The SCR has two states, On and Off, and allows current to flow in only one direction. An SCR unit is composed of two SCRs arranged to control AC power. SCRs can remain in the off state even though the applied potential may be several thousand volts; in the on state, they can pass several thousand amperes.
Semiconductor power devices (SCR’s and Diodes)
A control circuit normally referred to as the firing or trigger circuit
A means to dissipate the generated heat
Protective circuits (fuses and transient suppressors)
Selection Guide for Payne Eng. SCR Controls
Features and Benefits
Because the SCR power controller is a solid-state device, it provides virtually limitless, trouble free operation with a minimum of maintenance.
Power, current or voltage can be controlled from zero to 100% with infinite resolution.
Extremely fast response
The SCR controller can toggle load power on and off rapidly, providing the means to respond quickly to command, load, and power supply changes.
Zero Firing and Phase Angle Firing
Zero firing is a method of proportional voltage control in which the SCRs are fired only when the voltage wave form crosses the zero voltage level at half cycles. Zero fired power controls are useful when radio frequency interference (RFI) and line voltage distortions characteristic of phase angle power controls must be avoided.
Within a given time base, the trigger circuit will energize the SCR gate for a certain number of cycles and then the trigger circuit will withdraw the pulse from the gate for a number of cycles. Power is regulated to the load by the ratio of the number of cycles ”ON” to the number of cycles “OFF” over the given time period (tbase).
Phase angle fired controls vary the voltage to the load by controlling the point of conduction within each ½ sine wave, by applying a timed trigger pulse to the SCRs-hence the name phase angle controlled. Therefore this method provides infinitely variable AC voltage control.
The result is that only a part of each cycle is “ON” and the remaining part of the cycle is “OFF” providing proportional RMS voltage control. This method provides a finer resolution of AC power control, but can generate RFI and line voltage distortions.