Two transistors make up high-precision, ac-mains ZCD

Article By : Djessas Zoheir

Here is a simple two-transistor circuit that can accurately detect the zero crossing of the ac mains voltage.

Many applications that employ 110V/230V-ac mains call for a zero-crossing-detection (ZCD) circuit for the ac-line voltage, for example, to synchronise the switching of loads. One method of ZCD uses a high-value current-limiting resistor or a voltage-resistive divider to sense the ac voltage at the controller's I/O pin. However, depending on whether the I/O pin is in TTL or Schmitt-trigger mode, the ZCD has a delay that depends on the threshold swing of the I/O pin and the slew rate of the power line. For example, assume a 230V, 50Hz ac system voltage and a voltage divider of 100—that is, 230V/100=2.3V. Further, assume that the I/O pin triggers at 1V. This trigger level implies 1V×100=100V referenced to the 230V-ac mains. Thus, 100=230×sin(2×π×50×t) yields a delay of 1.43 msec, which represents 14.3% of the half-cycle period—a significant error.

Figure: This simple two-transistor circuit accurately detects the zero crossing of the input ac mains.

The figure shows a low-cost, efficient ZCD using two standard transistors. Coming directly from the ac mains, the supply network comprising C₁, C₂, D₁, D₂, and R₁ forms a simple half-wave rectifier, which powers the ZCD. Q₁ toggles with the ac-mains-voltage ZCD. To compensate for the base-emitter gap, Q₂ acts as a diode to block the ac-positive cycle. For efficiency, the detector must sense the ac-mains cycles at as high a voltage as possible. This requirement drives the choice of the transistor. Q₂ and Q₁, low-noise, small-signal BC549B transistors, have collector-to-emitter-voltage limits of 30V. With this choice, you must attenuate the ac-mains voltage from 230 to 30V. (For a BC546 transistor, you can attenuate 230 to 80V.) Thus, the voltage-divider ratio is 30V/230V=13.4%, and the values of the divider resistors are R₂/(R₃+R₂)=13.4/100, or R₃=6.46×R₂. R₂ and R₃ must be high enough for current limiting. The normalized value of R₃, 820 kΩ, means that R₂ is 820 kΩ/6.46=126.9 kΩ or 120 kΩ, the nearest standard-value resistor. With these values, Q₂ can block 230V×R₂/(R₂+R₃)=29.3V, which is less than the transistor's maximum rating of 30V.
Upon the ac-positive cycle, the base of Q₁ rises to approximately 0.6V through R₄. Q₂ acts as a simple diode. So, when the cycle voltage is higher than 0V, Q₂ is reverse-biased and blocks any current flow. At 0V, Q₂ is forward-biased, but it maintains 0.6V across the base-emitter junction, V_BE. Thus, the collector, or base, of Q₂, which connects to the base of Q₁, stays at 0.6V. Q₁ is saturated for the positive cycle, and the output voltage is low. At the ac's negative cycle, when the ac voltage is less than 0V, current flows through Q₂. Consequently, the base of Q₁, which connects to Q₂'s collector, falls to less than 0.6V, which leads to the blocking of Q₁ and the output voltage's becoming high. Note that the base of Q₁ can reach about –30V from Q₂; you can add clamp diode D₃ for Q₁ junction protection higher than –1V.

This article is a Design Idea selected for re-publication by the editors. It was first published on June 7, 2007 in EDN.com.

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