Extech Instruments 420 User manual

User's Guide
Digital Multimeter
Extech 420

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Introduction
Congratulations on your purchase of the Extech 420 (part number EX420) Autoranging
Multimeter. This meter measures AC/DC Voltage, AC/DC Current, Resistance, Capacitance,
Frequency, Duty Cycle, Diode Test, and Continuity plus Thermocouple Temperature. Proper
use and care of this meter will provide many years of reliable service.
Safety
International Safety Symbols
This symbol, adjacent to another symbol or terminal, indicates the user must refer
to the manual for further information.
This symbol, adjacent to a terminal, indicates that, under normal use, hazardous
voltages may be present
Double insulation
This WARNING symbol indicates a potentially hazardous situation, which
if not avoided, could result in death or serious injury.
This CAUTION symbol indicates a potentially hazardous situation, which
if not avoided, may result damage to the product.
This symbol advises the user that the terminal(s) so marked must not be
connected to a circuit point at which the voltage with respect to earth
ground exceeds (in this case) 600 VAC or VDC.
WARNING
CAUTION
MAX
600V

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CAUTIONS
Improper use of this meter can cause damage, shock, injury or death. Read and
understand this user manual before operating the meter.
Always remove the test leads before replacing the battery or fuses.
Inspect the condition of the test leads and the meter itself for any damage before
operating the meter. Repair or replace any damage before use.
Use great care when making measurements if the voltages are greater than 25VAC
rms or 35VDC. These voltages are considered a shock hazard.
Warning! This is a class A equipment. This equipment can cause interferences in the
living quarters; in this case the operator can be required to carry out adequate
measures.
Always discharge capacitors and remove power from the device under test before
performing Diode, Resistance or Continuity tests.
Voltage checks on electrical outlets can be difficult and misleading because of the
uncertainty of connection to the recessed electrical contacts. Other means should be
used to ensure that the terminals are not "live".
If the equipment is used in a manner not specified by the manufacturer, the protection
provided by the equipment may be impaired.
This device is not a toy and must not reach children’s hands. It contains hazardous
objects as well as small parts that the children could swallow. In case a child
swallows any of them, please contact a physician immediately
Do not leave batteries and packing material lying around unattended; they can be
dangerous for children if they use them as toys
In case the device is going to be unused for an extended period of time, remove the
batteries to prevent them from training
Expired or damaged batteries can cause cauterization on contact with the skin.
Always, therefore, use suitable hand gloves in such cases
See that the batteries are not short-circuited. Do not throw batteries into the fire.

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OVERVOLTAGE CATEGORY III
This meter meets the IEC 610-1-2001 standard for OVERVOLTAGE CATEGORY III. Cat III
meters are protected against overvoltage transients in fixed installation at the distribution
level. Examples include switches in the fixed installation and some equipment for industrial
use with permanent connection to the fixed installation.
SAFETY INSTRUCTIONS
This meter has been designed for safe use, but must be operated with caution. The rules
listed below must be carefully followed for safe operation.
1. NEVER apply voltage or current to the meter that exceeds the specified maximum:
Input Protection Limits
Function Maximum Input
V DC or V AC
10
00V DC
/
750V
AC
,
200Vrms on 4
00mV range
mA
AC/
DC
5
00
mA
250V fast acting fuse
AAC/DC 20A 250V fast acting fuse(30 seconds max every
15 minutes)
Frequency, Resistance, Capacitance,
Duty Cycle, Diode Test, Continuity 250Vrms for 15sec max
Temperature 60V DC/24V AC
2. USE EXTREME CAUTION when working with high voltages.
3. DO NOT measure voltage if the voltage on the "COM" input jack exceeds 600V above
earth ground.
4. NEVER connect the meter leads across a voltage source while the function switch is in
the current, resistance, or diode mode. Doing so can damage the meter.
5. ALWAYS discharge filter capacitors in power supplies and disconnect the power when
making resistance or diode tests.
6. ALWAYS turn off the power and disconnect the test leads before opening the covers to
replace the fuse or battery.
7. NEVER operate the meter unless the back cover and the battery and fuse covers are in
place and fastened securely.

