Thermolec Application &Technical Notes - Manual (Page 27)

THREE EASY STEPS
STEP 3 - (continued)
Pulse Width Modulation Diagram
In the above example:
10% = 12 cycles ON, 108 cycles OFF 0.2 seconds of heating on 2 seconds
50% = 60 cycles ON, 60 cycles OFF 1 second of heating on 2 seconds
80% = 96 cycles ON, 24 cycles OFF 1.6 second of heating on 2 seconds
100% = 120 cycles ON 2 seconds of heating on 2 seconds,
Principle of the Pulse Width Modulation (PWM)
The input signal is compared with an internally generated reference signal (triangle wave) that has a
time base of 2 seconds (2 x 60 cycles or 120 cycles), and the controller activates the SCR output in a
range of 0 to 100%. On this graphic, every time the horizontal line corresponding to the input (in the
above example, 10%, 50% and 80%), crosses the reference signal, the heating elements are switched
ON or OFF. The total heat output is an average between the ON and OFF periods, and for low heat
demand, the heater is not "dimmed" but switched ON for very shorts periods.