Electronics
Voltage and Its Measurements
Copyright © Texas Education Agency, 2013. All rights reserved.
1
Lesson Overview










Terms and Definitions
Common Voltage Sources
Voltage Symbols or Abbreviations and Definitions
Principal Parts of a Voltmeter
Procedures for Using a Voltmeter
Ohm’s Law
Formulas for Voltage Drops in Resistive Circuits
Polarity in a Resistive Circuit
Kirchhoff’s Law of Voltage
Current Flow in a Resistive Circuit
Copyright © Texas Education Agency, 2013. All rights reserved.
2
Terms and Definitions




Voltage- electrical force or
pressure that causes the flow of
electrical current (electrons)
Volt- the unit of measurement of
electromotive force (Note: One
volt forces one ampere of
current through one ohm of
resistance)
Voltage drop- difference in
voltage measured across a
component in a circuit
Voltmeter- instrument used to
measure voltage
Copyright © Texas Education Agency, 2013. All rights reserved.
3
Common Voltage Sources
Common Voltage Sources



Batteries
Generators/alternators
Electronic power supplies
Other Alternatives




Static
Friction
Heat
Light
Copyright © Texas Education Agency, 2013. All rights reserved.
4
Voltage Symbols or Abbreviations
and Definitions








EMF (or emf)- electromotive force
(Note: EMF is the same as voltage)
E (or e)- voltage source or applied voltage
V- voltage or voltage drop
kV- kilovolt (one thousand volts)
MV- megavolt (one million volts)
mV- millivolt
(one thousandth of a volt)
µV- microvolt
(one millionth of a volt)
VM- voltmeter
Copyright © Texas Education Agency, 2013. All rights reserved.
5
Basic Electrical Measurement

Using handheld
electronic test
tools
Copyright © Texas Education Agency, 2013. All rights reserved.
6
Types of Measurement

The most common electrical measurements





Voltage
Resistance
Current
Each of these types of measurement has
different characteristics.
Newer meters measure many more things.
Copyright © Texas Education Agency, 2013. All rights reserved.
7
Measurement Devices

Historically, devices could measure only one
electrical characteristic.




Voltmeters, ammeters, meggers, etc.
Different types of measurement require a specific
circuit.
Newer meters combine all these functions into a
single device.
This device is called a digital multimeter, or
DMM.
Copyright © Texas Education Agency, 2013. All rights reserved.
8
Principal Parts of a Voltmeter
(Multimeter)

Test leads or probes


Red is positive
Black is negative or
ground
Copyright © Texas Education Agency, 2013. All rights reserved.
9
Principal Parts of a Voltmeter
(Multimeter)

Function Switches
 Voltage, current,
resistance
 Direct current,
alternating current,
 Measurement range
Copyright © Texas Education Agency, 2013. All rights reserved.
10
Function Switch Positions
Copyright © Texas Education Agency, 2013. All rights reserved.
11
Function Switch Positions
Volts AC
Alternating Current
Copyright © Texas Education Agency, 2013. All rights reserved.
12
Function Switch Positions
Volts DC
Direct Current
Copyright © Texas Education Agency, 2013. All rights reserved.
13
Function Switch Positions
Resistance
Continuity
Copyright © Texas Education Agency, 2013. All rights reserved.
14
Function Switch Positions
Current
AC or DC
Copyright © Texas Education Agency, 2013. All rights reserved.
15
Current Measurement
Switch the lead
from here
Copyright © Texas Education Agency, 2013. All rights reserved.
16
Current Measurement
Switch the lead
To here
Copyright © Texas Education Agency, 2013. All rights reserved.
17
Principal Parts of a Voltmeter
(Multimeter)

An analog meter has
multiple use scales

Range switch (to select
proper range)

Note: A range position should be
selected, when possible, for
middle-third region of a scale
where the meter is most
accurate.

