Page 310 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Digital multimeters

Objectives: At the end of this lesson you shall be able to:
• explain the method of measurement of voltage by using digital multimeter
• list and explain the types of digital multimeter
• state the application of digital multimeters.

Digital Multimeter
In a digital multimeter the meter movement is replaced by a digital read out (Fig 1 and 2). This readout is similar
to that used in electronic calculators. The internal circuitry of the digital multimeter is made up of digital, integrated
circuits. Like the analog-type multimeter, the digital multimeter has a front panel switching arrangement.

The quantity measured is displayed in the form of a four digit number with a properly placed decimal point. When
DC quantities are measured the polarity is identified by ‘+ve’ or ‘-ve’ sign displayed to the left of the number
indicating the probes are connected correctly by +ve sign and probes are reversely connected by -ve sign.
DMM functions: The basic functions found on most DMMs are the same as those on analouge multimeters.
That is it can measure:-

• ohms
• DC voltage and current
• AC voltage and current
Some DMMs provide special functions such as transistor or diode test, power measurement, and decibel
measurement for audio amplifier tests.
DMM displays: DMMs are available with either LCD (liquid -crystal display) or LED (light-emitting diode) read-
outs. The LCD is the most commonly used read-out in battery-powered instruments due to the fact that it draws
very small amount of current.
A typical battery-powered DMM with an LCD read-out operates on a 9V battery that will last from a few hundred
hours to 2000 hours and more. The disadvantages of LCD read-outs are that (a) they are difficult or impossible
to see in poor light conditions, and (b) they are relatively slow response to measurement changes.
LEDs, on the other hand, can be seen in the dark, and respond quickly to changes in measured values. LED
displays require much more current than LCDs, and, therefore, battery life is shortened when they are used in
portable equipment.

Both LCD and LED-DMM displays are in a seven segment format (Fig 3).
Multimeter: Safety precautions: The following safety precautions should always be taken.
• Never use the ohmmeter section on a live circuit.
• Never connect the ammeter section in parallel with a voltage source.
• Never overload the ammeter or voltmeter sections by attempting to measure currents or voltages far in excess
of the range switch setting.
• Check the meter test leads for frayed or broken insulation before working with them. If damaged insulation is
found the test leads should be replaced.
• Avoid touching the bare metal clips or tips of the test probes.
• Whenever possible, remove the supply before connecting the meter test leads into the circuit.
Applications of Digital multimeter: A multimeter is used for testing and fault finding in electrical/electronic
circuits, electrical appliances and machines. A multimeter is a portable handy instrument used for
• checking continuity of circuit, appliances and devices.
• measuring/checking the supply presence at the source
• for testing components like capacitors, diodes, and transistors for checking their condition.
• measuring the current drawn by the circuit.
• measuring resistance of the electrical appliances and devices.

Note: Some meters have provision also for temperature measurement with suitable sensing probes.

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CITS : Power - Electrician & Wireman - Lesson 50-53 CITS : Power - Electrician & Wireman - Lesson 50-53