-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-2665K 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 if 100VDC, it is reduced to 100mV DC by
selecting a 1000:1 divider.
Should the input be
100VDC, 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-2665K 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 circuit assembly.
Section E - Capacitance and transistor testing
circuit assembly.
Section F - Final assembly.
1. The function of the A/D converter is to . . .
A. convert digital to analog.
B. divide analog signal by 2.
C. convert analog to digital.
D. convert AC to DC.
2. What type of divider network is used for voltage
measurements?
A. Divide by 20.
B. Capacitance.
C. Divide by 5.
D. Resistor.
3. When the AC voltage is measured, it is first . . .
A. divided down by 2.
B. converted to DC.
C. coupled to a halfwave rectifier.
D. low voltage.
4. When measuring current, the shunt resistors
convert the current to . . .
A. +0.190 to –0.190.
B. –1.199 to +1.199.
C. –0.099 to +0.099.
D. –199 to +0.199.
5. Which IC drives the LCD?
A. 358.
B. LM324.
C. 7106R.
D. 1N5232.
6. Resistance measurements are made by . . .
A. comparing voltage drops in the unknown
resistor and a reference resistor.
B. measuring the current in the unknown
resistor.
C. measuring the current in the reference
resistor.
D. equalizing the voltage drop in the unknown
and reference resistor.
7. Measurement cycles performed by the A/D
converter can be divided into what types of time
periods?
A. Long, short.
B. Auto zero, integrate, read.
C. Zero, read, interphase.
D. Autozero, read, cycle phase.
8. A resistor with band colors green-black-green-
brown-green is what value?
A. 50.5k
Ω
+ 5%.
B. 5.15k
Ω
+ 10%.
C. 5.05k
Ω
+ .5%.
D. 5.05k
Ω
+ 1%.
9. When checking a transistor, the selector knob
should be in the . . .
A. farad position.
B. ohm position.
C. diode position.
D. h
FE
position.
10. Where do the leads need to be on the meter
when measuring 450mA?
A. A20, COM.
B. V, COM.
C. A, A20.
D. A, COM.
-34-
QUIZ
Answer
s:
1.C
, 2.
D, 3.
B, 4.
B, 5.
C, 6.
A, 7.B
, 8.
C, 9.
D, 10.
A
