AC 555 no trans Sync Motor Speed Controller

Mains powered ac sync motor speed is controlled using 555 IC without a transformer Synchronous AC motor such as TA-50 works with the mains at 50hz. It operates poorly with other frequencies. This circuit modulates the mains supply with a square wave and a duty cycle controlled with a potentiometer First two capacitors and resistors on the left are used to divide and smoothen the voltage used to power 555 and also lower the current through zener diode. It is hard to get high W zeners. Pulsating voltage makes 555 behave badly. On the other hand 555 has no trouble handling wide voltage range - feel free to replace 5v zener with higher btv one. 555 works in an astable mode with a fixed frequency (resistance of the potentiometer) and variable duty cycle set by the potentiometer wiper. 50% setting on the wiper gives 50% duty cycle. Output has limited source current - but can deliver voltage. High input impedance transistor must be used to trigger the SCR (yes, the second transistor should be replaced by an SCR) Circuit is optimised for cheap jellybeans components - low V caps, std low W resistors, low W zener, small FET etc...

published 10 years ago