_______________________________________________________________________ Special Devices
Figure 3-21. Comparison of an SCR and a Thyratron
3-46. There are many applications of the SCR as a rectifier. In fact, its many applications
as a rectifier give this semiconductor device its name. When AC is applied to a rectifier,
only the positive or negative halves of the sine waves flow through. All of each positive or
negative half cycle appears in the output. However, when an SCR is used, the controlled
rectifier may be turned on at any time during the half cycle, thereby controlling the amount
of DC power available from zero to maximum (see Figure 3-22). Since the output is
actually DC pulses, suitable filtering can be added if continuous DC is needed. Therefore,
any DC operated device can have controlled amounts of power applied to it. Notice that the
SCR must be turned on at the desired time for each cycle.
Figure 3-22. SCR Gate Control Signals
3-47. When an AC power source is used, the SCR is turned off automatically, since
current and voltage drop to zero every half cycle. By using one SCR on positive
alternations and one on negative, full-wave rectification can be accomplished, and control
is obtained over the entire sine wave. The SCR serves in this application just as its name
implies (as a controlled rectifier of AC voltage).
23 June 2005
TC 9-62
3-15
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