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Electrical Skills

Voltage and Its Measurements

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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

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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

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Common Voltage Sources

Common Voltage SourcesBatteriesGenerators/alternatorsElectronic power supplies

Other AlternativesStaticFrictionHeatLight

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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

Basic Electrical Measurement

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Using handheld electronic test tools

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.

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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.8

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Principal Parts of a Voltmeter (Multimeter)

Test leads or probes Red is positive Black is negative or

ground

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Principal Parts of a Voltmeter (Multimeter) Function Switches

Voltage, current, resistance

Direct current, alternating current,

Measurement range

Function Switch Positions

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Function Switch Positions

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Volts ACAlternating Current

Function Switch Positions

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Volts DCDirect Current

Function Switch Positions

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Resistance

Continuity

Function Switch Positions

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Current

AC or DC

Current Measurement

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Switch the leadfrom here

Current Measurement

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Switch the lead

To here

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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.

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Principal Parts of a Voltmeter (Multimeter)

Voltage Measurement The circuit is energized

Dangerous voltages may exist! Two types of measurement

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

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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.

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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.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.

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Other Measurements Continuity Diode test

also used for transistors Temperature Capacitance Frequency

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Procedures for Using a Voltmeter (Multimeter)1. Hold probes by insulated part.2. Select AC or DC voltage.3. Set range switch for correct

range.4. 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

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Procedures for Using a Voltmeter (Multimeter)

5. Connect voltmeter in parallel with load.

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Procedures for Using a Voltmeter (Multimeter)

6.Touch the probe tips to the circuit.7.Read voltage on meter.8.Remove probes.

Follow The Law! All electrical measurements are based on Ohm’s

Law. Ohm’s Law describes the relationships between

voltage, current, and resistance.

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V

I R =

= I

I

The Ohm’s Law Circle Cover the value you want to solve for. The formula to use is the two

remaining values.

V = I x R

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Formulas for Voltage Drops in Resistive Circuits

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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)

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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 +…

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Current Flow in a Resistive Circuit Negative to positive Resultant potential across resistance (voltage drop)

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.

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Voltage Measurement Summary

1. Select AC or DC voltage as necessary.2. The black (ground) test probe goes into the COM

input connection; the red test probe goes into the V input connection.

3. If the DMM has manual ranging only, select the highest range, so as not to overload the input.

4. Touch the probe tips to the circuit across a load or power source (in parallel to the circuit).

5. View the reading, being sure to note units.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

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.

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.

Components need to have a power rating that is the same for AC or DC voltage.

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5 VRMS = heat = 5 VDC

Measuring Resistance

1. Turn the power OFF to the circuit.2. Select the resistance setting (Ω).3. Plug the black test probe into the COM input jack; plug

the red test probe into the Ω input jack.4. Connect the probe tips across the component or portion

of the circuit for which you want to measure the resistance value.

5. View the reading, and make sure to note the unit of measurement.

ohms (Ω), kilohms (kΩ), or megohms (MΩ).39

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

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Measuring Current

1. Turn off power to the circuit.2. Cut or unsolder the circuit, creating a place

where the meter probes can be inserted.3. Select A~ (AC) or A (DC) as desired.4. 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).

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Measuring Current (continued)

5. Connect the probe tips to the circuit across the break so that all current will flow through the DMM (a series connection).

6. Turn the circuit power back on.7. View the reading, being sure to note the unit

of measurement.8. Unplug the leads when done.

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DMM Display A DMM display is not like a calculator.

A four digit calculator can read from 0 to 9999. A multimeter that can display four digits could

historically have only a 1 in the leftmost (or most significant) decimal place. 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.

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Example 3 ½ digit display

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Example 199.9 volts is displayed using all four digits.)

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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).

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