Figure 4-2. Voltage at cathode of silicon-controlled rectifier.

TM11-6130-381-14

Figure 4-2. Voltage at cathode of silicon-controlled rectifier.

b. Bridge rectifiers CR6001 through CR6004,

resistor R6001, Zener diode CR6116, and RF filter

capacitors C6001 and C6002 (fig. FO-1 ) provide a

clipped sine wave voltage (B, fig. 6-2) that is in

phase with the charging path ac voltage. This

clipped sine wave is used as a positive voltage

supply for a multivibrator circuit that consists of

R6122 and C61 14, and unijunction transistor

(UJT) Q6104. The UJT has three terminals: the

emitter, base (B1 ), and base (B2). Between B1 and

B2, the UJT has the characteristics of an ordinary

resistance. The emitter is reverse-biased until the

emitter -base voltage is greater than the emitter

peak voltage (Vp). The emitter peak voltage (Vp)

is dependent on the B 1 to B2 voltage (V b), and is

approximately equal to 0.6 Vbb. When the emitter

voltage (Ve) exceeds the emitter peak Voltage Vp),

the UJT turns on and the resistance between the

emitter and B1 is very low, allowing emitter

current to flow. Voltage divider R6123 and R6124

Figure 4-3. Basic UJT miltivibrator circuit.

(fig. FO-1) provide the Q6104 B1 to B2 bias

voltage.

d. In the ac phase-controlled circuit (fig. 4-1),

c. Figure 4-3 shows a basic UJT multivibrator

the clipped sine wave voltage is used as the supply

circuit. Capacitor C6114 is charged to the potential

voltage for the UJT multivibrator. The voltage

at resistor R6122 until the emitter voltage (Ve)

across R6003 is proportional to the current flowing

reaches the emitter peak Voltage (VP). At this

t h r o u g h the battery, a n d is compared to a

time, the UJT turns on and discharges C6114

reference voltage and amplified in differential

through RL O A D. When the emitter voltage falls

comparator A6102. The output of A6102 is coupled

to approximately 2 volts, conduction ceases and

to capacitor C6114 in the multivibrator circuit,

the cycle is repeated.

4-3