Control Devices 459
ing the anode positive and the cathode negative.
When the circuit switch is turned on initially, the load is not ener-
gized. In this situation the VBo is in excess of the VS voltage. Power control
is achieved by turning on SW-1 which forward biases the gate. If a suitable
value of IG occurs, it will lower the VBO and turn on the SCR. The IG can
be removed and the SCR will remain in conduction. To turn the circuit off,
momentarily open the circuit switch. With the circuit switch on again, the
SCR will remain in the off state. It will go into conduction again by closing
SW-1.
Dc power control applications of the SCR require two switches to
achieve control, but this application of the SCR is not practical. The cir-
cuit switch would need to be capable of handling the load current. The
gate switch could be rated at an extremely small value. If several switches
were needed to control the load from different locations, this circuit would
be more practical. More practical dc power circuits can be achieved by
adding a number of additional components. Figure 17-6 shows a dc pow-
er control circuit with one SCR being controlled by a second SCR. SCR 1
would control the dc load current. SCR 2 controls the conduction of SCR 1.
In this circuit, switching of a high-current load is achieved with two small,
low-current switches. SCR 1 would be rated to handle the load current.
SCR 2 could have a rather small current-handling capacity. Control of this
type could probably be achieved for less than a circuit employing a large
electrical contactor switch.Figure 17-6. Dc power control circuit