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Theory of Operation 2-9
Power Supply Theory of Operation
Introduction
This section provides a general description of the power supply, which includes the power
module, part number 09895-67910, and the associated ac input circuitry mounted on the
chassis and fan compartment.
General Description
Ac power enters through fuse A3Fl and is switched by SI, the front panel power switch. It
passes through S2 and S3, the voltage selection switches, into the proper windings of
transformer Tl. Tl has three secondary windings.
One secondary winding supplies rectifier CRI0 through fuses AIFI and AIF2. CRI0
supplies unregulated + 12 volts and -12 volts. The + 12 volt line is regulated to +5 volts by
Ul, an LM723 voltage regulator, which functions by changing the bias on Ql, a Darlington
pass regulator. Ul receives inputs from the current limiter (U2 pin 1), regulated +5V line
and regulated + 12V line (see next paragraph). U2 senses the voltage across R4. As the
current through R4 increases, the voltage across it increases. When it reaches a predeter-
mined threshold, pin 1 changes rapidly from zero to + 12 volts, turning VOUT of Ul off,
,. -
which turns Ql off. +5V enters the inverting input of Ul and + 12 volts biases CR7, a 4.99
volt precision zener diode at the non-inverting input. Note that in this configuration there
will be no +5 volts until the + 12 volt line is up. CR4 and CR5 form a crowbar. CR5
conducts when the potential across it reaches 6.19 volts, turning on CR4 which blows fuses
Fl and/ or F2, or current limits the +5 volt supply. U2 and U3 produce the PON signal. U3
produces a power-up signal when the potential on pin 2 reaches 4.75 volts (which occurs
after a time delay to charge CI2).
Another secondary winding supplies rectifier CRI through fuse F3. CRI supplies unregu-
lated +24 volts. U4 is a + 12 volt regulator. CR8 and CR9 form a crowbar. When the
potential on CR9 exceeds 14.7 volts, it conducts, turning CR8 on, which shorts the + 24 volt
line to ground, blowing fuse F3. Note that the crowbar operates from the + 12 volt line, but
affects the +24 volt line. CR2 and CR3 are negative voltage protection.
The third secondary winding provides 120 Vac for the fan motor and the drive motors.
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