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Controls and Jacks
1. 4000 count LCD display
2. HOLD
3. SELECT button
4. Function switch
5. mA, uA and A input jacks
6. COM input jack
7. Positive input jack
8. RANGE hold button
9. RELATIVE button
10. Backlight button
11. Protective holster
Note: Tilt stand and battery compartment are on rear of unit.
Symbols and Annunciators
•))) Continuity
Diode test
Battery status
Test lead connection
error
n nano (10-9) (capacitance)
µ micro (10-6) (amps, cap)
m milli (10-3) (volts, amps) A Amps
k kilo (103) (ohms) F Farads (capacitance)
M mega (106) (ohms)  Ohms
Hz Hertz (frequency) V Volts
% Percent (duty ratio) REL Relative
AC Alternating current AUTO Autoranging
DC Direct current HOLD Display hold
ºF Degrees Fahrenheit ºC Degrees Centigrade

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Operating Instructions
WARNING: Risk of electrocution. High-voltage circuits, both AC and DC, are very
dangerous and should be measured with great care.
1. ALWAYS turn the function switch to the OFF position when the meter is not in use.
2. If “OL” appears in the display during a measurement, the value exceeds the range you
have selected. Change to a higher range.
NOTE: On some low AC and DC voltage ranges, with the test leads not connected to a
device, the display may show a random, changing reading. This is normal and is caused
by the high-input sensitivity. The reading will stabilize and give a proper measurement
when connected to a circuit.
DC VOLTAGE MEASUREMENTS
CAUTION: Do not measure DC voltages if a motor on the circuit is being switched ON or
OFF. Large voltage surges may occur that can damage the meter.
1. Set the function switch to the green V position.
2. Press the SELECT button to indicate “DC” on the display.
3. Insert the black test lead banana plug into the negative COM
jack.
Insert the red test lead banana plug into the positive Vjack.
4. Touch the black test probe tip to the negative side of the
circuit.
Touch the red test probe tip to the positive side of the circuit.
5. Read the voltage in the display.

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AC VOLTAGE MEASUREMENTS
WARNING: Risk of Electrocution. The probe tips may not be long enough to contact the
live parts inside some 240V outlets for appliances because the contacts are recessed
deep in the outlets. As a result, the reading may show 0 volts when the outlet actually has
voltage on it. Make sure the probe tips are touching the metal contacts inside the outlet
before assuming that no voltage is present.
CAUTION: Do not measure AC voltages if a motor on the circuit is being switched ON or
OFF. Large voltage surges may occur that can damage the meter.
1. Set the function switch to the green V position.
2. Press the SELECT button to indicate “AC” on the display.
3. Insert the black test lead banana plug into the negative COM
jack.
Insert red test lead banana plug into the positive Vjack.
4. Touch the black test probe tip to the neutral side of the circuit.
Touch the red test probe tip to the “hot” side of the circuit.
5. Read the voltage in the display.
DC CURRENT MEASUREMENTS
CAUTION: Do not make current measurements on the 20A scale for longer than 30
seconds. Exceeding 30 seconds may cause damage to the meter and/or the test leads.
1. Insert the black test lead banana plug into the negative
COM jack.
2. For current measurements up to 4000µA DC, set the
function switch to the yellow µA position and insert the
red test lead banana plug into the µA/mA jack
3. For current measurements up to 400mA DC, set the
function switch to the yellow mA position and insert the
red test lead banana plug into the µA/mA jack.
4. For current measurements up to 20A DC, set the
function switch to the yellow 20A range and insert the
red test lead banana plug into the 20A jack.
5. Press the SELECT button to indicate “DC” on the
display.
6. Remove power from the circuit under test, then open up
the circuit at the point where you wish to measure
current.
7. Touch the black test probe tip to the negative side of the
circuit.
Touch the red test probe tip to the positive side of the circuit.
8. Apply power to the circuit.
9. Read the current in the display.