If unknown voltage, start at high
range and work down one step
at a time.
Copyright © Texas Education Agency, 2013. All rights reserved.
18
Principal Parts of a Voltmeter
(Multimeter)
Copyright © Texas Education Agency, 2013. All rights reserved.
19
Voltage Measurement

The circuit is energized


Two types of measurement



Dangerous voltages may exist!
The measurement is taken across a component using
both test leads
Measurement from a test point to a reference point
(like ground) using both test leads
The meter is NOT part of the circuit
Copyright © Texas Education Agency, 2013. All rights reserved.
20
Resistance Measurement



The circuit is de-energized.

The meter provides an internal reference voltage.

Caution: measuring an energized circuit can be
dangerous or lead to error.
The component to be measured may need to be
removed from the circuit.
The meter has an internal reference circuit that
the component value is compared to.
Copyright © Texas Education Agency, 2013. All rights reserved.
21
Current Measurement



The meter needs to be set up before the
measurement takes place.
Leads are moved to the Amp position.
With circuit power off, the meter leads are
placed in series with the circuit.


A component lead or jumper may need to be removed
to allow this.
Once leads are set up and the proper current
range is selected, the circuit is energized.
Copyright © Texas Education Agency, 2013. All rights reserved.
22
Current Measurement

The meter becomes part of the circuit.


Current is the same everywhere in a series circuit.
NEVER measure across a component when the
meter is in current mode.


The meter has low internal resistance in this
measurement mode.
If you do, the internal fuse WILL blow.

A common problem with multimeters is the blown fuse,
which will prevent future current measurement.
Copyright © Texas Education Agency, 2013. All rights reserved.
23
Other Measurements


Continuity
Diode test




also used for transistors
Temperature
Capacitance
Frequency
Copyright © Texas Education Agency, 2013. All rights reserved.
24
Procedures for Using a Voltmeter
(Multimeter)
1.
2.
3.
4.
Hold probes by insulated part.
Select AC or DC voltage.
Set range switch for correct
range.
Use correct polarity of leads
or probes.
A.
negative or common probe
(black) toward negative of
power supply
B.
positive probe (red) toward
positive of power supply
Copyright © Texas Education Agency, 2013. All rights reserved.
25
Procedures for Using a Voltmeter
(Multimeter)
5.
Connect voltmeter in parallel with load.
Copyright © Texas Education Agency, 2013. All rights reserved.
26
Procedures for Using a Voltmeter
(Multimeter)
6.
7.
8.
Touch the probe
tips to the circuit.
Read voltage on
meter.
Remove probes.
Copyright © Texas Education Agency, 2013. All rights reserved.
27
Follow The Law!


All electrical measurements are based on Ohm’s
Law.
Ohm’s Law describes the relationships between
voltage, current, and resistance.
Copyright © Texas Education Agency, 2013. All rights reserved.
28
The Ohm’s Law Circle


V
I
Cover the value you want
to solve for.
The formula to use is the
two remaining values.
V=IxR
R
I=
=
I
Copyright © Texas Education Agency, 2013. All rights reserved.
Formulas for Voltage Drops in
Resistive Circuits
Copyright © Texas Education Agency, 2013. All rights reserved.
30
Polarity in a Resistive Circuit

End nearer the negative of supply is negative
(use black lead)

End nearer the positive of supply is positive
(use red lead)
Copyright © Texas Education Agency, 2013. All rights reserved.
31
Kirchhoff’s Law of Voltage
The algebraic sum of the voltage drops
around a closed loop must equal the applied
voltage.

