Composition "220V Cooling Fan" speed control circuit consists of a bridge circuit D1, optocoupler 6N137, SCR thyristors, transistors Q1 and so on. PIC16F873A microcontroller generates thyristor controlled phase pulse motor to achieve change by changing the phase angle conduction angle, enter 90 to change the output voltage of the circuit to control fan rotation. When the conduction angle is 0 the motor stops, the greater the conduction angle, "220V Cooling Fan" the faster the speed. P1.6 is low Q1 is turned on, 6N137 optocoupler turns and then control the fan turn circuit. Since mutations in digital IC power level up and down transitions, prone to power glitches, usually switching power supply large impact on the ratio of linear, because the switching power supply can not respond within the switching cycle current mutation, but only to provide current from the capacitor Changes section. General "220V Cooling Fan" more complex digital circuits, the higher the data rate, the more intense the cumulative current transition, richer high frequency components. With 95 optocoupler helps reduce noise interference and improve signal to noise ratio.