A digital multimeter is a microprocessor based multi functional device which display its readings on an LCD screen. It looks just like an analog multimeter.
A digital multimeter differs from the analog meter in its ability to display measured electrical quantities quickly on an LCD display unit without any delay. It has a processor built into the meter which allows the user to take measurements of frequency, the inductance of a coil, capacitance of a capacitor, and a host of other high functional electrical measurements.
Various digital multimeters presently in use are of following two types –
1) Scalable digital multimeter –
This type of device require an expert operator to use it. Operator must have a rough idea of the approximate values of voltage, current, or resistance that has to be measured. The device is then adjusted accordingly before using, otherwise inaccurate readings will be recorded or the device will get damaged.
2) Auto ranging digital multimeter –
Auto ranging digital multimeter is more widely used due to its ease to use. It need not require precise adjustments. It has high functionality and quick display readings achieved without the employment of an expert operator for any calculations.
A schematic diagram of a digital multimeter with all the functional points, range and sockets is shown in figure.
For voltage measurement using a digital multimeter following steps are followed –
- First know about that supply is AC or DC. Range selection knob of the meter dial is placed to the suitable range to AC Voltage or DC voltage.
- Range selector is adjusted in the range nearest to the predictive value.
- Test probe leads are plugged into the respective sockets. Black lead is inserted into common socket and red lead inserted into voltage socket.
- Leads are applied to the respective points of circuit.
- Position and reposition of the test lead is prompted depending on the reading appears on the meter LCD.
- While measuring AC voltage, variations may happen in the reading. As the test continues, later the measurement will get steady.
A block diagram showing internal circuit and functional blocks for measurement of AC voltage using a digital multimeter is shown in figure.
When current is measured using a digital multimeter, following steps are followed –
- Probing lead is plugged into the mA marked socket for measuring low current or into the A marked socket for currents over 500 mA .
- Rotary range selector dial is set to AC or DC current depending on the circuit being measured.
- Probing leads are then applied to the open circuit current to take the readings.
A particular type of digital multimeter is known as Clamp Meter or Tong Tester which is used for measurement of higher currents. A typical clamp meter is shown in figure.
A clamp meter is basically a combination of basic digital multimeter enhanced with a current sensor. It is capable of measuring currents of higher values. It has two jaws. One jaw is fixed and the other jaw can be operated with the help of a lever. The jaws are made of ferrite iron and are engineered to detect the magnetic field being generated around a conductor carrying current. Rest of the design feature is similar to a digital multimeter.
The clamp meter can be clamped with the help of its jaws around a wire, cable or other conductor at any point in an electrical system.
- Thus, the current in a circuit can be measured without disconnecting or de-energizing the circuit.
- A digital multimeter is useful only up to 10 A of current measurement whereas, a clamp meter is capable of measuring current in the range of 0 A to 1000 A
The advantage of using a clamp meter is that, measurements can be obtained even without opening the test circuit. Proper protective equipment must be worn before testing. For measuring the current, following steps are followed –
- First know about the type of current if it is AC or DC.
- Adjustment of the meter dial to the suitable function to DC current or AC current is made.
- Range is selected on the rotating dial nearest to the value of current in the circuit.
- Movable Jaw is opened by pressing the lever and the meter is clamped in the circuit by placing the wire to be measured, in annular space between the jaws.
- The meter directly reads the value of current flowing through the wire.
Measurement of resistance is done to ensure the load or circuit being tested is complete. A complete circuit means there is no break or opening in the wires connected to the load or the internal components of the device being tested.
When a digital multimeter is used for measuring resistance, following steps are followed –
- Switch off the power supply in the circuit being tested.
- Meter dial is adjusted to the resistance mode.
- A suitable range is selected on the dial.
- Probe leads are plugged into the suitable terminals or sockets.
- The leads are connected to the element being tested.
Resistance testing is sometimes called as continuity testing. Continuity testing does the same action as resistance testing with the exception that continuity testing emits an audible sound indicating that the circuit or wires are complete.
- Adjustment of the dial is made for continuity testing function.
- Probe test leads are plugged into the suitable terminal.
- Leads are contacted at the relevant points for checking continuity.
The multimeter beeps under good continuity that allows the flow of current. At this condition resistance of the circuit will show zero value. If the circuit is broken or no continuity exists, the meter does not beep and the value of resistance will show infinity.
The resistance testing and continuity testing are also a good way to check for short-circuits and the ground fault which are events that cause circuit breakers to trip, fuses to blow, and possible injury to workers in the field.
A digital multimeter has following advantages over an analog multimeter –
- It display the measured electrical quantities quickly on an LCD display unit.
- The results are displayed instantaneously without any delay.
- User need not require to do any calculations to get the values of measurement.
- It has a processor built into the meter which allows the user to take measurements of frequency.
- A digital meter has additional measuring capacity to measure inductance of a coil, capacitance of a capacitor and a host of other high functional electrical measurements.