A Few Components to Drive a Zero-Crossing Detector

The circuit in this design generates a zero-crossing pulse of the AC mains and provides electrical isolation. The falling edge of the output pulse occurs about 200μs before the zero-crossing point. This circuit can be used to safely stop the triggering of a thyristor gate, giving it time to shut down properly. The circuit only generates short pulses when the mains voltage is about 0V, so it consumes only 200mW at 230V, 50Hz input.

The circuit charges capacitor C1 until it reaches the upper limit of 22V Zener diode D3 (Figure 1 and Reference 1). Resistors R1 and R5 limit the input current. When the input rectified voltage drops below the voltage of C1, Q1 begins to conduct, generating a pulse several hundred microseconds long. The coupling of IC1 causes Q1's square wave generator to respond. Only R1 and R5 are required for rms operating voltage. SMD 1206-type resistors can generally withstand 200V rms. This design divides the input voltage in half between R1 and R5, giving a total voltage rating of 400V rms. D3 limits the bridge voltage to 22V, so all subsequent components have lower voltage ratings. The 22V Zener can clamp to 30V, so this design uses a 50V, 470nF ceramic capacitor. Ceramic capacitors have better reliability than electrolytic or tantalum capacitors, especially at high temperatures. If you prefer to use cheaper and smaller 25V components, you can change the Zener voltage to 18V and still have a good safety margin. R4 is used to limit the peak current in the LED. The main limitation on the LED current is the slope of the rectified AC input. A slow slope prevents Q1 from producing a current spike when C1 releases the stored energy.

Figure 1. This zero-crossing detector uses low-voltage components and consumes very little power.

The operation of this circuit can be simulated in LTspice Version IV (Figure 2 and Reference 2). At 230V, 50Hz, the simulation shows a 17mA peak across the optocoupler LED. The simulation gives good results for inputs from 90V to 250V (50Hz and 60Hz). At 110V, 60Hz input, the LED current peaks at 8.5mA, so IC1 still works. If higher LED drive current is needed, the value of R3 can be reduced, or the value of C1 can be increased.

Figure 2. This is an LTspice simulation. When the input voltage drops below 0V, the LED current generates a pulse with edges leading and lagging the crossover point. The peak current of the optocoupler LED is 17mA.

Figure 3. The results of the prototype circuit correlate well with the simulation.

Testing of an actual circuit shows good correlation with simulation (Figure 3). Using a 5V logic supply to drive the isolated outputs, a good pulse waveform is obtained (Trace 1). For safety, the main input is fed to the oscilloscope through a 15V isolation transformer (Trace 2). The zero crossing points of the transition can be shown using the oscilloscope's persistence feature (Figure 4). This approach allows accurate measurement of the timing of the pulses relative to the input zero crossings.

Figure 4. Using the persistence function of an oscilloscope, the exact zero crossing is shown.