It is a common practice in many areas to control AC electrical input power to a load by reducing line supply voltage. Most commonly, this has been accomplished by means of a step-down transformer.
Such transformers, while effective, cause numerous problems in some circuits. Their major disadvantages lie in their size, i.e., they are greatly larger than any other circuit component, and in their inability to regulate power in the face of varying supply voltage.
With the advent of solid state devices the step-down transformer has been largely replaced with such devices as thyristors. These devices regulate power input not by voltage change, as with the step-down transformer, but rather by limiting the application of voltage to a part of the AC waveform.
While thyristors are effective for this purpose, they also have characteristics which do not always make them acceptable as power regulators. First, these devices, simply with a resistive gate circuit, can regulate power only in the rising quarter of the AC waveform, since the thyristors permit current to flow therethrough once a firing current is reached, which current must eventually be reached within the first quarter of the AC waveform. In addition, these devices have no ability to compensate for supply voltage fluctuations. As such, these devices have a tendency to fire early with increased supply voltage, resulting in oversupply of power to the load, and to fire late with reduced supply voltage, resulting in insufficient power to the loads.
It is known to allow for second quarter of the AC waveform firing of the thyristor in a circuit by means of a ramp circuit, in which a charging current controlling resistor limits current to a firing capacitor. The capacitor is then charged and, when sufficiently charged, discharges through a breakover voltage device, such as a diac, to fire the thyristor. By choosing the proper resistance and capacitance, firing of the thyristor can be selected to occur at any point along the positive, or negative halfwave, with rectification, of the AC waveform. However, after the firing point has been set, variations in supply voltage will still cause over and under power to the load based upon fluctuations of the supply voltage, as there is no means for controlling the voltage applied to the breakover device.
It is also known to stabilize the voltage applied to the ramp circuit, and thus stabilize the slope of the ramp supplied to the breakover device, by means of a zener diode bleed off circuit. In this circuit, excess current bypasses the ramp circuit through the parallel zener diode. Such a circuit guarantees that firing of the thyristor will occur at the same point of the positive or negative halfwave of the AC waveform, without regard to the amplitude of the supply voltage or fluctuations thereto.
There remains, however, the problem of controlling the power supply to the load with voltage fluctuations. None of the known circuits can compensate for supply voltage change and provide the same level of power to the load. It is desirable, therefore, to provide a circuit which, while maintaining stabilization of the firing of the thyristor, can also advance or delay firing to compensate for variations in supply voltage and thereby provide an unchanged level of power to the load.