Electrical Power Systems Technology

Control Devices 455

lead determines when the device switches from its off to on state. An SCR

will usually not go into conduction by simply for-ward biasing the anode

and cathode. The gate must be forward biased at the same time. When

these conditions occur, the SCR becomes conductive. The internal resis-

tance of a conductive SCR is less than 1 Ω. Its reverse or off-state resistance

is generally in excess of 1 MΩ. This allows the device to be similar to a me-

chanical switch.

A schematic symbol and the crystal structure of an SCR are shown

in Figure 17-2. Note that the device has a PNPN structure from anode to

cathode. Three distinct P-N junctions are formed. When the anode is made

positive and the cathode negative, junctions 1 and 3 are forward biased.

J 2 is reverse biased. Reversing the polarity of the source alters this condi-

tion. J 1 and J 3 would be reverse biased and J 2 would be forward biased

and would not permit conduction. Conduction will occur only when the

anode, cathode, and gate are all forward biased at the same time.

Some representative SCRs are shown in Figure 17-3. A few of the

more popular packages are shown here. As a general rule, the anode is

connected to the largest electrode if there is a difference in their physical

size. The gate is usually smaller than the other electrodes. Only a small

gate current is needed to achieve control. In some packages, the SCR sym-

bol is used for lead identification.

To turn off a conductive SCR, it is necessary to momentarily remove

or reduce the anode-cathode voltage. The device will then remain in this

state until the anode, cathode, and gate are forward biased again. With

Figure 17-2. SCR crystal: (a) symbol; (b) structure