CD4017 simple and easy to make zero-crossing power regulation circuit

The circuit introduced in this article has a simple structure but can work reliably. It is very effective in regulating the power of electric heating appliances or the speed of DC series-excited motors.

The circuit is shown in the attached figure. An op amp of ICl LM358 is used for zero-crossing detection of AC power, and the in-phase terminal ③ of ICl is grounded. The pulsating DC voltage obtained by full-wave rectification of the AC secondary voltage by D3 and D4 is added to the in-phase terminal ② of ICl.

When the pulsating voltage passes through zero (that is, the AC voltage passes through zero), the LM358 pin ① outputs a zero-crossing pulse. IC2 CD4017 is used to count the zero-crossing pulses and distribute the pulses to generate a trigger signal for the thyristor. Switch K is a power regulating switch, which adjusts the AC load power by changing the counting method of CD4017. For example, when K is in gear 3, CD4017 performs quaternary counting. For every 4 zero-crossing pulses input, only 2 trigger pulses are generated to trigger the thyristor to turn on. Therefore, this gear is a half-power gear. It is worth mentioning that CD4017 has ten output terminals, QO-Q9. By combining these output terminals appropriately, different power gears can be obtained.

The circuit's component parameters are marked in the figure. This circuit is used for power regulation of electric heating appliances. The thyristor should have sufficient power and be equipped with a heat sink. If the thyristor is difficult to trigger, the resistance of R3 should be appropriately reduced.