CIRCUIT ANALYSIS
4.0 CIRCUIT ANALYSIS
In the 60 Hz power supply (19A149979P1), the ON/OFF switch (S1) provides line
voltage to the power supply through the primary line fuse F1. Line current flows through
F1 to the courtesy receptacle prior to S1. This allows line voltage to always be available
at the receptacles. Current then flows through the primary of stepdown transformer (T1)
via the 200°C thermal fuse. The thermal fuse would only open in the unlikely event that
an internal short would develop in the transformer. The varistor (VR1- 150 V rating)
provides addition input line voltage suppression.
In the 50 Hz power supply (19A149979P2) and the 60 Hz power supply (19A149979P3),
the ON/OFF switch (S1) is a DPST type switching both primary AC lines. In addition,
both input lines have 5 amp fuses (F1 and F4). The varistor (VR1-275 V rating) provides
additional input line voltage suppression. When power is applied, current flows through
the primary of step-down transformer (T1) via the 200°C thermal fuse. As in the 60 Hz,
19A149979P1 model, the thermal fuse would only open if the transformer develops an
internal short.
The step-down transformer (T1) is a ferroresonant type, which has inherently good input
line voltage regulation. This eliminates the need for additional high current regulators.
Capacitor C9 serves as a resonating capacitor across the secondary taps of the
transformer.
The transformer steps the input voltage down to approximately 28 VAC across two
secondary windings. Each winding drives two separate full wave bridge rectifiers
consisting of D1A, D1B through D4A and D4B. The rectifiers are dual diode packages
and are mounted on heat sink HS 1. During the first half of the period diodes D1B, D2A,
D3B, and D4A are conducting and delivering current, which is summed at the input to
the high current filter. During the second half of the period diodes D1A, D2B, D3A, and
D4B are conducting and also delivering current, which is summed at the input to the high
current filter. The high current filter consists of capacitors C1-C4, and C7, inductor L1,
and resistor R1. It is designed to reduce the output ripple to less than 100 mV p-p for any
current load up to 15 amps. It also keeps transient responses greater than 22 volts and less
than 30 volts. Resistor R1 is a 30-ohm, 50 watt resistor that serves two functions. One, it
acts as a bleeder resistor to discharge the capacitors when the supply is turned off. Two, it
provides a minimum current load to prevent the output voltage from ever rising above 30
volts under any load condition. The high current filter sources up to 15 amps through the
20 amp fuse F2 to the high current output port F801A on the rear wall of the chassis.
F801A-1 and F801A-2 are A + and A-, respectively, and connect to the transmitter power
amplifier. The two secondary windings are also center tapped to produce a step-down
voltage of around 14 VAC, which is also fed to the two full wave bridge rectifiers.
During the first half of the period, diodes D2A and D4A provide a conduction path for
current going to both the high current filter and the low current filter. During the second
half of the period, diodes D1A and D3A provide the conduction path for current going to
the high current filter and the low current filter. The low current filter consists of C5, C6,
L2, and R2. It is designed to reduce the output ripple to less than 100 mV p-p for any
current load up to 3 amps. It also keeps transient responses greater than 11 volts and less
than 18 volts. Resistor R2 is a 100-ohm, 10 watt resistor that serves two functions. One, it
acts as a bleeder resistor to discharge the capacitors when the supply is turned off. Two, it
provides a minimum current load to prevent the output voltage from ever rising above 18
volts under any load condition. The low current filter sources up to 3 amps through the 5
amp fuse F3 to the low current output port J801 on the rear wall of the chassis. J801-1, 2,
LBI-38551D 9
