SCR Controls

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SCR Controls

Features and Benefits of SCR’s


High reliability
Because the SCR power controller is a solid-state device, it provides virtually limitless, trouble free operation with a minimum of maintenance.

Infinite resolution
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.

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. When a small signal is applied the SCR will turn on in 10-100 microseconds. Once turned on it will remain on until the current through it is reduced below a very low value called the holding current. Basically, an SCR power controller consists of the following:

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
Zero Firing and Phase Angle Firing

How the solid state AC switch controls any given load depends on how and when the SCR gates are triggered.  The firing “pulse” to the SCR gate is generated by a trigger circuit which in turn is usually controlled by a manual potentiometer, instrument controller, milliamp signal or ON/OFF control voltage.  For proportional control of resistance heating loads, there are two types of SCR firing methods available:
Zero 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 Firing

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.