Schiller CARDlOVlT AT-6 Service Manual page 81

The current limit threshold at the input of the 3524 current sense comparator monitors the voltage drop
across the 0.12R sense resistor, the activation threshold is circuit defined to be active (regulator shut-off)
by the condition U.sense
= 2
than 200mV. Thus when the power supply provides a maximum available
current of 1.6 A, voltage drop of 200 mV across R19 (0.12R) would therefore correspond to I= U/R
=
1.667 Adc.
NOTE: The lead-acid battery is automatically charged, even
if
the AT-6 has been switched off (key-
board ON/OFF fnc). Unless the main switch (green front switch) has been switched off, the
charging circuit is inoperate.
3. Charging Circuit Ovetvoltage Protection
In order to protect the battery from an excessive charging voltage (lead-acid batteries are very sensitive
to overvoltage), the circuit also consists of a power input crowbar which short-circuits the DC input
(18..30 Vdc) thus tripping Si2 (1.25 A). The circuit works by means of sensing the output charging voltage
at the output of diode D4 (cathode, see schematic point B). OpAmp U4b is connected as a voltage
follower (impedance converter) and feeds the "sensed" voltage back to the input of OpAmp U4a (compa-
rator) via voltage divider network (R20
& R21).
The comparator reference corresponds to 5 Vdc and is
generated by the LM 336 bandgap reference circuit Z5.0V. As soon as the charging voltage exceeds
215.8 Vdc, the potential at the comparator non-inverting input (pin 3) will correspond
to
5Vdc thus causing
the comparator output to saturate the gate of crowbar SCR T2 (thyristor TIC
11
6, point B on the sche-
matic power supply MK2-6 '13.8V Batt charger). As a result of firing the SCR, the DC unregulated output
will instantly short to ground thus blowing Si2 fuse.
4.
Temperature Compensation
The charging circuit is also temperature compensated by means of an NTC thermistor resistance (R11)
connected to the voltage feedback path, if the ambient
to
increases, the NTC resistance decreases thus
affecting the divider network and reducing the output voltage charge.
NOTE: The NTC (negative temperature coefficient) resistance value is given by 1.5
k ! 2
at room tem-
perature (20°C).
5.
Battery Charge Calibration
Lead-acid batteries are high quality, maintenance-free accumulators and have a long operational life in
standby or cyclic applications. Lead-acid batteries are charged by a constant VOLTAGE, unlike NiCd
cells which are charged by a constant current regardless of the voltage. The manufacturer of these
batteries specifies that the EMF charge to the element is defined as 13.6 Vdc at 23°C (see the table
'U
=
f(to)l. The calibration of the correct charging voltage requires great care and should not be considered
as a
5
minute job (see service manual procedure), otherwise the life-time of the accumulator will be
greatly reduced. The fundamental conditions for a successful calibration are:
1. Record the ambient temperature with a digital temperature instrument by placing the sensor in the
case of the AT-6 (in the vicinity of the NTC thermistor). The reading should be stable and within 0.5"
of max. tolerance.
CAUTION: make sure that the temperature probe does not touch any components or heat sink of
power devices, otherwise the probe or the circuit may be destroyed.
2. The calibration of the specific charge voltage (according to the tables) has to be done with the battery
connected to the machine, the battery acts as a current regulator (variable load) !
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