User's Manual Part 5
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Refer to the diagram to the rear of this Section.
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The mains supply is connected to the primary of the transformer via the supply cord,
input fuse and on/off switch.
A conventional bridge rectifier is used across the secondary winding of the transformer,
with both positive and negative leads being isolated from earth. Five PCB mounted
smoothing capacitors are used to produce a 23V DC bus from the 18Vrms secondary of
the transformer. The ripple on the DC bus is 4V peak to peak, with an output current of
6A.
The front panel LED is illuminated when mains power is applied.
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Voltage regulation is provided by the complementary Darlington configuration of Q1
and Q2. The switching of this pair is derived directly from IC1 (TL494).
When Q1 is turned on, current flows in inductors L1, L2 and L3 to supply the output.
Capacitors C6 and C7 hold the output voltage at a nominal 13.8V. When Q1 turns off,
the current flowing in the inductors continues to flow, via diode D1.
C8, C9 and L3 form a common mode filter to suppress conducted noise at the output.
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A pulse width modulating IC (IC1) controls the switching of Q1 and thus the regulation
of the output voltage. A voltage divider (R4, R5 and RV1) converts the output voltage to
a 5V nominal level. It is then compared to IC1s temperature compensated internal 5V
reference. The length of time Q1 is turned on is proportional to the difference between
the reference and the output voltage. Feedback compensation is provided by C10, C11,
R6 and R7, while R10 improves the transient response for the feedback circuit.
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Current limiting is provided by monitoring the voltage across the current sense resistor
(R18). When the current is increased to approximately 9.5A, the voltage on pin 15 of IC1
decreases to a point where the pulse width of the switching waveform decreases. In
addition, a 10A fuse is provided in the secondary circuit of the power supply.