The art of controlling an electrical load has long recognized the advantage in switching loads at or near the zero crossing point of the power supply waveform. In response to this knowledge, many circuits have been designed for producing a control signal to achieve that end. More recently, the use of optical isolation has resulted in the design of zero crossing circuits which produce an optically coupled output signal in the vicinity of the zero crossing point.
In many of these applications, the accuracy with which the control signal matches the actual zero crossing point of the voltage supply waveform is not critical. For this reason, many circuits merely drop the input waveform, or a replica thereof, across a switching means such as a diode, and employ the diode's conduction as the output signal. Due to the forward voltage drop of the diode, and the diode photodetector gain, the relationship between the actual zero crossing point and the signal produced by the prior art zero crossing circuits are a function of the diode's characteristics, and since, typically, these devices are manufactured to relatively loose tolerances, so, too, is the tolerance between the output signal and the actual zero crossing point.
It is also very desirable to apply some filtering to the input line voltage signal to reduce the effect of line noise. In previous circuits, the amount of filtering that could be used was limited because the filter circuit introduces a signal time delay that makes the circuit tolerance problem worse.
Notwithstanding the fact that the circuits are useful in many applications, there are applications in which such loose tolerances are not acceptable. For example, in an application where the current through an incandescent lamp is to be phase controlled (by a triac, for example) and where the lamp current must be gradually increased from zero to the desired level, it is essential that the zero crossing circuit deliver an output signal which is no later than the actual zero crossing point of the supply voltage. Were the signal to be retarded slightly after the actual zero crossing point of the supply voltage, the signal which enables the triac would result in the lamp being illuminated for a full half cycle directly negating the desire to gradually bring the lamp current up to the desired level.
It is therefore an object of the invention to provide a zero crossing circuit in which an output signal is produced in the vicinity of the zero crossing point of a supply voltage waveform, but which is no later than the zero crossing point. It is another object of the invention to provide such a circuit which includes an optical isolator and in which the output signal timing is not a function of the isolator characteristics. It is still another object of the present invention to provide a zero crossing circuit providing an optically coupled output which is capable of compensating for the delays introduced into the voltage waveform by conventional filtering. It is yet another object of the invention to provide a zero crossing circuit which produces an output signal in the vicinity of zero crossing point of a supply voltage waveform, but no later than that point, which automatically compensates for the changes in line voltage. It is still another object of the present invention to provide a zero crossing circuit of the foregoing type which includes an impedance means including a capacitor, and in which the timing of the output signal is not sensitive to capacitor tolerances.