Download DVOM Not-So Basics - Myers Equipment Corp

DVOM Not-So Basics
Digital High Impedance VOM
Meter Selection Features
• Voltage DC
V
• DC is Direct Current
• Reads the circuit pressure
• Best meter for troubleshooting
• Voltage AC
~
V
• AC is Alternating Current
• Use for Magnetic Pickup Outputs
Meter Selection Features
• Diode Check
• Applies voltage to turn on Diode in Forward
Bias
• .3 to .5v in one direction and OL in the other
means the diode is OK
• Resistance / Ohms Ω
• Last meter to use
• OL is open or over the range of the meter
• 0.1 – 0.3Ω circuit is shorted or completed
Meter Selection Features
• Frequency Hz
• Measures the Hertz or pulses, cycles per sec.
• Percent Duty Cycle %
• Measures the percent “on” time for Pulse
Width Modulated signals
• Continuity Buzzer
• “Beep” to indicate continuity (maybe)
• Be careful – some meters will sound at 100Ω
Meter Selection Features
• Amperes (Amps) A
• Measures the current through the circuit
• CAUTION - The probes complete the circuit
• MilliAmperes
mA
• Same as Amps only lesser current
• Auto Range
• Auto is easier – Manual is harder
Digital High Impedance VOM
Probe Sockets
• COM Black cord goes here
• VΩ Red cord here for Voltage and Resistance
• A
• Red cord here for Amps – 10A - 20A
• Fuse Protected
•
mA
• Red cord here for Lower Amps
• Protected with a smaller fuse
Probe Safety
Voltage Check 1
12.5v
Open
M
12.5
V
-
+
Since there is no
current flow, there
is no voltage drop
Electricity Compared to Hydraulics
12.5
• Voltage = Pressure
V
-
+
.15
• Current = Flow (gpm)
A
-
+
240
• Resistance = Restriction
Ω
* Electricity behaves very much like fluid *
+
Electricity Compared to Hydraulics
Battery = Reservoir + Pump
Voltmeter = Pressure Gauge
Ammeter = Flow meter
Load/Resistance = Load/Restriction
* Electricity behaves very much like fluid *
Pressure Check 1
1000psi
Pump w/1000psi
internal relief valve
Control valve
1000psi
Motor
blocked
P
M
1000psi
Reservoir
Since there is no flow,
there is no pressure drop
Voltage Check 2
12.5v
Completed Circuit
M
12.5
12.47
0.00
12.45
V
-
+
1200 RPM
Voltage Drop is normal on
Non-Load components as
long as the maximum drop
is ≤ 0.5v on 12v systems
Pressure Check 2
1000psi
Pump w/1000psi
internal relief valve
Control valve
P
998psi
Motor
M
0psi
Reservoir
Minor pressure drops across NonLoad components are normal
1200 RPM
Voltage Check 3
12.5v
Green Connection
M
12.5
12.47
9.45
V
-
+
300 RPM
Voltage Drop that is
Significantly less than full
system voltage indicates
High Resistance
Pressure Check 3
1000psi
Pump w/1000psi
internal relief valve
Control valve
275psi
Motor
P
Reservoir
M
Major pressure drops across
Non-Load components indicate
restriction
300 RPM
Pressure Check - Example
20psi
P
18psi
0psi
Pressure drop across filter
indicates the level of Restriction
Tank
Other Concepts of Operation
• Ghost voltage & zero voltage indicate
opens and shorts and can be used in
place of an ohmmeter
• A digital VOM will indicate full system
voltage, even through high resistance
• Continuity buzzer will buzz or beep even if
there is up to 100Ω in circuit – must check
ohms to verify
• Circuits must be isolated to check ohms
Ghost Voltage
Ghost
The meter is so sensitive
that it is picking up voltage
from the atmosphere
V
-
+
Your body even generates
an electrical field!!
Ghost Voltage
0.00
V
-
+
This is eliminated by
touching the probes together
giving you Zero volts.
Voltage Check 4
12.5v
+-
0.00
0.00
V
-
+
If the meter reads
“True Zero” you
have Continuity
V
-
+
Voltage Check 5
Open Connector
12.5v
+-
Blown Fuse
Ghost
V
-
+
Ghost Voltage
indicates that you
have an Open
Circuit
Ghost
V
-
+
Voltage Checks Review
12.55
Could you make these checks
with an Ohmmeter??
V
-
+
12.55 V
Ghost
V
-
0.00
+
V
-
+
Voltmeters
12.5v
V
-
+
Voltmeters are the safest
of all meters. They don’t
complete the circuit. The
meter has an extremely
high internal resistance or
impedance and therefore
draws extremely low
amperage.
Low Amps – Low Danger.
Other Concepts of Operation
• Ghost voltage & zero voltage indicate
opens and shorts and can be used in
place of an ohmmeter
• A digital VOM will indicate full system
voltage, even through high resistance
• Continuity buzzer will buzz or beep even if
there is up to 100Ω in circuit – must check
ohms to verify
• Circuits must be isolated to check ohms
High Resistance
12.55
Without Current Flow
There is No Voltage Drop
V
-
+
12.55 V
12.55
V
-
+
Corroded Terminal
Voltage Check 3
12.5v
(Review)
Green Connection
M
High Resistance causes a
Significant Voltage Drop but
only if there is Current Flow
9.45
V
-
+
Problem:
How do I insert the meter’s
Probe into the circuit?
