Display; Acpowersupply; Batterychargingcircuit - Keithley 177 Instruction Manual

4.9 DISPLAY
Five light-emitting diodes (LED) are driven by lJ203, which is
a CMOS BCD-to-seven segment decoder/driver with bipolar
current-sourcing outputs. Segment currents are. limited to ap-
proximately 20mA peak by resistor network R202. The LED
readout is a multiplexed, common-cathode configuration
with Darlington array U202 sequentially sinking current from
each digit. Blanking of the overrange digit is accomplished by
gates UlO4C and U104D. U104A and U104B ensure that
CMOS-compatible levels are maintained on U104D.
regardless of the loading of U202. The minus polarity readout
is blanked on AC voltage and resistance ranges by normally
open contacts on the AC and 0 pushbuttons.
4.10 AC POWER SUPPLY
When the Model 177 is operated from AC line power, the
power supply furnishes +5, + 15, and -15V from regulators
VR,lOl, VR102 and VR103, respectively. Full-wave rectified
AC from bridge rectifiers CR103 and CR104 is filtered by
capacitors C120, Cl23 and Cl25 and is applied to the linear
voltage regulators.
4.11 MODEL 1788 BAlTERY PACK
When the Model 1788 Battery Pack is installed in the Model
177. S102 must be set to the BAT position to provide addi-
tional secondan/ voltage for battery charging. SlO2 also swit-
ches the input to VRlOl from bridge rectifier CR103 to bat-
teries BT301. Four 2V. 2.5 ampere-hour lead-acid calls supply
approximately 9.8V at full charge. After six hours of use on
battery power, the battery pack should be recharged to en-
sure long battery life.
4.11.1
Battery Charging Circuit
While the Model 177 is plugged into line power and the bat-
tery pack is installed, battery charging proceeds as follows:
I, Full-wave rectified from CR103 is applied to the
anode of 0301, which is an SCR which regulates charg-
ing voltage. When Q301 is triggered on by a sufficient
gate-cathode voltage differential, the batteries receive
charge. Charging continues as long as the bridge output
voltage exceeds battery voltage by 1V or more. Resistor
R304 limits charging currant when recharging a set of
completely discharge calls, a filtered positive output from
CR104 (or T301) provides the necessary gate turn-on bias
through I7306 and diode CR301. Resistor R303 ensures
proper high-temperature operation of CI301.
2. When the battery voltages reach the present float voltages
of 9.8V, zener VR301 conducts sufficient current to turn
on Q302 and thus remove the gate trigger voltage from
Q301. Float voltage is adjusted with R301. This is a factory
adjustment which will normally not need field readjust-
ment.
4.11.2 Battery Operation and Shutdown Circuit
The Model 177 operates as follows on battery power:
I. When the power is turned on, the batteries are connected
to the input of VRlOl to supply + 5V for the logic, display
and the clock circuit. The clock output is applied to the
A/D converter as described in paragraph 4.7, and U301
which is a divide-by-four binary counter. The outputs of
U301 drive a DC-to-DC inverter which is sychronized to the
A/D converter to filter out inverter noise. The 25kHz
operating frequency is optimal for the small transformer
size and results in low switching losses. Blocking
capacitors C301 and C302 protect 0307 and 0308 from
damage if the drive is lost. Two half-wave rectifiers (CR304
and CR3051 on the secondary of T301 provide rectified AC
to filter capacitors C304 and C305, which provide power to
+ 15 and -15V regulators VR102 and VR103, respectively.
2. To prevent permanent loss of battery capacity caused by
deep discharge, a shutdown circuit stops operation on bat-
tery power when the battery voltage drops below approx-
imately 7.2V. Shutdown is performed by micropower
voltage detector U302. The open-collector output (U302,
pin 41 saturates low and turns off pass transistor Q309
when the input voltage
(at
U302. pin 31 drops below 1.15V
(typical). Resistor R314 provides sufficient hysteresis to
prevent discharge from resuming when the battery voltage
rises following disconnection of the load.
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