There are many circuits published showing zero-crossing detectors for use with 50- and 60-Hz power lines. Though the circuit variations are plentiful, many have shortcomings. This Design Idea shows a circuit that uses only a few commonly available parts and provides good performance with low power consumption.

In the circuit shown in Figure 1, a waveform is produced at V O with rising edges that are synchronized with the zero crossings of the line voltage, V AC . The circuit can be easily modified so that it produces a falling-edge waveform that is synchronized with V AC .



Figure 1 The zero-crossing detector uses few components and consumes very little power. The V O signal has a rising edge that is coincident with each zero crossing of the line voltage, V AC .

The circuit operates as follows. At the zero crossings of V AC , the current through the capacitor and the LED of the HCPL-4701 optocoupler satisfies Equation 1 below. Equation 2 shows the standard conversion between radians per second and hertz; it also shows the derivation and explanation for v i (t). Equations 3 and 4 show the simplification used in Equation 1. Because the voltage across the LED is close to constant, differentiation of that value with respect to time results in a zero value.

The peak value of the current through the LED is a function of the capacitor, C, so you must choose a value for C under the constraint that at the initial time (t=0) and for a given minimum supply-voltage value, the intensity exceeds the triggering threshold value for the optocoupler. In the case of the HCPL-4701, it is I F(ON) =40 μA.

Diode D 1 not only allows for the capacitor to discharge but also prevents the application of a reverse voltage on the LED. The maximum reverse input voltage of the HCPL-4701 is 2.5V.

Resistor R 1 is included in order to discharge the energy stored in the capacitor in the latter portion of each cycle of v i (t) when i c (t)<0 (Figure 1). Its maximum value is limited by the capacitor, by the peak value of the supply voltage (V AC-PEAK ), and by the maximum acceptable time delay of the current rising edges through the LED with respect to the corresponding ac-voltage zero crossing (Figure 2). Its minimum value is limited by the maximum allowable power dissipation in R 1 ([V AC-RMS ]2/R 1 ). A practical compromise has to be reached.



Figure 2 The relationship between v i (t) and I LED (t) is a function of the value of R 1 . The time delay between the zero crossing and the LED current is shown.

Table 1 shows the time delay (t DELAY ) of the current rising edges through the LED and the power dissipation for three different values of R 1 . Notice that the time delay of the rising edges of V O with respect to the zero crossings of V AC must include an additional delay for the optocoupler’s propagation time delay. The HCPL-4701 has a typical propagation time delay of 70 μsec.

Based on the previous information, the following practical values for C and R 1 are obtained:

For V AC =230V RMS ±20% ( Figure 3 ): C=0.5 nF/400V (MKT-HQ 370 polyester metallized, MKT series), R 1 =560 kΩ/0.25W, t DELAY =114 μsec (the time delay in the rising edges of V O with respect to the zero crossings of V AC ), and P≈100 mW (average power from the ac line).

=230V ±20% ( ): C=0.5 nF/400V (MKT-HQ 370 polyester metallized, MKT series), R =560 kΩ/0.25W, t =114 μsec (the time delay in the rising edges of V with respect to the zero crossings of V ), and P≈100 mW (average power from the ac line).

Figure 3 Empirical results are shown for V AC =230V RMS , C=0.5 nF, and R 1 =560 kΩ.



For V AC =115V RMS ±20% ( Figure 4 ): C=1 nF/200V, R 1 =220 kΩ/0.25W, t DELAY =130 μsec (time delay in the rising edges of V O with respect to the zero crossings of V AC ), and P≈65 mW (average power from the ac line).

=115V ±20% ( ): C=1 nF/200V, R =220 kΩ/0.25W, t =130 μsec (time delay in the rising edges of V with respect to the zero crossings of V ), and P≈65 mW (average power from the ac line).

Figure 4 Empirical results are shown for V AC =115V RMS , C=1 nF, and R 1 =220 kΩ.



For operation from 80 to 280V RMS : C=1 nF/400V and R 1 =330 kΩ/0.25W.

Empirical results are shown for V AC =267V RMS , C1=1 nF, and R 1 =220 kΩ (Figure 5). See Figures 6 and 7 for additional empirical results.



Figure 5 Empirical results are shown for V AC =267V RMS , C=1 nF, and R 1 =220 kΩ.



Figure 6 Empirical results are shown for V AC =114V RMS , C=1 nF, and R 1 =560 kΩ.



Figure 7 Empirical results are shown for V AC =228V RMS , C=1 nF, and R 1 =560 kΩ.

Note that as with any device connected directly to the mains, exercise extreme caution while bench testing the circuit. Follow proper guidelines when laying out a printed circuit board.