| THEORY OF OPERATION |
Return to Section TOC
Return to Section TOC
Return to Section TOC
Return to Master TOC
Return to Master TOC
Return to Master TOC
SCR OPERATION
A silicon controlled rectifier (SCR) is a three terminal device used to control large cur- rents to a load. An SCR acts very much like a switch. When it is turned on there is cur- rent flow from anode to cathode. In the ON state, the SCR acts like a closed switch. When the SCR is turned OFF, there is no current flow from anode to cathode, thus the device acts like an open switch. As the name suggests, the SCR is a rectifier, so it passes current only during positive half cycles of the AC supply. The positive half cycle is the portion of the sine wave in which the anode of the SCR is more posi- tive than the cathode.
When an AC supply voltage is applied to the SCR, the device spends a certain por- tion of the AC cycle time in the ON state and the remainder of the time in the OFF state. The amount of time spent in each state is controlled by the Gate.
An SCR is fired by a short burst of current into the gate. This gate pules must be more positive than the cathode voltage. Since there is a standard PN junction between gate and cathode, the voltage between these terminals must be slightly greater than 0.6V. Once the SCR has fired, it is not necessary to continue the flow of gate cur- rent. As long as current continues to flow from anode to cathode, the SCR will remain on. When the anode to cathode current drops below a minimum value, called hold- ing current, the SCR will shut off. This nor- mally occurs as the AC voltage passes through zero into the negative portion of the sine wave. If the SCR is turned on early in the positive half cycle, the conduction time is longer, resulting in greater SCR output. If the gate firing occurs later in the cycle, the conduction time is less, resulting in lower SCR output.
Return to Section TOC
Return to Master TOC
Note: As the gate pulse is applied later in the cycle, the SCR output is decreased.
