must find the exact point where the fan is turned on. When you do so, you are done with the 'low speed' set-point.

The next and last step is to set the 'full speed' temperature. In continue to the above procedure, put the thermo-gun closer to the NTC-thermometer pair. The temperature will rise. Using my gub, at about 15 cm i get around 46 ï¿½C. When the temperature is stable for about a minute or so, start turning the R6 potentiometer. Do it slowly and find the exact point where you hear the fan rotating at full speed. If you have done this correctly, the circuit is ready to operate. The fan will turn on at about 40 ï¿½C and revolve at 'low speed'. If the temperature goes above 46 ï¿½C, the fan will revolve at full speed. That's it!

Selecting a different NTC

I have not try it but i suppose the circuit will work with another NTC as well. The R1 should have resistance near the resistance of the NTC. Be careful not to select a low resistance NTC because the current that will go through will be large enough to destroy the NTC and the resistor.

Relative pages
• 555 timer basic circuits
• 555 theory of operation
• Dr.Calculus: 555 Astable multivibrator calculator
• How to make a PWM fan controller / LED dimmer using a 555
• How to make a light / dark activated switch - 3 different circuits under the microscope
• Create PWM pulses with variable duty cycle controlled by a DC voltage input
• PWM signal theory
• Control a 3-wire Fan with PWM pulses and use the Pulse Stretching method to get clear rpf feedback
• An intelligent self-tunned fan PWM controller
• Learn about the most popular PC Cooling methods
• PIC Thermometer and thermostat circuit, designed for etching baths
• Re-use and/or extend your molex connectors