Operating Digital Multimeters
Red Lead
(1) a volts, resistance, and diode measurement jack (for the red or "+" test lead).
Describe the two-step process a standard DMM uses to measure an AC voltage waveform.
A standard DMM measures an AC voltage waveform by: (1) sampling the high and low points of the measured waveform; and (2) multiplying the average voltage by 1.11 to obtain the effective or RMS voltage of the AC waveform
Sinusoidal AC waveform
A waveform with a sine wave shape; for example, the waveform of the 115 VAC input voltage to a broadband cable converter or power supply.
What are two reasons to manually select a fixed meter measurement range on a DMM?
Two reasons to manually select a fixed meter measurement range on a DMM are: (1) to potentially speed up the measurement when you know an approximate value ahead of time; and (2) to prevent too much current passing through the test leads and damaging a sensitive electronic circuit that is the subject of a resistance measurement.
loop length
dc loop resistance/ footage
total resistance
loop resistance + terminator resistance = total resistance
True root mean square (RMS) digital multimeter (DMM)
A digital multimeter that measures nonsinusoidal AC waveforms accurately.
Standard digital multimeter (DMM)
A digital multimeter that samples the high and low points of a measured waveform and multiplies the average voltage by 1.11 to obtain the effective AC root mean square (RMS) voltage.
Which DMM jacks should the red and black test leads be connected to for measuring AC voltage?
Connect the DMM black test lead to the common test lead jack, and connect the red test lead to the "volts" test lead jack for measuring AC voltage.
Which DMM jacks should the red and black test leads be connected to for measuring resistance?
Connect the DMM red test lead to the "ohms" test lead jack, and connect the black test lead to the common test lead jack, for measuring resistance.
measuring for ac voltage
If any of the readings are not proper, inform the customer that a licensed electrician must correctly wire the electrical AC outlet before you can plug any power cords into that outlet.
What type of batteries do most DMMs use?
Most DMMs use two, 1.5 volt AA batteries.
Name the three basic types of measurements made by a DMM.
The three basic types of measurements made by a DMM are voltage, resistance, and current.
List the three resistance specifications of a typical DMM.
The three resistance specifications of a typical DMM are: (1) maximum resistance; (2) maximum resolution; and (3) basic meter accuracy.
What is the function of a DMM's manual range selector switch?
A DMM's manual range selector switch steps through fixed meter measurement ranges.
Which of the following is a characteristic of a true RMS digital multimeter (DMM)?
Measures AC voltage through an analog computation by continuously sampling the entire waveform.
Which of the following is a characteristic of a standard digital multimeter (DMM)?
Samples only two points on the AC waveform.
what is the most common waveform measured ?
Sinusoidal AC waveform
List six maintenance precautions for DMMs.
Six maintenance precautions for DMMs are: (1) avoid storing the DMM where someone could step on it or drop it; (2) avoid excessive bending or stretching of the test leads; (3) avoid exposing the DMM to extreme heat, prolonged direct sunlight, or high humidity; (4) avoid using the DMM in wet conditions; (5) remove the DMM's batteries if no one will use it for an extended period of time; and (6) keep the DMM in a protective case when not in use.
List six measurement functions found in most DMMs.
Six measurement functions found in most DMMs are: (1) DC voltage; (2) DC amperes; (3) AC voltage; (4) AC amperes; (5) resistance; and (6) a diode measurement function.
Most DMMs have the following functions:
(1) DC voltage, for measuring in parallel with the voltage source; (2) DC amperes, for measuring the direct current in series with the current source; (3) AC voltage, for measuring in parallel with the source voltage; (4) AC amperes, for measuring the alternating current in series with the current source; and (5) a resistance measurement function for measuring resistance in parallel.
Making resistance measurements using a digital multimeter (DMM) on coaxial drop cables at the tap location can help determine which drop cable belongs to a particular customer's house. When trying to identify the desired customer drop, in what order should you perform the following steps?
(1) Terminate the drop cable at the ground block; (2) Calculate the expected total resistance of the drop cable with a terminator; (3) Test for voltage on each drop cable at the tap; and (4) Measure the resistance of each drop cable at the tap. Calculate the expected total resistance of the drop cable with a terminator; (2) Measure the resistance of each drop cable at the tap; (3) Test for voltage on each drop cable at the tap; and (4) Terminate the drop cable at the ground block. Calculate the expected total resistance of the drop cable with a terminator; (2) Test for voltage on each drop cable at the tap; (3) Measure the resistance of each drop cable at the tap; and (4) Terminate the drop cable at the ground block. (1) Terminate the drop cable at the ground block; (2) Calculate the expected total resistance of the drop cable with a terminator; (3) Measure the resistance of each drop cable at the tap; and (4) Test for voltage on each drop cable at the tap.
