KEPCO JQE 150-1.5MVPY-26956 Half Rack - Manual (Page 44)

5-4 JQE SPECIAL SVC 081111
amplified voltage developed across R
S
(due to load current flow) is less than the preset refer-
ence level, the current limit amplifier will be biased to its inactive state and will not affect the out-
put. If the output current increases however, the voltage drop across R
S
will overcome the
reference level set by the current limit control. The current limit amplifier will be activated and
produce a drive signal at the diode gate greater than the output of the voltage error amplifier,
thereby taking control away from the voltage error amplifier and transferring the power supply
into current limit mode. Since both the current limit amplifier and voltage error amplifier are cou-
pled through the diode gate circuit to the pre-driver stage A1Q2, the amplifier with the greater
(negative) output will control the pass transistors and therefore the output.
Current Limit "A" uses A1IC3 as the current error amplifier, while Current Limit "B" uses A1IC4.
The reference for Current Limit "A" is delivered by A1IC2B, using R101 (front panel Current
Limit "A" control) as the controlling potentiometer. Current Limit "B" uses A1IC5 as the reference
stage using R103 (front panel Current Limit "B" control) as the controlling potentiometer. If the
link between terminal block TB502, terminals 1 (REF) and 2 (RPC1) is installed, both current
limit "A" and "B" loops are enabled, however the one with the lowest current limit value controls
the unit. If Current Limit "B" is in control, this value may be modified by a remote resistance con-
nected across TB502, terminals 2 (RPC1) and 3 (RPC2). If the link between block TB502, termi-
nals 1 (REF) and 2 (RPC1) is removed, Current Limit "B" is disabled.
5.2.6 OVERVOLTAGE PROTECTION CIRCUIT AND CURRENT MONITOR
Input and output connections for the overvoltage protection circuit (VP) are made via a single
connector, P401/J407. The circuit receives a-c voltage derived from a secondary of the main
transformer (T201). The a-c is rectified by a conventional bridge rectifier (A4CR1), capacitor fil-
tered by the input filter capacitor (A4C1) and shunt regulated by a 15V zener diode (A4CR3). A
6.5-Volt source, derived from the pre-regulated DC voltage by means of shunt regulator A4IC1,
provides a stable reference source referenced to C1 (–) (minus).
The overvoltage protection circuit contains output sensing voltage differential amplifier IC2 and
voltage comparator IC3A. This circuit compares the output sensing voltage with the adjustable
voltage limit level. Once triggered, the comparator’s output is integrated (A4CR14, A4C6, and
A4C9), providing an adjustable delay through R(C)403 and A4IC3B.
In the event of an overvoltage, resulting either from external causes or from internal power sup-
ply failure, the circuit become active. A4CR402 will be turned on, producing an output pulse.
As the first S.C.R. (CR402) is conducting, it performs two functions: It shorts out the DC supply
for the voltage protection circuit, thereby creating an overcurrent in one of the circuit breaker
coils. Simultaneously, CR402 generates a firing pulse for the main S.C.R. (CR203), which
through its ”crowbar” action short-circuits the power supply output. The a-c circuit breaker
(CB101) trips due to the action of the first S.C.R. (CR402), disconnecting the power supply from
the power line. The protection circuit is automatically re-activated with the ”turn-on” of the power
supply via CB101.
The current monitor circuit (A4IC4) amplifies the signal across the second current sensing resis-
tor (A4R24) connected in the –OUTPUT leg of the power supply. The transfer factor is 50mV/A.
The current monitor signal is available at terminals +C MON and –C MON of terminal block
TB503 at the rear panel.
5.2.7 AUXILIARY SUPPLIES
AMPLIFIER POWER SUPPLY. This full wave, bridge rectified source is derived from a second-
ary winding on the auxiliary transformer (T202). Rectified by A1CR8, the supply is capacitor fil-