Why NOT to Puncture the Wire
1. Puncture the Insulation
2. Remove the Probe, a Hole
is Left
Moisture
3. As the Wire Heats, the
Hole allows the Insulation
to Expand
4. As the Wire & Insulation
Cools and Heats, it Draws
in Moisture thought the
Opening
5. Corrosion Develops
High Resistance Checking Tool
TESlite Voltmeter Leads
TESlite ® Enhanced Voltmeter Leads - www.teslite.com
High Resistance Checking Tool
Without Current Flow there is No Voltage Drop
12.55
3.75
Corroded Terminal
V
-
+
TESlite ®
Leads
12.55 V
PUSH the Button
Now a Circuit is Completed, there is Load,
Current flows and Voltage Drop can be read
First Corrosion Check
Corrosion
12.55v
4.75v
V
-
1. Remove load
2. Test across Load Connection
3. Low Voltage – Bad Circuit
M
4. Retest w/ Good Ground
+
5. Low Voltage – Power Circuit is Bad
6. If Good Voltage – Ground is Bad
M
Power Side Corrosion Check
Corrosion
12.55v
12.53v
12.52
3.78v
12.55v
M
V
-
+
TESlite ®
Leads
Ground Side Corrosion Check
12.55v
12.53v
4.75v
V
-
+
M
Corrosion
Other Concepts of Operation
• Ghost voltage & zero voltage indicate
opens and shorts and can be used in
place of an ohmmeter
• A digital VOM will indicate full system
voltage, even through high resistance
• Continuity buzzer will buzz or beep even if
there is up to 100Ω in circuit – must check
ohms to verify
• Circuits must be isolated to check ohms
Continuity “Beeper”
To safely check for Continuity,
use the Ohmmeter function or
Voltmeter, don’t trust the Beeper
BEEP
100.0Ω
Ω
-
+
Other Concepts of Operation
• Ghost voltage & zero voltage indicate
opens and shorts and can be used in
place of an ohmmeter
• A digital VOM will indicate full system
voltage, even through high resistance
• Continuity buzzer will buzz or beep even if
there is up to 100Ω in circuit – must check
ohms to verify
• Circuits must be isolated to check ohms
Ohmmeter Limitations
• For an Ohmmeter to work accurately you MUST
isolate the circuit – EVERYWHERE HOW??
∞
• “OL” or
indicates that the meter is not able to
read the resistance in the circuit. Go to a higher
scale.
• Ohmmeters use about 3 volts to operate – over 50
feet of wire this is not much to work with
• Minimum Resistance is usually about 0.3 – 0.5 Ω
which can approach actual values on some
components
Ohmmeters
OL
2.03Ω
254.0Ω
87.0Ω
Sensor
Ω
-
+
Ohmmeters are used for
checking separate
components for shorts,
opens or proper values.
However, it is not the
best troubleshooting
tool for circuits.
M
Relay
Motor
Troubleshooting Relays
75 – 110 Ω
Coil Resistance
(12V Estimates)
87A
87
86
85 30 87 87A 86
20 – 40 Ω
30
85
~ 130 Ω
Why use a Relay??
If I Short 87 to 30
What Would Happen?
87A
12 V+
87
Horn
30
86
(Socket)
87
87A
85
85 30 87 87A 86
86
30
(1Ω)
85
(80Ω)
A Volt Meter
across Socket
positions 85 & 86
should show
what when the
switch is closed ?
85 & 86 = Coil
30 = Common
87 = N.O.
87A = N.C.
A=V/Ω
Ohm’s Law
Switch A=12/80 = .15A
Horn
A=12/1 = 12A
Troubleshooting Relays
(Base of Relay)
87
87A
86
85
30
With the Battery Across 85 & 86
30 to 87 Should Read 0Ω
30 to 87A Should Read OL
With the Battery Removed
30 to 87 Should Read OL
30 to 87A Should Read 0Ω
87A
9 Volt
87
86
30
85
Troubleshooting Micro Relays
(Base of Relay)
With the Battery on Brass Pins
Copper Pins Should Read 0Ω
With the Battery Removed
Copper Pins Should Read OL
9 Volt
Ammeters
9.0a
A
-
+
M
Ammeters complete a
circuit. It is a jumper.
Whatever is connected
will become active. It
is therefore the most
dangerous meter. The
meter is protected with
a fuse but you aren’t.
Amps Kill.
Relay Amperage Ratings
12V
30A/40A
Coil Voltage - 12 Volts
85 30 87 87A 86
30A for N.C. Contacts
40A for N.O. Contacts
Checking Amp Flow with a Compass
Align the needle with the cable to start
Current Flow
Current Flow
Crank
Negative
Magnetic
North
Run
We just proved that the
alternator is charging
Power
Power Off
On
Solenoid Electro-Magnetic Actuator
Arc Suppression in Relays
-
+
+
( 700Ω )
No Polarity
The Diode
Points to the
Positive Wire
85 30 87 87A 86