458 Electrical Power Systems Technology
IG will cause an increase in the VBO needed to produce conduction.
The gate current characteristic of an SCR shows an important elec-
trical operating condition. For any value of IG there is a specific VBO that
must be reached before conduction can occur, which means that an SCR
can be turned on when a proper combination of IG and VBO is achieved.
This characteristic is used to control conduction when the SCR is used as
a power control device.
DC Power Control with SCRs
When an SCR is used as a power control device, it responds primarily
as a switch. When the applied source voltage is be-low the forward break-
down voltage, control is achieved by increasing the gate current. Gate cur-
rent is usually made large enough to ensure that the SCR will turn on at the
proper time. Gate current is generally applied for only a short time. In many
applications this may be in the form of a short-duration pulse. Continuous
IG is not needed to trigger an SCR into conduction. After conduction occurs,
the SCR will not turn off until the IAK drops to zero.
Figure 17-5 shows an SCR used as a dc power control switch. In this
type of circuit, a rather high load current is controlled by a small gate cur-
rent. Note that the electrical power source (VS) is controlled by the SCR.
The polarity of VS must forward bias the SCR, which is achieved by mak-
Figure 17-5. Dc power control switch