Circuit Description—Type 422 AC-DC
As the Battery Pack charges, its output potential in
creases. This increase in voltage is reflected at the base of
Q1033 through zener diode D1022 and emitter follower
Q1023. Therefore, as the output potential of the Battery
Pack increases, the bias on Q1033 and its charge current
remain fairly constant throughout the charge cycle. For this
reason, it is important to reduce the charge rate to trickle
charge after the recommended period to prevent damage to
the Battery Pack due to overcharging. (See Operating
Instructions.)
Fuse F I014 protects the instrument from damage if
excessive current is demanded by the Battery Pack during
the charge cycle. Diode D1016 is reversed biased when the
AC power to the instrument is shut o ff to disconnect the
Battery Pack from the Battery Charger stage.
DC Power Input. DC power is applied to the Power
Selection stage through filter C1010-C1011-C1012-T1010,
Power Mode switch SW1030 and DC fuse F I014. Filter
C1010-C1011-C1012-T1010 reduces the EMI radiation con
ducted out of the instrument through the DC power cord.
The Power Mode switch SW1030 connects the external DC
source to the Power Selection stage only in the OPERATE
11.5-35V DC position. Fuse F I014 protects the instrument
if excess current is demanded from the external DC source.
D1014 provides reverse-voltage protection for the power
supply. If the external DC voltage is inadvertently con
nected to the AC-DC Power Supply with the wrong polar
ity. D1014 appears essential^/ as a short circuit across the
input connector. The excess current flowing through
D1014 under this condition causes F1014 to open to pro
tect the instrument.
Battery Pack. The Battery Pack consists of 20 size D
rechargable nickel-cadmium battery cells. When Power
Mode switch SW1030 is in the OPERATE I NT BATT posi
tion, the battery potential is connected to the Power Selec
tion stage through F I014. Fuse F I014 protects the Battery
Pack if excessive current is demanded during battery opera
tion. The Battery Pack can be recharged from an AC line
with the correct setting of the Power Mode switch (see
Battery Charger).
Power Selection. The power source from which the
Type 422 AC-DC Power Supply is operated is selected by
the Power Mode switch SW1030. This switch also deter
mines the battery charge rate as discussed under Battery
Charger. The power from the source selected by SW1030 is
connected to the Low-Voltage Indicator stage and the
DC-DC Regulator circuit through TK1039, POWER switch
SW1001 and the power interlock. Thermal cutout TK1039
provides thermal protection for this instrument, primarily
for external DC or battery operation. If the internal tem
perature of the instrument exceeds a safe operating level,
TK1039 opens to interrupt the power. When the tempera
ture returns to a safe level, TK1039 automatically closes to
re-apply the power. The power interlock disconnects the
output power from the Power Selection stage when the
AC-DC Power Supply is disconnected from the indicator.
Since the Battery Charger stage is not affected by the
power interlock, the Battery Pack can be charged even
when the AC-DC Power Supply is disconnected from the
indicator.
Power Mode switch SW1030 also determines the level on
the zero-reference voltage line. In all positions of SW1030
except OPERATE 11.5-35V DC, this line is connected to
chassis ground to provide a ground reference to all stages
connected to this line. However, in the OPERATE
11.5-35V DC position, the zero-reference voltage line is dis
connected from chassis ground and connected to the nega
tive terminal of the DC input connector. This allows the
instrument to be operated from DC voltages which are
elevated above earth (ground).
CAUTION
See the precautions given in Section 2 for operation
from elevated DC voltages.
Low-Voltage Indicator. The Low-Voltage Indicator
stage monitors the unregulated power output to the DC-DC
Regulator circuit when the Power Mode switch SW1030 is
in the OPERATE I NT BATT position. The unregulated
power at the output of this circuit is connected across
divider R1041-D1041 to the zero-reference voltage line.
Zener diode D1041 maintains a level at the anode of D1042
which is about 11 volts more positive than the zero-
reference voltage line (ground for battery operation). The
voltage at the base of Q1045 is determined by voltage
divider R1046-R1047-R1048 between the unregulated out
put of this circuit and the zero-reference voltage line. Low-
Voltage Indicator adjustment R1047 sets the base level of
Q1045 for correct operation of this stage. The base level of
Q1055 is determined by voltage divider R1043-R1044
between the collector of Q1045 and the zero-reference line.
When the Battery Pack is fully charged, the unregulated
output level of this circuit is about +24 volts. The voltage
at the base of Q1045 (with correct adjustment of R1047) is
held more positive than the voltage at the anode of D1042
as set by zener diode D1041. Therefore Q1045 and D1042
are both reverse biased. With no current flowing through
Q1045, the base of Q1055 drops negative to about the level
on the zero-reference voltage line (ground) and Q1055 is
off also. The collector of Q1055 is connected to the junc
tion of D1057-R1057 in the OPERATE INT BATT'posi-
tion by SW1030. However since Q1055 is off when the
Battery Pack is charged, the level at its collector is set by
the level at the junction of D1057-R1057 and D1054 is
reverse biased to disconnect the collector circuit of Q1055
from the base circuitry of Q1045. With Q1055 off, current
flows from the +95-volt supply through R1057 and D1057
to ignite the POWER neon in the indicator to indicate that
power is applied.
As the Battery Pack discharges, the voltage applied to the
Low-Voltage indicator stage becomes less positive. Low
Voltage Indicator adjustment R1047 is set so that Q1045 is
biased on when the unregulated output voltage from this
circuit drops to 22 volts. When Q1045 is forward biased, it
conducts through R1041, D1042, R1043 and R1044. The
3-22
To Next Page
Loading...