The DC volts specifications are:
(1) maximum DC voltage — the highest DC voltage the meter can measure; (2) maximum resolution — the smallest unit of DC voltage the meter can accurately measure; and (3) basic meter accuracy — the meter's accuracy in measuring a DC voltage (which is dependent on the voltage range selected).
The resistance specifications are:
(1) maximum resistance — the highest resistance the meter can accurately measure; (2) maximum resolution — the smallest unit of resistance the meter can accurately measure; and (3) basic meter accuracy — the meter's accuracy in measuring resistance (which is dependent on the resistance range selected).
The AC volts specifications are:
(1) maximum voltage (RMS) — the highest RMS AC voltage the meter can measure; (2) maximum resolution — the smallest unit of AC voltage the meter can accurately measure; (3) frequency range — the AC signal frequency range in which the meter can measure AC voltage; and (4) basic meter accuracy — the meter's accuracy in measuring AC voltage (which is dependent on the voltage range selected).
Two basic types of DMMs exist:
(1) the standard DMM A digital multimeter that samples the high and low points of a measured waveform and multiplies the average voltage by 1.11 to obtain the effective AC root mean square (RMS) voltage. , calibrated to accurately measure only the AC voltage of a stable sinusoidal waveform; and (2) the true root mean square (RMS) DMM A digital multimeter that measures nonsinusoidal AC waveforms accurately. , designed to measure the AC voltage on all types of waveforms accurately.
To perform accurate AC voltage measurements, you need to know:
(1) whether or not the AC voltage measured is a sinusoidal AC waveform A waveform with a sine wave shape; for example, the waveform of the 115 VAC input voltage to a broadband cable converter or power supply. ; and (2) which meter to choose to accurately measure the RMS AC voltage.
Which of the following is a typical digital multimeter (DMM) specification range?
(A)DC volts, maximum resistance - 2-40 VDC (B)DC volts, maximum voltage - 12 volts (RMS) (C)Resistance, maximum resistance - 1,000 VDC (D)AC volts, maximum voltage - 750-1,000 volts (RMS)
Which of the following is a characteristic of the manual range selector switch on a digital multimeter?
(A)Selects between AC or DC voltage. (B)Checks diodes and continuity from one point to another. (C)Adjusts for measurements that vary between distinct values. (D)Selects different internal circuits in the DMM for measuring different functions.
What is indicated by a DMM measurement of 115 VAC ±10% between the long receptacle slot and the AC wall plate screw, when checking the wiring polarity of a polarized two-slot AC receptacle?
A DMM measurement of 115 VAC ±10% between the long receptacle slot and the AC wall plate screw, when checking the wiring polarity of a polarized two-slot AC receptacle, indicates the hot and neutral wires are reversed.
How does a true RMS DMM obtain the correct average voltage of any AC waveform?
A true RMS DMM continuously samples the entire waveform (not just the high and low points) to obtain the correct average voltage of any AC waveform.
The true RMS DMM measures what?
AC voltage through an analog computation by continuously sampling the entire waveform to first obtain the correct average voltage of the AC waveform.
NOTE DMM
Always follow the manufacturer's recommended operating procedures for the specific DMM in use. When testing voltage, always start on the highest range setting to avoid damaging the meter.
What level of AC current may indicate a hot chassis condition, when using a DMM to measure AC current between the TV set VHF input connector and the wall plate?
An AC current level of more than 100 μA may indicate a hot chassis condition when using a DMM to measure AC current between the TV set VHF input connector and the broadband cable wall plate.
Describe the difference between analog and digital multimeter displays.
An analog multimeter display uses a mechanical needle moving across a scale. A digital multimeter uses a liquid crystal display (LCD).
RMS voltage for a sinusoidal waveform
Average voltage * 1.11 or Peak Voltage *0.707
How does a standard digital multimeter (DMM) measure average AC voltage?
By sampling the high and low points on the AC waveform.
Most digital multimeters (DMM) have a diode measurement function for checking diodes. What else does the diode function check for?
Continuity from one point to another.
To check for proper voltage and wiring polarity at a polarized three-hole AC receptacle using a DMM
First set up the DMM for measuring AC voltage: (1) turn the function selector switch to the "AC voltage" function; (2) connect the black test lead to the "common" test lead jack; (3) connect the red test lead to the "volts" test lead jack; and (4) turn on the DMM, if necessary.
Why is it important to properly orient the grounding adapter before plugging in any CPE to a nonpolarized two-slot receptacle?
