Steps for measuring ac voltage with a digital multimeter. How to measure ac voltage 1. Turn the dial to ṽ. Some digital multimeters (DMMs) also include m ṽ . If voltage in the circuit is unknown, set the range to the highest voltage setting and set the dial on ṽ. Note: Most multimeters power up in Autorange mode. This automatically selects a measurement range based on voltage present. 2. First insert the black lead into the COM jack. 3. Next insert the red lead into the VΩ jack. When finished, remove the leads in reverse order: red first, then black. 4. Connect the test leads to the circuit: black lead first, red second. Note: ac voltage does not have polarity. Caution: Do not let fingers touch the lead tips. Do not allow the tips to contact one another. 5. Read the measurement in the display. When finished, remove the red lead first, black second. Other useful functions when measuring ac voltage 6. Press the RANGE button to select a specific fixed measurement range.

7. Press the HOLD button to capture a stable measurement. It can be viewed after the measurement is complete. 8. Press the MIN/MAX button to capture the lowest and highest measurement. The DMM beeps each time a new reading is recorded. 9. Press the relative (REL) button to set the multimeter to a specific reference value. Measurements above and below the reference value are displayed. Note: Avoid this common and serious mistake: inserting test leads into incorrect input jacks. Doing so can lead to a dangerous arc flash. If measuring ac voltage, be certain to insert the red lead into the input jack marked V, not A. The display should show the ṽ symbol. Placing test leads in A or MA inputs and then measuring voltage will create a short in the measurement circuit. Analysis of ac voltage measurements In general, all ac voltage sources vary from fluctuation in ac voltage over power distribution systems. When different from an expected measurement, voltage is more likely to be lower than normal.

Generally speaking, voltage measured in ac power systems should be within -10% and +5%. Voltage measurements taken at various points in a system vary. Refer to the chart below. Point of Use Range Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers. An in-depth look at an indispensable tool How to measure frequency How to measure capacitance How to test for continuity How to test diodes How to measure current with a clamp accessory How to measure duty cycle How to measure resistance Why ruggedness matters in a digital multimeter How to measure dc voltage with a digital multimeter Accuracy, resolution, range, counts, digits and precision How to verify electrical test tool operation with a proving unit How to troubleshoot a pump control panel with a thermal multimeter How to validate a pump control panel repair with a thermal multimeter What is a thermal multimeter?

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The following table gives a list of some of the standard electrical units of measure used in electrical formulas and component values. Unit of Electrical PotentialV = I × R Unit of Electrical CurrentI = V ÷ R Unit of DC ResistanceR = V ÷ I Reciprocal of ResistanceG = 1 ÷ R Unit of CapacitanceC = Q ÷ V Unit of Electrical ChargeQ = C × V Unit of InductanceVL = -L(di/dt) Unit of PowerP = V × I  or  I2 × R Unit of AC ResistanceZ2 = R2 + X2 Unit of Frequencyƒ = 1 ÷ T There is a huge range of values encountered in electrical and electronic engineering between a maximum value and a minimum value of a standard electrical unit. For example, resistance can be lower than 0.01Ω’s or higher than 1,000,000Ω’s. By using multiples and submultiple’s of the standard unit we can avoid having to write too many zero’s to define the position of the decimal point. The table below gives their names and abbreviations.

So to display the units or multiples of units for either Resistance, Current or Voltage we would use as an example: –  which is equal to 1,000 Volts. –  which is equal to one thousandths (1/1000) of an Ampere. –  which is equal to 47 thousand Ohms. –  which is equal to 100 millionths (1/1,000,000) of a Farad. –  which is equal to 1,000 Watts. –  which is equal to one million Hertz. To convert from one prefix to another it is necessary to either multiply or divide by the difference between the two values. For example, convert 1MHz into kHz. Well we know from above that 1MHz is equal to one million (1,000,000) hertz and that 1kHz is equal to one thousand (1,000) hertz, so one 1MHz is one thousand times bigger than 1kHz. Then to convert Mega-hertz into Kilo-hertz we need to multiply mega-hertz by one thousand, as 1MHz is equal to 1000 kHz. Likewise, if we needed to convert kilo-hertz into mega-hertz we would need to divide by one thousand.

A much simpler and quicker method would be to move the decimal point either left or right depending upon whether you need to multiply or divide. As well as the “Standard” electrical units of measure shown above, other units are also used in electrical engineering to denote other values and quantities such as: •  Wh – The Watt-Hour, The amount of electrical energy consumed by a circuit over a period of time. Eg, a light bulb consumes one hundred watts of electrical power for one hour. It is commonly used in the form of: Wh (watt-hours), kWh (Kilowatt-hour) which is 1,000 watt-hours or MWh (Megawatt-hour) which is 1,000,000 watt-hours. •  dB – The Decibel, The decibel is a one tenth unit of the Bel (symbol B) and is used to represent gain either in voltage, current or power. It is a logarithmic unit expressed in dB and is commonly used to represent the ratio of input to output in amplifier, audio circuits or loudspeaker systems. For example, the dB ratio of an input voltage (Vin) to an output voltage (Vout) is expressed as 20log10 (Vout/Vin).

The value in dB can be either positive (20dB) representing gain or negative (-20dB) representing loss with unity, ie input = output expressed as 0dB. •  θ – Phase Angle, The Phase Angle is the difference in degrees between the voltage waveform and the current waveform having the same periodic time. It is a time difference or time shift and depending upon the circuit element can have a “leading” or “lagging” value. The phase angle of a waveform is measured in degrees or radians. •  ω – Angular Frequency, Another unit which is mainly used in a.c. circuits to represent the Phasor Relationship between two or more waveforms is called Angular Frequency, symbol ω. This is a rotational unit of angular frequency 2πƒ with units in radians per second, rads/s. The complete revolution of one cycle is 360 degrees or 2π, therefore, half a revolution is given as 180 degrees or π rad. •  τ – Time Constant, The Time Constant of an impedance circuit or linear first-order system is the time it takes for the output to reach 63.7% of its maximum or minimum output value when subjected to a Step Response input.