Tektronix PS 503A, Instruction Manual

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Circuit Description—PS 503A
power switch is turned off, or power fails, the shut-down
equalizer circuit will cause the supply (+ or
—) that has the
\ lightest load to reduce its output voltage to prevent spiking
' of the lightly loaded supply as the filter capacitor is
discharged.
Power
Supplies
+27 V Integrated Circuit Pre-regulator. Zener diode
VR20 and emitter follower Q20 form a voltage regulator to
supply
U35 and U135.
+5 V Ground-Referenced Supply. The power module
supplies +11.5 V through pins 2A and B and 3A and B on
the plug-in rear connector to pins 1 and 3 of integrated
circuit U1. U1 provides a reg5 V output limited at
1 A from pins 2 and 3 to output connectors J1 and J2 on
the front panel.
Over-voltage Protection
When the voltage at the PS 503A front-panel + output
terminal reaches about 26 volts, over-voltage protection
silicon controlled rectifier (scr) Q65 turns on. Q65 turning
on shorts the +33 volts supply to the COMMON terminal,
causing the power supply to shut down.
NOTE
By changing Zener diode VR64, different over­
voltage levels can be obtained.
Output Current Selection
If the PS 503A is operated in any compartment of a
TM 500 Series Power Module except the high power
compartment of the TM 504 and TM 506 Power Modules,
S95 reduces the current limit to 400 mA and turns on
400 mA limit indicator LED.
5-3