Digital Multimeter (DMM)
A digital multimeter (DMM) is an electronic testing device that is used to determine specific information about the current, resistance and voltage of electrical signals travelling through a line. A digital multimeter is differentiated from an analog multimeter by the display and accuracy of the device. Analog multimeters use a less-accurate needle and gauge to display results, whereas a digital multimeter converts the signals into a digital format that is shown on an LED display. A DMM is useful in resolving electrical problems in many types of equipment, ranging from automobiles to mobile electronics.
2/4 Wire Resistance
Wire resistance measurements made by a DMM come in two basic types: 2-wire measurement and 4-wire measurement. A 2-wire measurement is made using two leads from the DMM to the device being tested. It measures only the resistance of the signal through the device. A 4-wire measurement eliminates resistance that may be created by the DMM, and it allows for the testing of lower values of resistance than can be accomplished with 2-wire measurement.
AC + DC Voltage/Current Measurements
A DMM can measure the voltage of both alternating current (AC) and direct current (DC). Most generated electricity is AC, whereas most stored electricity is DC. However, electricity at certain voltages is both AC and DC.
CE, CSA Certification
Certification of an electronic device shows that the device meets all safety, health and environmental standards set forth by a government or independent agency. CE certification is a standard created by the European Commission. CE is an abbreviation of Conformité Européenne, or European Conformity. CSA certification comes from the Canadian Standards Association, a nonprofit organization dedicated to public safety.
Conductance is a measurement of how easily an electrical signal can be transmitted through a specific medium. Conductance is measured by the standard unit known as siemens, which is the equivalent of 1 ampere per volt.
A continuity check is one of the basic tests that can be performed with a DMM. Continuity determines whether an electrical circuit is open or closed. A closed circuit is one in which an electrical signal may be transmitted, whereas an open circuit is one that is broken or blocked, impeding the signal. A circuit that does not check out as having continuity may often be fixed by replacing a broken or disconnected wire.
Diodes are connected to an electrical circuit to ensure that electricity flows in only one direction through the circuit. A diode check can reveal if a diode is malfunctioning by conducting electricity in the opposite direction. It can also discover the voltage loss experienced by electricity as it passes through the diode.
The display count of a DMM refers to how large or how accurate a measurement can be taken and shown on the LED display. Display count is stated in terms of x00. The x is a whole number that represents the maximum top digit before a unit of precision is lost. For instance a 2000 display count has four total digits that will measure voltage up to 19.99. At 20 volts, the precision drops, so the display would read 020.0.
Display digits are the number of digits the display of a DMM will show. Half digits mean only a 1 or 0 will be displayed as the final digit.
Form Factors, Types
Digital multimeters are available in a variety of sizes and shapes, known as form factors. Some of the most common form factors for DMMS include bench/free-standing, handheld and rack-mounted.
Frequency, Period, Duty Cycle Measurements
Many DMMs can measure more than current, resistance and voltage. Frequency is a measurement of the number of cycles of the electrical signal, expressed in hertz. Period refers to the time it takes for the signal to complete one cycle. Duty cycle is the ratio of electrical pulses to the duration of the pulses, which is equivalent to the ratio of average power to peak power.
The hold function stops the DMM from making a new reading while freezing the currently displayed value.
The min/max function of a DMM records the lowest and highest readings taken since the function was started.
The relative function of a DMM is used to correct a reading for resistance created by the leads on the DMM. Once the lead resistance is measured, new measurements are made without its inclusion.
Digital multimeters are available as combination scopemeters. A scopemeter, because it is also an oscilloscope, can measure the amplitude and frequency of electrical signals.
Temperature Measurements, J-Type/K-Type
Temperature can be measured by some DMMs through the use of a thermocouple. J-type thermocouples measure lower temperatures than do K-type thermocouples. J-type thermocouples only measure temperatures up to 600° C, while K-type thermocouples measure temperatures up to 1,273° C.
TRMS stands for true root mean squared. TRMS multimeters apply a compensating factor that accounts for distortion to achieve a more accurate reading.
Voltage/Current Measurement Accuracy
The voltage and current measurements taken by a DMM are not always totally accurate. The potential for variation is given in the specific device’s accuracy rating, which is usually around 0.5 percent.