87 V/AN
Service Manual
10
Low
Pass
Filter
Function
Selection
&
Overload
Protection
V/Ohms
mA/
µ
A
AMPS
Common
RMS
Converter
Back-
light
Beeper
Keypad
Reference
Power
Supply
Analog
Signal
Conditioning
Analog
to
Digital
Converter
Microprocessor
&
Support
Circuits
Active
Filter
Secondary
Analog
Circuits
LCD
ama101f.eps
Figure 1. 87 V/AN Block Diagram
Power Supply and Voltage Reference
A 9-V alkaline battery supplies power to the Meter and is connected via a cable assembly
to J3 on the A1 printed circuit assembly (PCA). Diode CR3 protects the Meter from
damage due to accidental polarity reversal at J3.
Q13 is used as a remote power switch that connects the battery to the power supply
regulators when the Meter is turned on. When S1 is in the OFF position, Q14 is off to
allow the battery voltage to turn off Q13 via R73. When the Meter is turned on, S1
makes a momentary contact to the battery voltage via R80, turning on Q14, which turns
on Q13, which connects the battery voltage to R74, which holds Q14 until S1 is returned
to the OFF position.
When Q13 is on, battery voltage is applied to regulators U6 and U10, bypass capacitors
C12 and C14 and the low battery detect divider resistors R15 & R16. Note that since the
power supply levels are referenced to the COMMON input terminal of the Meter, the
negative end of the battery is the -2.5 V power supply.
U6 is a 3.3-V regulator that generates the +0.8-V (-2.5 V + 3.3 V) power supply. U10 is
a 5.0-V regulator that generates the +2.5-V (-2.5 V + 5.0 V) power supply. U10 is
