Current Walk-In Circuit - Marconi LORAIN RHM800E50 Manual

magnitude but opposite in polarity. The cosine voitage
applied to the non-inverting input of comparator A3 at
pin 5 is compared to the positive enor signal applied to
the comparator at pin 4. When the cosine vokage at pin
pin 4. the output signal of the comparator at pin 2
unclamps the base of driver transistor A8. causing ban-
is applied to the respective thytistor. At the same time.
the positive going cosine voltage applied to comparator
A3 at inverting input 6 is compared to the negdtiva error
signal applied to the comparator non-inverting input 7.
entire half cycle, the output of the comparator at pin 1
remains damped to ifs negative power supply voitage
which is sufficient to biis Is respective driver transistor
A8 off. During the negative half cycle, tfte first driver
transistor is cut off and the second driver transistor con-
ducts, generating a firing pulse which is applied to its
6.16 The synchronizing comparators of A3 receive a
signal from a secondary winding of power transformer
This signal is voltage divided by resistors Al2 (4-
13) and Al6 (3-4) and applied to the synchronizing
comparators where the signal is referenced to common.
The sampled voitage is applied to the inputs of the syn-
type comparators. The comparator inputs are paralleled
but opposite in polarity. The opposite polarity is
achieved by appfying the sampled voltage to tf~e non-
inverting input of the first comparator at pin 9 and apply-
ing tf~e sampled voltage to the inverting input of the sec-
ond comparator at pin 10. During a positive half cycle,
the output of one comparator is damped to the negative
supply and the output of the second comparator is
allowed to float. The two comparator outputs are
applied to the base of their respective driver transistor
A8. In this way, the comparators are used to clamp the
base of the respective driver transistor to assure no fir-
ing pulses are applied to the Mytistors when the thytts-
CR19, diodes CR6 and CR7 and associated compo-
nents are used in single phase rectffirs onfy, and have
no function in this application.
Current Walk-in Circus
6.17 A current walk-in drcuft on Regulator and Alarm
circuit card A7 prevents the rapid application of initial
load. A timer circuit provides an approximate 8 second
delay from the time the rectifier is started until full rated
output current is avaflable to the load. The current walk-
in feature of me rectifier is achieved by suppressing the
current regulator reference voltage and allowing it to
increase at a fiied rate. When the rectifier is turned off,
the walk-in circuit is reset to time 0. The walk-in circuit
consists of amplifiers A4 and A5 and associated compo-
nents. Ampliiier A5 transistor Q9, zener die CR21
Signal at
and associated components form a walk-in reset circuit
which assures that the walk-in circuit resets to zero in
the event of momentary AC power failures.
6.18 When the rectifier is started, a positive DC output
from Power Supply A8 is applied to the cathode of zener
diode CR21 This voltage causes transistor Q9 to con-
duct. Conduction of transistor Q9 provides 0 volts to the
inverting input of amplifier A5 (1.2.3) which produces a
positive output. This positive voltage is applied to volt-
age follower A5 (12. 13. 14). The voltage follower
charges capacitor C6 which is connected to the input of
a second voltage follower A4. the output of which follows
the capacitor voltage directly. This voltage is diode cou-
pled through CR14 to the current limiting voltage divider
gradually increases as capacitor C8 charges. Once
capacitor C8 charges to a level higher than the refer-
ence voltage, if is decoupled by diode CR14. The time
required to charge capacitor C6 to the reference voltage
will be the same as the time required for the walk-in.
6.19 When the rectifier is turned off or the commercial
AC input fails momentarily, the positive DC input from
Power Supply A8 is removed from CR21 This biases
transistor Q9 into cutoff: subsequently, amplifier A5 (1,
2.3) produces a 0 vok output which is applied to voltage
follower A5 (12, 13, 14). When the 0 volt signal is
applied. the outpuf of the voltage follower seeks corn-
discharged through diode CR2 into the amplifier. reset-
ting the walk-in circuit.
6.20 As mentioned in the current sensing and current
regulation description praviousiy given. shunt amplifier
A8 provides a voltage which is dependent on rectifier
output current. This voltage is applied to amplifier A7 at
non-inverting input pin 10. Amplifier A7. transistor Ql,
potentiometer R28 comprise the load sharing cinuif. A
negative load sharing lead at J2 pin 16 of the cinxif
card is connected at the junction of resistors R9 and
R20. This load sharing lead is connected to the similar
load sharing lead on the other paralleled rectifiers. The
output of amplifier A7 at pin 8 causes transistor Ql to
conduct. Conduction of Ql inserts resistor R29 and
load sharing adjustment potentiometer R28 into a voft-
Section 4307
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