Ericsson MASTR III 344A3168P1, Руководство по обслуживанию

Страница: 5 / 14

Charger overvoltage sensing depends on whether the
charger is in the trickle or full charge mode. The first mode
of operation is for a trickle charge condition in which the
battery charging current is less than 200 milliamps. The al-
lowable line voltage range for this mode of operation is
±
20% (P1) or
±
15% (P2). The second mode of operation is
for charging of the battery at current up to the maximum out-
put current of 10 amps. The line voltage tolerance range for
this mode of operation is
±
10%. It is critical for the charger
to be able to operate over both ranges yet protect itself from
excessive series pass transistor heat dissipation.
The method by which the charger discerns the appropri-
ate line voltage tolerance is by monitoring the amount of
charging current flowing through the current sense resistors
R4, R5, and R6. The current sense voltage developed across
these resistors is applied across the differential amplifier of
U8B. The output of U8B is normally biased at 2.5 Vdc.
However, as the amount of current through the sense resis-
tors increases, (sense voltage increases) the output of U8B
begins to decrease. At approximately 6 amps of charging
current, the output of U8B is low enough to trip the output
of comparator U6B. This normally low impedance output
goes to a high impedance state removing resistor R41 from
its parallel placement with resistor R40. With R40 removed,
the line voltage sense voltage is now only divided by the se-
ries combination of resistors R38 and R40. This decreases
the amount of line sense voltage needed to trip the overvol-
tage comparator U7B. This sets up the
±
10% line voltage
tolerance range. When the charging current is less than 6
amps, the output of comparator U6B remains a low imped-
ance, placing R41 in parallel with R40 and setting up the
±
20% (
±
15%) line voltage tolerance.
OVERTEMPERATURE PROTECTION
To protect the charger from abnormal ambient tempera-
ture operating conditions it is equipped with overtemperature
protection. A thermostat, S1, has been attached to the heat
sink in order to monitor the operating temperature of the se-
ries pass transistors. This thermostat is normally closed.
When an abnormal operating temperature is reached, the
thermostat’s switch contacts open and remove base drive to
Q7. As described earlier, removing base drive to Q7 causes
shutdown of the charging regulator. When the temperature of
the heat sink returns to a safe value, the contacts of S1 close
and operation resumes.
BATTERY SWITCHING OPERATION
When the AC line voltage drops below 70% of nominal
voltage, the charger interrupts the charging mode of opera-
tion and switches the battery on line for emergency power
operation. The AC line voltage is sensed as described under
overvoltage protection. The U8A buffered sense voltage is
applied to undervoltage comparator U7A. When the sense
voltage drops below the specified limit, the normally low
impedance output of U7A switches to a high impedance
state. This interrupts base drive current being sourced from
transistor Q5. With the interruption of its base drive current,
Q5 shuts off removing the 24 volt signal coming from U4.
Switching transistor Q5 off also removes drive to relay
K1 and K3. With removal of relay drive the contacts associ-
ated with K1 and K3 switch to their normally closed states.
When this happens, battery charging current can no longer
flow through K1 to the battery. With the K1 relay contacts in
their normally closed position, current flows from the battery
through K2 to the system. In addition, the K3 relay contacts
open thus isolating the switched ground signal from ground.
DEEP DISCHARGE PROTECTION
The battery voltage is sensed at the charger’s A + port.
This voltage provides both bias voltage and signal input to
voltage comparator U6A. The battery voltage is stepped
down by resistors R20 and R21 and compared with the 2.5
Vdc output of voltage reference U5. When the base station
reverts to emergency power, the fully charged battery voltage
starts at around 12.7 Vdc and slowly drops as the battery dis-
charges. The output of U6A remains an open circuit, allow-
ing pull up resistors R24 and R26 to provide drive to Q6 to
energize relay K2. When the battery has discharged to
around 10.5 Vdc the voltage comparator output pulls low,
disabling the drive to Q6 and de-energizing K2. The charger
and base station remain "in limbo" until the line voltage is
restored to the station power supply and charger. The charger
A + port must be greater than 12.25 Vdc before K2 is re-en-
ergized to await the next emergency power state.
