Zero-crossing is also zero-crossing detection, which means that the zero-crossing point of the AC voltage is automatically detected to generate a driving signal, so that the electronic switch starts to turn on at this moment. Modern zero-crossing technology has been combined with photoelectric coupling technology. The following figure is a zero-crossing detection circuit that can be used in a single-chip CNC AC voltage regulator.

As shown in the figure:

Figure 1: Zero-Crossing Detection

After the 220V AC voltage is limited by the resistor R1, it is directly applied to the input terminals of the two anti-parallel photocouplers GD1 and GD2. In the positive and negative half cycles of the AC power supply, GD1 and GD2 are turned on respectively, and U0 outputs a low level. At the moment when the sine wave of the AC power supply crosses zero, both GD1 and GD2 are not turned on, and U0 outputs a high level. The pulse signal can be used as the interrupt request signal of the single-chip microcomputer and the zero-crossing synchronization signal of the thyristor after the anti-gate shaping.


1. In the input and output parts of the optocoupler, separate power supplies must be used respectively. If the two ends share a power supply, then the isolation effect of the optocoupler will be meaningless.

2. When using optocouplers to isolate input and output channels, all signals (including digital signals, control signals, and status signals) must be completely isolated, so that there is no electrical connection between the two sides to be isolated, otherwise this isolation It just doesn’t make sense.

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