It important to properly orient the grounding adapter before plugging in any CPE to a non-polarized two-slot receptacle, to avoid damaging the equipment or creating a hot chassis condition.
Keep your digital multimeter (DMM) clean, dry, and properly repaired, as well as follow the manufacturer's recommended operating procedures and precautions to ensure a long operating life. Which one of the following choices is a precaution you need to take to prevent damage to your DMM?
Keep your DMM in a protective case when not in use.
When measuring ac voltage
Measure the AC voltage between adjacent receptacle slots by placing the black test lead in the long slot and the red test lead in the short slot (Figure 10), and verify the reading on the display is 115 VAC ±10%.
When measuring ac voltage:
Measure the AC voltage between the long slot and the ground hole by placing the black test lead in the ground hole and the red test lead in the long slot (Figure 9), and verify the reading on the display is 0.0 VAC.
when measuring ac voltage
Measure the AC voltage between the short slot and the ground hole by placing the black test lead in the ground hole and the red test lead in the short slot (Figure 11), and verify the reading on the display is 115 VAC ±10%.
Where should you store a DMM to prevent theft?
Store a DMM out of sight in a locked area to prevent theft.
List the four AC volts specifications of a typical DMM.
The four AC volts specifications of a typical DMM are: (1) maximum voltage (RMS); (2) maximum resolution; (3) frequency range; and (4) basic meter accuracy.
You are at a customer's residence checking a polarized two-slot alternating current (AC) receptacle for proper voltage and wiring polarity. The digital multimeter (DMM) display reads 115 VAC ± 10% when you measure between the longest receptacle slot and the AC wall plate screw. What does this indicate?
The hot and neutral wires are reversed.
What is the maximum DC voltage that a typical DMM can measure?
The maximum DC voltage that a typical DMM can measure is 1,000 VDC.
What is the maximum resistance in ohms that a typical DMM can measure?
The maximum resistance that a typical DMM can measure is 2-40 MΩ.
What should the voltage range upper limit be higher than when setting the range switch before measuring AC voltage with a true RMS DMM?
The selected voltage range upper limit should be higher than the largest anticipated voltage value when setting the range switch before measuring AC voltage with a true RMS DMM.
Describe the three test lead jacks on a typical DMM.
The three test lead jacks on a typical DMM are: (1) a volts, resistance, and diode measurement jack (for the red or "+" test lead); (2) a common ground jack (for the black or "-" test lead); and (3) a current measurement jack (for the red or "+" test lead%
Briefly describe the two basic types of DMMs.
The two basic types of DMMs are: (1) the standard DMM, which is calibrated to accurately measure only the AC voltage of a stable sinusoidal waveform; and (2) the true root mean square (RMS) DMM, which is designed to measure the AC voltage on all types of waveforms accurately.
What are five practices to ensure accurate resistance measurements using a DMM?
To ensure accurate resistance measurements using a DMM: (1) make sure that power is removed from the circuit you intend to measure; (2) make sure that one end of the circuit or component you plan to measure is not connected to anything; (3) ensure that the contact areas are clean and free of corrosion; (4) hold the test lead probe ends securely on the contact areas; and (5) avoid body contact with the test lead metal probe ends.
You are at a customer's residence and have confirmed the correct voltage and wiring polarity of an alternating current (AC) receptacle. You now need to check for indications of a hot chassis condition at the TV set or at another piece of customer equipment. When checking for a hot chassis condition, where do you touch the black test lead probe?
To the cable wall plate barrel threads.
Where are the two DMM test lead probes touched to measure AC voltage when checking for a hot chassis condition at a customer's TV set?
Touch the black test lead probe to the broadband cable wall plate barrel threads, and touch the red test lead probe to the threads of the TV set's VHF input port to measure AC voltage when checking for a hot chassis condition at a customer's TV set.
When checking for proper voltage and wiring polarity at a polarized three-hole receptacle, what is the range of acceptable voltage when measuring between the short slot and the ground hole?
When checking for proper voltage and wiring polarity at a polarized three-hole receptacle, the range of acceptable voltage when measuring between the short slot and the ground hole is 115 VAC ±10%
What occurs when you select the DMM "auto range" feature?
When you select the DMM "auto range" feature, the DMM automatically selects a measurement range based on what is sensed at the meter input.
What type of damage should you periodically inspect DMM test leads for?
You should periodically inspect DMM test leads for broken wires, loose connections, and worn wire insulation.
Black lead
a common ground jack (for the black or "-" test lead)
Common broadband cable applications of DMMs
measuring AC and DC voltages, a "hot chassis" condition, and resistance.
