Section VIII. POWER SUPPLY STAGE ANALYSIS AND SWITCHING
This section presents a detailed discussion of the various stages of the main power supply and the operational
control circuitry which switches it. The main power supply consists of PRIMARY POWER connector input Jack J8, circuit
breaker CB1, a transient suppressor, a +22-volt preregulator, a +20-volt regulator, and MODE SELECTOR switch S4F.
The special purpose power supplies in modules A2 and A5 are discussed in paragraph 1-62 and 1-40, respectively.
1-64. POWER INPUT AND TRANSIENT SUPPRESSOR STAGE ANALYSIS
The transient suppressor consists of polarity diode A9CR1, overvoltage reference A9VR1, switches A9Q1, Q2, Q3,
CR2, and zener diode VR7 and associated resistors and capacitors. The transient suppressor protects against overloads,
reverse polarities, and transient voltages which would cause damage to the following circuitry.
Reverse Polarity. When the 27 volt supply is connected backwards, polarity diode A9CR1 conducts, tripping
circuit breaker CB1. The auxiliary power output is not protected by CB1.
Overvoltage Protection. When the primary power exceeds 33 volts (± 5 percent), overvoltage reference zener
A9VR1 conducts, forward biasing switch Q1, which reverse biases switch A9Q2. This action causes the voltages at the
Junction of A9R4, R5 to rise, charging A9C1 through A9R4, R5. (Normally, A902 Is forward biased, grounding A9C1
through A9R5.) Transistor A9Q3 Is silicon unilateral switch (SUS) which has a discrete switching voltage. When not
conducting, the impedance of A9Q3 Is high, but when Its switching voltage Is reached, the impedance is low. If the
duration of the overvoltage Is long enough to damage following circuitry, A9C1 charges through A9R4 and A9R5 to the
switching voltage of A9Q3 (8 ± 2 volts). Transistor A9Q3 conducts, applying a grating voltage to silicon-controlled (scr)
switch A9CR2. When A9CR2 Is gated on, the overvoltage input to the modem is grounded through A9R7. The excess
current drawn trips CB1.
Transient Protection. If a high-voltage transient of short duration is applied to the modem, A9C1 may not have
time to charge through the overvoltage protection circuitry before the following circuitry is damaged. The following
circuitry can accept transients up to 115 volts, but protection is required against transients of higher voltages. When a
transient exceeds 110 volts ( ± 5 percent). A9VR7 conducts, rapidly charging A9C1 through A9R7. (Note that the charge
time through low value resistance A9R7 is faster than through high value resistors A9R4, R5.) When C1 reaches the
switching voltage of A9Q3, scr switch A9CR2 is gated on, and circuit breaker CB1 is tripped as described above.