What is a Digital Multimeter?
A Digital multimeter or DMM is a test equipment used for resistance, voltage, current measurement and other electrical parameters as per requirement and displaying the results in the mathematical digits form on an LCD or LED readout. It is a type of multimeter which functions digitally. Digital multimeters are widely accepted worldwide as they have better accuracy levels and ranging from simple 3 ½ to 4 ½ digit handheld DMM to very special system DMM.
Features of Digital Multimeter
Digital multimeter is most advanced instruments that make use of modern Integrated circuits for making electrical measurements. Some of its features which make it famous in the eyes of professional technicians are:
- It is light in weight.
- Capable of giving more accurate readings.
- It measures lots of physical quantities like voltage, current, resistance, frequency etc.
- It is less costly.
- It measures different electrical parameters at high frequencies with the help of special probes.
Block diagram of Digital multimeter
In digital multimeter, we can incorporate many types of meters like ohmmeter, ammeter, a voltmeter for the measurement of electrical parameters. Its block diagram is shown below in the figure. Let us have a look at its working and specification one by one.
(i) Digital voltmeter (DVM):
Digital voltmeter is the basic instrument used for measurement of voltage through the use of Analog to Digital converter. The basic principle behind the digital multimeters is the Analog to digital converter because without this we are not able to convert the analog output into digital form. There are several ADC available in the market, but we mainly use Flash type ADC due to its simplicity and fastest speed. Let’s have a look at its basic operation.
(a) Flash AD converter: It comprises of comparators, encoder, and digital display. Comparators are driven by resistor divider network, the encoder converts its inputs to corresponding outputs which drive the digital display.
As shown above, three resistors of value R drives the comparators C1, C2, C3. Let the input voltage Vi = 1v, +V= 4V and comparators i.e. C1 , C2, C3 voltages equal to 1V, 2V and 3V respectively. If the output of the C1 = +1 and C2=C3= 0, then we fed 001 as the input to the encoder which further converts it into 0001. This binary output drives the seven segment display to read 1V on it. With the help of this method, we read the voltages of magnitude 1V, 2V, 3V and we also add more comparators for more accurate readings as per our requirement.
(ii) Digital Ammeter (DAM):
Digital ammeter uses a shunt resistor to produce a calibrated voltage proportional to the current flowing. As shown in the diagram, to read the current we must first convert the current to be measured into a voltage by using a known resistance RK. The voltage so developed is calibrated to read the input current.
(iii) Digital ohm meter (DOM):
A digital ohmmeter is used to measure electrical resistance which obstructs the path to the flow of current.
As shown in the diagram, resistance network comprising a known resistance RK and unknown resistance Ru used to develop a voltage across the unknown resistance. The voltage is given by:
V = VB Ru / RK + Ru
where VB = Voltage of the built-in battery
After calibrating voltage, the meter can be calibrated in terms of ohms.
Working Principle of Digital Multimeter
As shown in block diagram, in a typical Digital multimeter the input signal i.e ac or dc voltage, current, resistance, temperature or any other parameter is converted to dc voltage within the range of the ADC. The analog to digital converter then converts the pre-scaled dc voltage into its equivalent digital numbers which will be displayed on the display unit. Sometimes, digital controller block is implemented with a microcontroller or a microprocessor manages the flow of information within the instrument. This block will coordinate all the internal functions as well as transferring information to external devices such as printers or personal computer. In the case of some hand held multimeter, some of or all of these blocks may be implemented in a VLSI circuit while A/D converter and display driver can be in the same IC.
Digital Multimeter symbols:
Some common Digital multimeter symbols and its description are given in the table below. These symbols are often found on the multimeter & its schematics are designed to symbolize components and reference values of electrical parameters.
DMM Parts and functions
A Digital Multimeter is divided into three parts:
(i) Display: The LCD screen present on the upper portion of the multimeter basically displays four or more digits and also shows negative value if necessary. A few or today’s multimeters have illuminated the display for better viewing in low light situations.
(ii) Selection Dial: It allows the user to set the multimeter to read different electrical parameter such as milliamps (mA) of current, voltage, resistance, capacitance etc. You can easily turn the dial anywhere for specific parameter measurement.
(iii) Ports: Two ports are available on the front of every multimeter except in some four ports are available for measuring current in mA or A. We plugged two probes into these ports which are of different colour i.e. one is of red colour and other is of black color.Ports are:
(a) COM: It stands for common and is almost connected to ground or considered as a -ve connection of a circuit. We generally insert the black color probe into COM port.
(b) mAVΩ: This port allows the measurement of current (up to 200 mA), voltage and resistance or considered as a +ve connection of a circuit. We generally insert the red color probe into mAVΩ port.
In the box of a digital multimeter, we got leads of different colors. Here we are going to explain these leads in detail. DMM leads are subdivided into four parts:
(i) Red lead
- Connected to voltage, resistance or ampere port.
- Considered as a +ve connection of a circuit
(ii) Black lead
- Connected to the common or ground port
- Considered as a -ve connection of a circuit
These are the handles used to hold the tip on the tested connection. There are different types of probes available, they are:
- Banana to Alligator Clips: These are great cables for connecting to large wires or pins on a breadboard. Good for performing longer term tests where you don’t have to hold the probes in place while you manipulate a circuit.
- Banana to IC Hook: IC hooks work well on smaller ICs and legs of ICs.
- Banana to Tweezers: Tweezers are handy if you need to test SMD components.
- Banana to Test Probes: If you ever break a probe, they are cheap to replace.
These are present at the end of the probes and basically, provide a connection point.
DMM Safety Precaution:
Before operating multimeters, we have to follow some safety precautions. Here we are going to explain you some safety information of DMM.
- If the DMM test leads are damaged then never use the meter.
- Always ensures that the test leads and dial are in right position for the desired measurement.
- When a test lead is plugged into the 10 A or 300mA input jack then never touch the probes to a voltage source.
- When power is applied never measure resistance in a circuit.
- While making measurements always keep your fingers behind the finger guards on the test probes.
- To avoid damage or injury, never use the meter on circuits that exceed 4800 watts.
- Replace the battery as soon as possible to avoid false readings which could lead to possible electric shock or personal injury.
- Be careful when working with voltages above 60 V DC or 30 V AC RMS. Such voltages pose a shock hazard.
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