3-35. Although it is not the same as either a diode or a transistor, the SCR combines
features of both. Circuits using transistors or rectifier diodes may be greatly improved in
some instances through the use of SCRs.
3-36. The basic purpose of the SCR is to function as a switch that can turn on or off
small or large amounts of power. It performs this function with no moving parts that wear
out and no points that require replacing. There can be a tremendous power gain in the SCR;
in some units a very small triggering current is able to switch several hundred amperes
without exceeding its rated abilities. The SCR can often replace much slower and larger
mechanical switches. It even has many advantages over its more complex and larger
electronic-tube equivalent, the thyratron.
3-37. The SCR is an extremely fast switch. It is difficult to cycle a mechanical switch
several hundred times a minute. However, some SCRs can be switched 25,000 times a
second. It takes just "microseconds (millionths of a second) to turn on or off these units.
Varying the time that a switch is on as compared to the time that it is off regulates the
amount of power flowing through the switch. Since most devices can operate on pulses of
power (AC is a special form of alternating positive and negative pulses), the SCR can be
used readily in control applications. Motor-speed controllers, inverters, remote switching
units, controlled rectifiers, circuit overload protectors, latching relays, and computer logic
circuits all use the SCR.
3-38. The SCR is made up of four layers of semiconductor material arranged PNPN (see
Figure 3-18, view (A). In function, the SCR has much in common with a diode. However,
the theory of operation of the SCR is best explained in terms of transistors.
3-39. Consider the SCR as a transistor pair, one PNP and the other NPN (see
Figure 3-18, view B). The anode "A", is attached to the upper P-layer; the cathode "C", is
part of the lower N-layer; and the gate terminal "G", goes to the P-layer of the NPN triode.
3-40. In operation (see Figure 3-18, view C)) the collector of Q2 drives the base of Q1,
while the collector of Q1 feeds back to the base of Q2. β1 (Beta) is the current gain of Q1
and β2 is the current gain of Q2. The gain of this positive feedback loop is their product
(β1 x β2). When the product is less than one, the circuit is stable; if the product is greater
than unity, the circuit is regenerative. A small negative current applied to terminal G will
bias the NPN transistor into cutoff, and the loop gain is less than unity. Under these
conditions, the only current that can exist between output terminals A and C is the very
small cutoff collector current of the two transistors. For this reason the impedance between
A and C is very high.
3-41. When a positive current is applied to terminal G, transistor Q2 is biased into
conduction, causing its collector current to rise. Since the current gain of Q2 increases with
increased collector current, a point (called the breakdown point) is reached where the loop
gain equals unity and the circuit becomes regenerative. At this point, collector current of
the two transistors rapidly increases to a value limited only by the external circuit. Both
transistors are driven into saturation, and the impedance between A and C is very low. The
positive current applied to terminal G, which served to trigger the self-regenerative action,
is no longer required since the collector of PNP transistor Q1 now supplies more than
enough current to drive Q2. The circuit will remain on until it is turned off by a reduction
in the collector current to a value below that necessary to maintain conduction.
23 June 2005