Fluke ScopeMeter 123 Service Manual page 37

3
Circuit Descriptions
3.3 Detailed Circuit Descriptions
line and filter R534-C534 to pin 80. A control loop in the control circuit adjusts the
actual charge current to the required value.
The filtered CHARCUR voltage range on pin 80 is 0... 2.7V for a charge current from
0.5A to zero. A voltage of 0V complies to 0.5A (fast charge), 1.5V to 0.2A (top off
charge), 2.3V to 0.06A (trickle charge), and 2.7V to 0A (no charge). If the voltage is > 3
Volt, the charger converter is off (V506 permanently non-conductive).
The D-ASIC derives the required charge current value from the battery voltage VBAT.
The P-ASIC converts this voltage to an appropriate level and supplies it to output pin 78
(BATVOLT). The D-ASIC measures this voltage via the Slow ADC. The momentary
value, and the voltage change as a function of time (-dV/dt), are used as control
parameters.
Charging process
If the battery voltage drops below 5.2V, and the battery temperature is between 10 and
45°C, the charge current is set to 0.5A (fast charge). From the battery voltage change -
dV/dt the D-ASIC can see when the battery is fully charged, and stop fast charge.
Additionally a timer in the D-ASIC limits the fast charge time to 6 hours. After fast
charge, a 0.2A top off charge current is supplied for 2 hours. Then a 0.06A trickle
charge current is applied for 48 hours maximum. If the battery temperature becomes
higher than 50°C, the charge current is set to zero
Battery temperature monitoring
The P-ASIC supplies a current to a NTC resistor in the battery pack (TEMP pin 5). It
conditions the voltage on pin 5 and supplies it to output pin 79 BATTEMP. The D-ASIC
measures this voltage via the slow ADC. It uses the BATTEMP voltage to decide if fast
charge is allowed (10-45°C), or no charge is allowed at all (<10°C, >50°C).
Additionally the temperature is monitored by the P-ASIC. The P-ASIC supplies a
current to reference resistor R512 (TEMPHI pin 4), and compares the resulting TEMPHI
voltage to the voltage on pin 5 (TEMP). If the battery temperature is too high, the P-
ASIC Control circuit will set the charge current to zero, in case the D-ASIC fails to do
this.
If the battery temperature monitoring system fails, a bimetal switch in the battery pack
interrupts the battery current if the temperature becomes higher then 70 °C
Maximum VBAT
The P-ASIC supplies a current to reference resistor R513 (VBATHIGH pin 7). It
compares the voltage on R513 to the battery voltage VBAT on pin 3 (after being
attenuated in the P-ASIC). The P-ASIC limits the voltage VBAT to 7.4V via its internal
Control circuit. This situation arises in case no battery or a defective battery (open) is
present.
Charger/Converter input current
This input current is sensed by R501. The P-ASIC supplies a reference current to R514.
The P-ASIC compares the voltage drop on R501 (CHASENSP-CHASENSN pin 14 and
15) to the voltage on R514 (IMAXCHA pin 6). It limits the input current (e.g. when
loading C503 and C555 just after connecting the power adapter) via its internal Control
circuit.
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