Ea = VR1 + VR2 + VR3 +…
Copyright © Texas Education Agency, 2013. All rights reserved.
32
Current Flow in a Resistive Circuit


Negative to positive
Resultant potential across resistance (voltage drop)
Copyright © Texas Education Agency, 2013. All rights reserved.
33
How DMMs Measure Voltage





There is an electronic gate inside the meter that opens
and closes while taking the measurement.
The time the gate is “open” is a function of the amount
of voltage being measured (e.g., the larger the voltage,
the longer the gate is held open).
These pulses are counted and the larger the count, the
greater the voltage being measured.
The output is then presented on an LED or LCD display.
This process works for both voltage and current.
Copyright © Texas Education Agency, 2013. All rights reserved.
34
Voltage Measurement Summary
1.
2.
3.
4.
5.
Select AC or DC voltage as necessary.
The black (ground) test probe goes into the COM
input connection; the red test probe goes into the V
input connection.
If the DMM has manual ranging only, select the
highest range, so as not to overload the input.
Touch the probe tips to the circuit across a load or
power source (in parallel to the circuit).
View the reading, being sure to note units.
Copyright © Texas Education Agency, 2013. All rights reserved.
35
AC Voltage



AC voltage is constantly changing.
We need a number to represent the value.
There are several types of values used



Peak to peak
Average
RMS
Copyright © Texas Education Agency, 2013. All rights reserved.
RMS vs. Average


RMS is the Root Mean Square or effective
heating value of any AC voltage or current
waveform.
RMS is the equivalent DC heating value of an
AC waveform.

Power consumed is the same for both AC and DC
sources when the Vacrms equals Vdc.

Average is the DC equivalent voltage of the AC
signal.
Copyright © Texas Education Agency, 2013. All rights reserved.
Why RMS?


The RMS voltage value determines how much
heat the voltage amount will create in a
component.
RMS is an AC voltage value that creates the same
heat as the same DC voltage value.
5 VRMS = heat = 5 VDC

Components need to have a power rating that is
the same for AC or DC voltage.
38
Copyright © Texas Education Agency, 2013. All rights reserved.
Measuring Resistance
1.
2.
3.
4.
5.
Turn the power OFF to the circuit.
Select the resistance setting (Ω).
Plug the black test probe into the COM input jack; plug
the red test probe into the Ω input jack.
Connect the probe tips across the component or portion
of the circuit for which you want to measure the
resistance value.
View the reading, and make sure to note the unit of
measurement.

ohms (Ω), kilohms (kΩ), or megohms (MΩ).
39
Copyright © Texas Education Agency, 2013. All rights reserved.
Current Measurement


An ammeter has low resistance in the current
measurement mode.
Current measurements are made with the circuit
energized.



creates safety hazards
makes it difficult to measure current
For these reasons, current is often measured
with a different type of probe.

clamp on ammeter
40
Copyright © Texas Education Agency, 2013. All rights reserved.
Measuring Current
1.
2.
3.
4.
Turn off power to the circuit.
Cut or unsolder the circuit, creating a place
where the meter probes can be inserted.
Select A~ (AC) or A (DC) as desired.
Plug the black test probe into the COM input jack.
Plug the red test probe into the amp or milliamp
input jack (depending on the expected value of
the reading).
41
Copyright © Texas Education Agency, 2013. All rights reserved.
Measuring Current (continued)
5.
6.
7.
8.
Connect the probe tips to the circuit across
the break so that all current will flow through
the DMM (a series connection).
Turn the circuit power back on.
View the reading, being sure to note the unit
of measurement.
Unplug the leads when done.
42
Copyright © Texas Education Agency, 2013. All rights reserved.
DMM Display

A DMM display is not like a calculator.


A multimeter that can display four digits could
historically have only a 1 in the leftmost (or most
significant) decimal place.


A four digit calculator can read from 0 to 9999.
If the MSD value was not 1, the digit is left blank (only
three digits are used).
This type of display is called a 3 ½ digit display.
43
Copyright © Texas Education Agency, 2013. All rights reserved.
Example

3 ½ digit display
44
Copyright © Texas Education Agency, 2013. All rights reserved.
Example

199.9 volts is displayed using all four digits.
)
45
Copyright © Texas Education Agency, 2013. All rights reserved.
Example


199.9 volts is displayed using all four digits.
200 volts is displayed using only three digits with
no decimal place (the “1” space is left blank).
46
Copyright © Texas Education Agency, 2013. All rights reserved.