UEi Test Instruments M20 - Manual (Page 2)

Introduction
The M20 has features you need for testing residential electrical and
HVAC/R systems. With its four AC and four DC voltage scales you can
measure typical control and source voltages with high resolution. It also
includes a low-ohm scale to test critical connections and a milliamps
function to verify many flame safeguard circuits.
Features include
• 12 ranges
• 500 Volts AC and DC
• DC milliamps
• Fused ohm circuit
• Resistance to 500 kilohms
• Color-coded and mirrored scale plate
Safety Notes
Before using this meter, read all safety information carefully. In
this manual the word "WARNING" is used to indicate conditions
or actions that may pose physical hazards to the user. The word
"CAUTION" is used to indicate conditions or actions that may
damage this instrument.
Operating Instructions
AC Voltage
Insert one test lead in the “+VΩM” jack. Insert the other test lead in the
“COM” jack. Set the function switch to the appropriate AC voltage.
NOTE: Always start with the 500V range if unsure of the magnitude of
voltage present.
CAUTION!
Always remove the test leads from the circuit under test before
disconnecting from front panel of M20.
DC Voltage
Insert the black test lead in the “COM” jack. Insert the red test lead in
to “+VΩM” jack. Set the function switch to the appropriate DC voltage.
The red test lead is connected to the positive voltage point. The black
test lead is connected to the negative voltage point.
NOTE: Always start with the 500V range if unsure of thee magnitude of
voltage present.
CAUTION!
Always remove the test leads from the circuit under test before
disconnecting from front panel of M20.
Direct Current
The M20 may be used to measure direct current up to a maximum
of 250 mA (0.250 Amps). To do this, the M20 must be connected in
series with the wire, or circuit element, in which the current is to be
measured. Remove power to the circuit under test before connecting
the M20. Set the function switch to the 250 mA position. Insert the black
test lead between the “COM” jack on the M20 and the ground, or low
voltage, side of the circuit test. Insert the red test lead between the
“+VΩM” jack on the M20 and the high voltage side of the circuit under
test. Apply power to the circuit under test.
Resistance
CAUTION!
Always remove power to any circuit in which resistance measurements
are to be made.
The M20 uses an internal battery to supply power to the circuit under
test. Access to the battery and the protective fuse is provided by
removing the single screw on the back of the case and removing the
case back. Observe polarity marking on battery.
Set the function switch to the appropriate OHMS setting, Rx1, Rx10, or
Rx100. Insert one test lead in the “COM“ jack and the other test lead
in the “+VΩM” jack. Touch the free ends of the test leads together and
note that pointer will swing to the right side of the scale. (Note: If the
pointer does not move all the way to the right the battery may be
weak and need replacing. If no reading can be made, check battery
and fuse) Use the green “OHM ADJ” knob to set the pointer to
zero on the green meter scale. This completes the calibration of the
resistance measuring circuit. This test should be performed each time
resistance tests are to be made to assure that the “OHM ADJ” knob
has not been inadvertently moved.
To make the resistance measurement, connect the free ends of the
test leads across the element to be measured. The measured resistance
value will be the green numeral on the resistance scale times the
resistance multiplier. For example, if the function switch is on Rx100
and the pointer is on the numeral 20, the resistance is 2,000 ohms
(100 x 20 = 2000).
Testing Diodes / Transistors
A simple check of diode or transistor quality may be made with the
M20. Using the same test procedure as for measuring resistance,
connect one test lead to one end of the diode and the other test lead to
the other end of the diode. Note the resistance reading. Then reverse
the test leads and again not the reading.
If the two readings differ by a factor of ten then the diode, (or transistor
junction) is probably good. If the two readings are approximately the
same then the diode is shorted. If a reading cannot be obtained in
either direction, the diode is probably open.
M20-MAN P. 1