ERROR FLAG SIGNALS
In the event of abnormal system operation, the charger
provides two error flag signals as output to the system con-
troller. The first, SHUTDOWN, is an indication of emer-
gency power operation. During normal operation from AC
line voltage the 24 volt signal from transistor Q5 is present
at connector J2-1. When the system is in the emergency power
mode, this signal is removed and becomes a high impedance,
greater than one Megohm.
The second error flag is provided at connector J2-2. This
signal, SHUTDOWN, indicates whether the charger is operat-
ing normally or has been shutdown due to excessive operating
temperature or high line voltage. During normal operation, the
24 VDC output of voltage regulator U4 is applied through R15
to and U3. Due to the U3 interface, this voltage becomes ap-
proximately 3 volts. This signal is independent of transistor
Q5. During shutdown of the charger, this output is pulled down
by transistor Q7 becoming an active low impedance signal.
With removal of the shutdown conditions, this signal automat-
ically returns to the normal operating state.
INTERNAL BIAS VOLTAGE SOURCES
There are three internal bias voltage sources implemented
within the charger to provide internal housekeeping supply
voltages and references. The first is the previously mentioned
24 volt source provided by voltage regulator U4. This source is
supplied directly off the AC line via transformer T1 and T2 as
well as rectifier D6 and filter C10. This source supplies power
for regulator control integrated circuit U3, relays K1, K2 and
also the error flag output signal.
The second source is provided by linear regulator U1. This
regulator supplies power for all the additional control circuitry.
It also acts as a buffer between the control circuitry and the un-
regulated charging voltage from which it draws its own power.
Voltage excursions on the charging bliss are not transmitted to
the control circuitry thus insuring the charger’s ability to pro-
tect itself.
Lastly, is the precision voltage reference provided by regu-
lator U2. This precision 2.5 volt source creates the reference
voltage that is used by all the AC line voltage comparators. The
ability to hold reasonable tolerances on line voltage sense point
requires the use of a high tolerance reference voltage.
FLOAT VOLTAGE ADJUST
Both the automobile batteries and gel cells used with the
new charger are lead acid based batteries. The chemical reac-
tion rates for converting the electrical energy to stored chemi-
cal energy during charging are functions of temperature.
Nominally either battery should be seeing a float voltage of
13.6 Vdc when the ambient temperature of the battery is 77°F
(25° C). As the temperature increases past nominal room tem-
perature the chemical reaction rates increase past nominal and
the float voltage should be lowered to compensate. Conversely,
the float voltage should be increased if the batteries are to see
an average temperature of less than 77°F, but the float voltage
should never exceed 14.4 Vdc. For a nominal 12 Vdc, 6 cell,
lead acid battery the slope of float voltage versus ambient tem-
perature is around -18 mV/°F (-32.4 mV/°C).
By use of the proper float voltage optimum battery usage
can be obtained. If the float voltage is set too high, the battery
can be overcharged, resulting in outgassing and reduction in
lifetime of the battery. If the float voltage is set too low the bat-
tery recharges at a slower rate but more importantly the battery
will permanently lose some of its storage capacity. Most manu-
facturers recommend a float voltage between 13.5 and 13.8
Vdc at room temperature.
The factory has preset the float voltage for 13.6 Vdc. If the
battery will be in an environment where the AVERAGE ambi-
ent temperature will not be 77°F, then the option exists to ad-
just the FLOAT VOLTAGE ADJUST pot for optimum float
voltage. Cable W1 (19B801937P3) is removed from J1 of
charger 344A3168 to present an open circuit load. A high im-
pedance DC voltmeter is attached to -1 (A +) and J1-2 (A-)
and the pot adjusted for optimum float voltage.
The first 344A3168P1, Rev. 0 chargers do not have re-
verse polarity protection designed into the circuitry.
This may allow blowing at transistor Q6 if customers
accidentally reverse the battery cable leads. This pre-
vents the charger from properly switching relay K2 for
emergency power. A production change on
344A3168P1, Rev. A chargers and all 344A3168P2
chargers adds a diode similar to a 1N4004 to the collec-
tor of Q6. Contact Ericsson GE Technical Support for
any additional information.
NOTE
LBI-38625
4