Ericsson MASTR III 344A3168P1, Maintenance Manual

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Several general rules can be applied to estimate charge
time of a lead acid battery system. There is almost a 100% con-
version of electrical energy to stored chemical energy for the
first 80% of battery capacity. If usable capacity is defined to be
at least 80 % of full charge, then the time to reach usable ca-
pacity is: T = 0.8 x AH/C, where T is in hours, AH is in amp-
hours, and C is the average charge rate in amps. To charge the
remaining 20 % to a full charge takes longer because the elec-
trical energy is no longer close to 100 % conversion to stored
chemical energy. The time to a full charge can be estimated as,
T = 1.1 x AH/C, where again T is in hours, AH is in amp-
hours, and C is the average charge rate in amps. Using these
estimates for a ten amp charger, a standard 55 Amp-Hour auto-
mobile battery and a gel cell system that uses four 25 Amp-
Hour batteries in parallel would recharge in the following
times:
Estimates can be provided for air time for a MII/MIII sta-
tion. Assuming a worst case scenario of a 100% transmit duty
cycle, the station air time with a 55 Amp-Hour automobile bat-
tery would be approximately one hour and considerably longer
for a smaller transmit duty cycle. With a four-in-parallel 25
Amp-Hour gel cell system, the station air time for a 100%
transmit duty cycle would be approximately three hours with
again a correspondingly longer air time for a smaller transmit
duty cycle.
MAIN CHARGING CIRCUITRY
Power to the charging circuitry is provided from a 120 Vac,
60 Hz (P1) or 230 Vac/50 Hz (P2) line source connected to the
main power cord (W801). Input power is passed through fuse
F1 (and F2 for P2), which limits input current to 5 amps, and
past varistor VR1. VR1 is a voltage transient surge protector
which clamps the line at approximately 150 Vac (P1) or 275
Vac (P2). This protects the internal circuitry from potentially
harmful line voltage surges.
Line voltage is then applied to transformers T1 and T2
which in parallel step the line voltage down to approxi-
mately 38 Vac. This voltage is then applied to rectifiers D1
and D2 as well as filter capacitors C1, C2, and C3. After rec-
tification and filtering, the unregulated DC voltage is ap-
proximately 20 Vdc.
Charging current then flows through the linear regulator
stage on its way to the battery. The linear regulator is com-
posed of two basic groups. These groups are the series pass
regulator group and the series pass control group.
The series pass regulator group consists of Q1, Q2, Q3,
R1, R2, and R3. In order to control the output voltage of the
charger, the series pass transistors are operated as variable
resistors. If the load on the charger is increased, causing a
drop in the output voltage, the resistance of the series pass
transistors is automatically decreased. With a decrease in se-
ries pass resistance, less voltage is dropped across the tran-
sistors thus increasing the output voltage back to the desired
value.
This implementation has one major drawback, a major
percentage of the total power drawn by the charger is dissi-
pated across the series pass transistors. In order to more ef-
fectively handle this dissipation, three transistors are used.
Resistor R1, R2, and R3 provide negative feedback to the
base of the appropriate transistor preventing unequal current
flow and unequal power dissipation.
The series pass control circuitry is comprised of U3, Q4
and their associated bias resistors and decoupling capacitors.
U3 continuously monitors the output voltage being devel-
oped by the interaction between the load and series pass
transistors. When more ouput voltage is required to maintain
regulation, U3 increases drive to transistor Q4. Q4 provides
the amount of series pass transistor base drive necessary to
decrease their resistance and boost the output voltage back
up to the desired value.
This continuous interaction between the control circuitry
and series pass state forms a closed loop control group
which provides the regulated output voltage to the battery.
Potentiometer R12 varies the amount of voltage feedback in
the control loop thus allowing precise adjustment of the out-
put voltage at which regulation is maintained.
OVERCURRENT PROTECTION
Overcurrent protection is provided via a current foldback
scheme. Resistors R4, R5, and R6 form a current sensing
element. The amount of voltage developed across these re-
sistors is directly proportional to the amount of current flow-
ing through them. This sense voltage is applied to the
regulator control integrated circuit, U3, by means of R9,
R10, and R50. As the current through these sense resistors
increases above approximately 10.5 amps, the sense voltage
and output current to decrease. This current foldback ap-
proach for overcurrent protection decreases the amount of
power dissipated across the series pass transistors during a
faulted condition. The maximum allowable short circuit cur-
rent is less than 3 amps.
INPUT OVERVOLTAGE PROTECTION
Overvoltage protection circuitry is provided to protect
the charger from abnormally high AC line voltages. These
voltages could cause premature failure of the series pass
transistors due to excessive power dissipation. When the line
voltage exceeds the limits specified for normal operation, the
charger senses an abnormal condition and reverts to the
SHUTDOWN mode. The charging current to the battery is
cut off by disabling the regulator control integrated circuit,
U3.
AC line voltage is applied to the input of transformer T3
which then steps down the voltage. This voltage is then recti-
fied and filtered into a DC voltage by D10 and C18. Resistor
R27 sets the response time of the filter to decreasing line
voltages. The resultant DC voltage is directly proportional to
the value of AC line voltage being seen by the charging cir-
cuitry. The DC voltage is then divided by the series combi-
nation of R28, R29, and R30. Potentiometer R29 is used to
adjust for transformer winding ratio tolerances from unit to
unit and is factory set. Capacitor C14 provides addition fil-
tering of the line sense voltage.
The same sense voltage signal provides information for
both the overvoltage and undervoltage sensing circuitry.
U8A provides current buffering to eliminate degradation of
the signal. This buffered signal is then applied to the over-
voltage comparator U7B. When a line overvoltage condition
is sensed, the output of U7B, normally a high impedance,
becomes a very low impedance. This low impedance re-
moves base drive to transistor Q7. When Q7 loses base drive
it turns off allowing the shutdown pin of U3 to go high and
disabling drive to the series pass transistors. This effectively
turns off all charging current to the battery.
Figure 1 - 344A3168 Charger
With the four-in-parallel gel cell battery system, if one gel
cell becomes defective before the other three, the cus-
tomer can run with only three gel cells in parallel (with re-
duced air time, of course). It is not advisable to run with
only two gel cells in parallel because of excessive charge
current from the charger which would damage the gel
cells. It is good practice when one gel cell battery be-
comes defective to replace all four gel cell batteries be-
cause of uneven charge and discharge characteristics of
new versus old gel cells. For that same reason it is also ad-
visable not to mix different brands of gel cells.
NOTE
Qty. Type Usable Capacity Full Capacity
1 55 Amp-Hour Auto Battery 4.4 Hours 6.0 Hours
4 25 Amp-Hour Gel Cell Batteries 8.0 Hours 11.0 Hours
Potentiometer R29 is specifically adjusted per internal
factory specs to set the proper trip voltages to send the
charger into the SHUTDOWN mode if the limits are ex-
ceeded. With the line voltage and the line frequency set
at nominal values, R29 is adjusted for 3.24
±
0.02 V(@
230 Vac 50Hz) or 3.20
±
0.02 Vdc (@ 121 Vac 60 Hz) at
TP1. AN IMPROPER TUNING OF R29 COULD
CAUSE THE CHARGER’S PASS TRANSISTORS TO
DISSIPATE EXCESSIVE HEAT, RESULTING IN
LOWERED RELIABILITY. THERE SHOULD BE NO
NEED TO ADJUST R29 IN THE FIELD.
CAUTION
LBI-38625
3