Lincoln Electric SVM132-A SCR Operation, 5E-5, Applied Later CYCLE, the SCR Output Is Decreased

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E-5E-5

THEORY OF OPERATION

FIGURE E.4 – SILICON-CONTROLLED RECTIFIER OPERATION

INPUT

CATHODE

OUTPUT

ANODE GATE

GATE

NOTE:￿ AS THE￿ GATE PULSE IS￿

￿ APPLIED LATER IN THE ￿

￿ CYCLE, THE SCR OUTPUT￿

￿IS DECREASED.

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SCR OPERATION

A silicon controlled rectifier (SCR) is a three terminal semi-conductor device used to control currents to a load. Refer to Figure E.4. An SCR acts very much like a switch. When a gate signal is applied to the SCR it is turned ON and there is current flow from anode to cath- ode. In the ON state the SCR acts like a closed switch. When the SCR is turned OFF there is no current flow, 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 posi- tive half cycle is the portion of the sine wave in which the anode of the SCR is more positive than the cathode.

When an AC supply voltage is applied to the SCR, the device spends a certain portion 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 the ON state is controlled by the gate.

An SCR is fired by a short burst of cur- rent into the gate. This gate pulse must be more positive than the cathode volt- age. Since there is a standard PN junc- tion between the gate and cathode, the voltage between these terminals must be slightly greater than 0.6 VDC. Once the SCR has fired, it is not necessary to continue the flow of gate current. 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 holding current, the SCR will shut off. This normally occurs as the AC supply 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 time is later in the cycle the conduction time is less, resulting in lower SCR output.

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LN-8