Heater and Ventilation Control

This is a guest post by our visitor Etienne F. For any questions please contact the author (email address link is given below). This circuit control a heater and a fan at variable speed and intensity with over temp and under temp alarm. The control work with a thermistor for the temperature reading and the setpoint is variable. The speed and intensity work with a difference between the temperature reading and a setpoint. At all time, only one of the system works (heat or ventilation) or if the temperature is near the setpoint, none of them work.

1st block: temperature reading and setpoint voltage:

The reading of the temperature to generate voltage

Difference between set point and temeprature reading:

For the first PWM voltage value, the input from the LM324 subtract the temp reading and the setpoint. The result is multiply by the pot between the out and the + input of op amp U2b (ratio from the resistance and the pot.) (Value in ohms). The result is send to the – input of Op amp U2c and the second 100K pot is use to set the offset voltage (2Vdc in my circuit). The result is send to the input of the PWM control block. The correct ratio should be around 2 :1 (multiply by 2 the voltage) The 1N4007 Diode keep the voltage over 0V (the result of the subtractions can be negative but the PWM need a voltage between 0 and 12V DC)

The voltage for the LM324 is +12 and -12VDC (pin 4 and 11) The second block subtract the setpoint voltage to the temperature reading. The result is multiply by the pot. (Depending of the value of the potentiometer). The ratio between the 56K resistance and the value of the potentiometer define the multiply ratio. The output equation is Vin X (Pot /R1) = -Vout The next op amp input add a voltage value to the result (2V) and the last amp is used to reverse the result.

3rd block: voltage generator proportional to a high temperature (PWM)

The block consists of an LM324 (quad op amp) which is used as a comparator (compare the temperature reading at set point), a square wave generator whose frequency varies according to the set point and the temperature reading. This square wave into an oscillator circuit which, after being compared to the set point (which is a triangle wave that varies between 4V and 9V has a frequency of 45Hz). The triangular wave enters into a circuit to be compared to the value of reading. When the reading exceeds the value of the triangular wave form is a square wave and the output is 12V.

More reading approaches the set point, the longer the 12V signal is activate and when the reading exceeds the maximum of the triangular wave (9V), the fan runs at maximum speed. The square wave into a MOSFET to be amplify (the ratio of on / off is increased). The duration of the excess voltage determines the duration of 12 V. The 0.1uF capacitor (C3) between the + and – of the load makes the signal clearer.