Zero-crossing power regulation (can be used for fan speed regulation, electric blanket temperature regulation, etc.)

The zero-crossing power regulation circuit introduced in this article is simple but can work reliably. It is suitable for power regulation of various electric heating appliances, power regulation of series-excited motors, etc. It can be used as a reference for electrical workers.

The circuit working principle of the device is shown in Figure 1. It consists of a power supply circuit, an AC zero-crossing detection circuit, a decimal counter/pulse distributor, and a bidirectional thyristor. After the 220V mains is stepped down by the power transformer T, it is rectified by the full-wave rectifier circuit composed of diodes VD1 and VD2, and then filtered by C to supply the whole circuit. After full-wave rectification by diodes VD3 and VD4, the pulsating DC voltage obtained is added to the inverting input terminal of the operational amplifier IC1 through R1. When the pulsating voltage passes through zero (that is, the AC voltage passes through zero), a zero-crossing pulse appears in IC1.

IC2 is used to count and distribute zero-crossing pulses, thereby generating a thyristor trigger signal. S is a power regulating switch, and the power of the AC load is adjusted by changing the counting method of IC2 through S. For example, when S is in the "3" position, IC2 performs quaternary counting, and only 2 trigger pulses are generated for every 4 zero-crossing pulses input to trigger the bidirectional thyristor to conduct, so this position is a half-power position. The figure shows 4 positions. Since IC2 has 10 output terminals, different power positions can be obtained by appropriately combining these output terminals. VT is connected as a high-current switch, which can trigger the thyristor VS with different flow rates with high current, so that it can work reliably.

IC1 uses a general-purpose operational amplifier integrated circuit (such as LM324N, T1082, etc.). IC2 uses CD4O17. VT can use 3DK4 or other medium-power switching transistors, β ≥ 100. VS should be selected according to the current of the load, and its withstand voltage should not be less than 600V. The withstand voltage value of the inductive load can be increased. T uses a 2-3W power transformer with a secondary voltage of 9V. S is a power adjustment selection switch, and a ceramic band switch can be selected. There are no special requirements for other components, and they can be selected according to the values ​​shown in the figure.