Elenco 3 1/2 Digit Cap. / Trans. Kit - Manual (Page 35)

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SCHEMATIC DIAGRAM
-1-
INTRODUCTION
Range
Switches
Current
Shunts
AC
Converter
Ohms
Converter
Voltage
Divider
Function
Switches
A/D
Converter
and
Display
Driver
Display
Decimal
Point
DC
Analog
Data
V
Ω
VAC
VAC/mA AC
mA
mA
Ω
mA
COMM
V/
Ω
V
Figure 1 Simplified Block Diagram
THEORY OF OPERATION
A block diagram of the M-2666K is shown in Figure 1.
Operation centers around a custom LSI chip. This IC
contains a dual slope A/D converter, display, latches,
decoder and the display driver. A block diagram of
the IC functions is shown in Figure 6. The input
voltage, current or ohm signals are conditioned by the
function and selector switches to produce and output
DC voltage between 0 and +199mV. If the input signal is 100VDC, it is reduced to 100mVDC by selecting a
1000:1 divider. Should the input be 100VAC, then
after the divider it is processed by the AC converter to
produce 100mVDC. If current is to be read, it is
converted to a DC voltage via internal shunt resistors.
For resistance measurements, an internal voltage
source supplies the necessary 0-199mV voltage to be
fed to the IC input.
The input of the 7106 IC is fed to an A/D (analog to
digital) converter. Here the DC voltage amplitude is
changed into a digital format. The resulting signals
are processed in the decoders to light the appropriate
LCD segment.
Timing for the overall operation of the A/D converter
is derived from an external oscillator whose
frequency is selected to be 40kHz. In the IC, this
frequency is divided by four before it clocks the
decade counters. It is further divided to form the three
convert-cycle phases. The final readout is clocked at
about three readings per second.
Digitized measurements data is presented to the
display as four decoded digits (seven segments) plus
polarity. Decimal point position on the display is
determined by the selector switch setting.
Assembly of your M-2666K Digital Multimeter Kit will
prove to be an exciting project and give much
satisfaction and personal achievement. If you have
experience in soldering and wiring technique, you
should have no problems. For the beginner, care
must be given to identifying the proper components
and in good soldering habits. Above all, take your
time and follow the easy step-by-step instructions.
Remember, “An ounce of prevention is worth a
pound of cure”. The meter kit has been divided into a number of
sections to make the assembly easy and avoid
major problems with the meter operation.
Section A - Meter display circuit assembly.
Section B - DC voltage and current circuit
assembly.
Section C - AC voltage and current circuit
assembly.
Section D - Resistance & buzzer circuit assembly.
Section E - Capacitance and transistor testing
circuit assembly.
Section F - Final assembly.