# Voltage and Its Measurements

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Voltage and Its Measurements

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, All rights reserved.

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, All rights reserved.

Common Voltage Sources

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, All rights reserved.

Basic Electrical Measurement

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, All rights reserved.

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, All rights reserved.

Principal Parts of a Voltmeter (Multimeter)

Principal Parts of a Voltmeter (Multimeter)

Function Switch Positions

Function Switch Positions

Function Switch Positions

Function Switch Positions
Resistance Continuity Image source: Continuity uses a buzzer sound to indicate a low resistance or short circuit. A user does not have to look at the meter, which can help with safety because the user does not have to look away from the circuit. Copyright © Texas Education Agency, All rights reserved.

Function Switch Positions

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, All rights reserved.

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 Internal meter resistance is high (ideally infinite). Copyright © Texas Education Agency, All rights reserved.

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. Internal meter resistance is medium. The known internal resistance forms a reference by which the circuit component resistance is compared. Copyright © Texas Education Agency, All rights reserved.

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. Internal meter resistance is low (ideally zero). Copyright © Texas Education Agency, All rights reserved.

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. Internal meter resistance is low because the meter is not supposed to change the amount of circuit current. The meter forms a series circuit with the circuit being measured, and resistance adds in a series circuit. If the meter were to add resistance, circuit current would decrease below the normal value. Copyright © Texas Education Agency, All rights reserved.

Other Measurements Continuity Diode test also used for transistors Temperature Capacitance Frequency Continuity is like a resistance measurement. A continuity test uses a buzzer sound to indicate that a circuit has continuity, which means there is a circuit path (low resistance) between the leads when taking a measurement. The sound is an easy way to tell if a fuse or other component is good or if there is a ground or short. You don’t have to look at the meter for the measurement, which can be safer because you are looking at the circuit and where the leads are placed instead of the meter. Copyright © Texas Education Agency, All rights reserved.

Procedures for Using a Voltmeter (Multimeter)
Hold probes by insulated part. Select AC or DC voltage. Set range switch for correct range. Use correct polarity of leads or probes. negative or common probe (black) toward negative of power supply positive probe (red) toward positive of power supply With analog meters, you must observe proper polarity or the meter might be damaged. With a newer digital meter, polarity can be reversed without damage and the meter will indicate either positive or negative values. Copyright © Texas Education Agency, All rights reserved.

Procedures for Using a Voltmeter (Multimeter)

Procedures for Using a Voltmeter (Multimeter)

The Ohm’s Law Circle Cover the value you want to solve for. The formula to use is the two remaining values. V V = I x R I R I = = I Copyright © Texas Education Agency, All rights reserved.

Formulas for Voltage Drops in Resistive Circuits

Polarity in a Resistive Circuit

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, All rights reserved.

Current Flow in a Resistive Circuit

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, All rights reserved.

Voltage Measurement Summary
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, All rights reserved.

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 Objective- Explain what is meant by average-sensing. Note, RMS and average voltage are AC equivalent voltage values. A DC voltage has only one value: the DC voltage. The average value of a sine wave = .637 x peak. The rms value of a sine wave = .707 x peak. The ratio .707 / .637 = 1.11. The circuitry to turn AC into DC average is relatively simple and inexpensive, hence the lower cost of average sensing meters. Image source: Copyright © Texas Education Agency, 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. Objective- Explain RMS AC voltage is not steady, so average and RMS voltages are used to indicate how much AC voltage is present over several cycles. Nominal voltages in distribution systems are given in RMS (e.g., 120 V, 208 V, 230/240 V, 480 V, 575 V, etc. are all RMS values). Equipment sizing relies on RMS values. Copyright © Texas Education Agency, 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. Components need to have a power rating that is the same for AC or DC voltage. 5 VRMS = heat = 5 VDC The key is that a component will have a power rating, not a voltage rating. The power rating needs to be a single value whether the voltage used is DC or AC. Copyright © Texas Education Agency, All rights reserved.

Measuring Resistance 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). Copyright © Texas Education Agency, 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 Copyright © Texas Education Agency, All rights reserved.

Measuring Current 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). Copyright © Texas Education Agency, All rights reserved.

Measuring Current (continued)
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. Copyright © Texas Education Agency, All rights reserved.

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. The 1 in the MSD position was historically used to save money and space for the display (no need to decode all of the decimal values, and a 1 takes less space than an 8). This type of display then became a characteristic of a multimeter and, to this day, remains even though we are no longer concerned with those limitations. Copyright © Texas Education Agency, All rights reserved.