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AC CURRENT MEASUREMENTS
CAUTION: Do not make current measurements on the 20A scale for longer than 30
seconds. Exceeding 30 seconds may cause damage to the meter and/or the test leads.
1. Insert the black test lead banana plug into the negative
COM jack.
2. For current measurements up to 4000µA AC, set the
function switch to the yellow µA position and insert the
red test lead banana plug into the µA/mA jack
3. For current measurements up to 400mA AC, set the
function switch to the yellow mA position and insert the
red test lead banana plug into the µA/mA jack.
4. For current measurements up to 20A AC, set the
function switch to the yellow 20A range and insert the
red test lead banana plug into the 20A jack.
5. Press the SELECT button to indicate “AC” on the
display.
6. Remove power from the circuit under test, then open up
the circuit at the point where you wish to measure
current.
7. Touch the black test probe tip to the negative side of
the circuit.
Touch the red test probe tip to the positive side of the
circuit.
8. Apply power to the circuit.
9. Read the current in the display.
RESISTANCE MEASUREMENTS
WARNING: To avoid electric shock, disconnect power to the unit under test and
discharge all capacitors before taking any resistance measurements. Remove the
batteries and unplug the line cords.
1. Set the function switch to the green  position.
2. Insert the black test lead banana plug into the negative
COM jack.
Insert the red test lead banana plug into the positive jack.
3. Press the SELECT button to indicate on the display.
4. Touch the test probe tips across the circuit or part under test.
It is best to disconnect one side of the part under test so the
rest of the circuit will not interfere with the resistance
reading.
5. Read the resistance in the display.

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CONTINUITY CHECK
WARNING: To avoid electric shock, never measure continuity on circuits or wires that
have voltage on them.
1. Set the function switch to the green position.
2. Insert the black lead banana plug into the negative COM jack.
Insert the red test lead banana plug into the positive jack.
3. Press the SELECT button to indicate on the display.
4. Touch the test probe tips to the circuit or wire you wish to
check.
5. If the resistance is less than approximately 150, the audible
signal will sound. If the circuit is open, the display will indicate
“OL”.
DIODE TEST
1. Set the function switch to the green position.
2. Insert the black test lead banana plug into the negative COM
jack and the red test lead banana plug into the positive
diode jack.
3. Press the SELECT button to indicate on the display.
4. Touch the test probes to the diode under test. Forward
voltage will typically indicate 0.400 to 0.700V. Reverse
voltage will indicate “OL”.. Shorted devices will indicate near
0V and an open device will indicate “OL” in both polarities.

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CAPACITANCE MEASUREMENTS
WARNING: To avoid electric shock, disconnect power to the unit under test and
discharge all capacitors before taking any capacitance measurements. Remove the
batteries and unplug the line cords.
1. Set the rotary function switch to the green position.
2. Insert the black test lead banana plug into the negative
COM jack.
Insert the red test lead banana plug into the positive jack.
3. Touch the test leads to the capacitor to be tested.
4. Read the capacitance value in the display
FREQUENCY MEASUREMENTS
1. Set the rotary function switch to the green “Hz” position.
2. Insert the black lead banana plug into the negative COM
jack and the red test lead banana plug into the positive Hz
jack.
3. Touch the test probe tips to the circuit under test.
4. Read the frequency on the display.
% DUTY CYCLE
1. Set the rotary function switch to the “Hz” position.
2. Insert the black lead banana plug into the negative COM
jack and the red test lead banana plug into the positive Hz
jack.
3. Press the SELECT key momentarily to select % in the
display.
4. Touch the test probe tips to the circuit under test.
5. Read the % duty cycle on the display.