Digital multimeter vs analog multimeter:– Multimeters are designed and mass produced for electronics engineers. Even the simplest and cheapest types may include features which you are not likely to use. Digital meters give an output in numbers, usually on a liquid crystal display. The diagram below shows a switched range multimeter.
The central knob has lots of positions and you must choose which one is appropriate for the measurement you want to make. If the meter is switched to 20 V DC, for example, then 20 V is the maximum voltage which can be measured, This is sometimes called 20 Vfsd, where fsd is short for full scale deflection. For circuits with power supplies of up to 20 V, which includes all the circuits you are likely to build, the 20 V DC voltage range is the most useful. DC ranges are indicated by v= on the meter. Sometimes, you will want to measure smaller voltages, and in this case, the 2 V or 200 mV ranges are used.
What does DC mean? DC means direct current. In any circuit which operates from a steady voltage source, such as a battery, current flow is always in the same direction. Every constructional project described in Design Electronics works in this way.
AC means alternating current. In an electric lamp connected to the domestic mains electricity, current flows first one way, then the other. That is, the current reverses, or alternates, in direction.
What do meters measure?
A meter is a measuring instrument. An ammeter
measures current, a voltmeter
measures the potential difference (voltage) between two points, and an ohmmeter
measures resistance. A multimeter
combines these functions, and possibly some additional ones as well, into a single instrument.
Before going into detail about multimeters, it is important for you to have a clear idea of how meters are connected into circuits. Diagrams below show a circuit before and after connecting an ammeter:
Think about the changes you would have to make to a practical circuit in order to include the ammeter. To start with, you need to break the circuit
so that the ammeter can be connected in series. All the current flowing in the circuit must pass through the ammeter. Meters are not supposed to alter the behaviour of the circuit, or at least not significantly, and it follows that an ammeter must have a very LOW resistance.
Diagram shows the same circuit after connecting a voltmeter:
This time, you do not need to break the circuit. The voltmeter is connected in parallel between the two points where the measurement is to be made. Since the voltmeter provides a parallel pathway, it should take as little current as possible. In other words, a voltmeter should have a very HIGH resistance.
Which measurement technique do you think will be the more useful? In fact, voltage measurements are used much more often than current measurements.
The processing of electronic signals is usually thought of in voltage terms. It is an added advantage that a voltage measurement is easier to make. The original circuit does not need to be changed. Often, the meter probes are connected simply by touching them to the points of interest.
An ohmmeter does not function with a circuit connected to a power supply. If you want to measure the resistance of a particular component, you must take it out of the circuit altogether and test it separately, as shown in diagram.
Ohmmeters work by passing a small current through the component and measuring the voltage produced. If you try this with the component connected into a circuit with a power supply, the most likely result is that the meter will be damaged. Most multimeters have a fuse to help protect against misuse.
An alternative style of multimeter is the auto ranging multimeter:
The central knob has fewer positions and all you need to do is to switch it to the quantity you want to measure. Once switched to V, the meter automatically adjusts its range to give a meaningful reading, and the display includes the unit of measurement, V or mV. This type of meter is more expensive, but obviously much easier to use.
Where are the two meter probes connected? The black lead is always connected into the socket marked COM, short for COMMON. Thered lead is connected into the socket labeled V OHMS mA. The 10A socket is very rarely used.
- 1. connect red test lead to ‘v’ input terminal and black test lead to ‘com’ input terminal.
- Set the function switch to DC/AC.
- CONNECT TEST LEAD TO THE DEVICE OR CIRCUIT.
- 2.CURRENT MEASUREMENTS; SET THE FUNCTION SWITCH TO ‘MA,OR Ampere measurement.
- 3. Resistance measurements; set the function switch to OHMS position.
- 4. Diode and continuity; press select switch to diode mode.
- 5. continuity; select switch to bazzer mode.
- Temperature measurement;set function switch to ‘TEMP’ position.1. Voltage measurements:Build the circuit shown below using prototype board and four 10k OHMS resistors:
To get the multimeter to function as an ohmmeter, you will need to select a resistance range. With a switched range meter, the 200 k position is usually suitable. You will see the resistance measurement change as the light level changes. Covering the LDR with your hand increases the resistance of the LDR.
If the meter reads this means that the resistance is more than the maximum which can be measured on this range and you may need to switch to a new position, 2000 k, to take a reading. (How many meg ohms is 2000 k?)
You can check the value of any fixed value resistor in the same way, and confirm that you have worked out the colour code correctly.
3. Current measurements:
The diagram below shows a prototype board set up for the measurement of current:
- IMPORTANT: AVOID HEAT DAMAGE TO THE METER,KEEP IT AWAY FROM SOURCES OF VERY HIGH TEMPERATURE.
- Check the battery. If don’t know any display check the battery. ( 1.5V AND 9V)
- Preview the operating instructions for possible mistakes in operating procedure.
- Inspect and test the Probes for a broken or intermittent connection.
- Inspect and test the fuse. If necessary to replace the fuse, be sure to install one of the proper current rating.
THE ANALOG MULTIMETER
The analog multimeter operates with a permanent magnet moving coil or the d’Arsonval movement. It is basically a combination of a voltmeter, an ammeter and ohmmeter using one movement. It thus contains, a basic circuit and a function switch to connect appropriate parts of the circuit to the basic movement.
FSD Means full scale deflection (current)
It corresponds to a current through the coil, which makes the pointer to move over the whole width of the scale and is usually 50 microA. That is, the basic movement is a micro ammeter.
|SENSITIVITY||FSD||INPUT RESISTANCE at 2.5V range|
|100 ohm/Volt||1 mA FSD||2500 ohm|
|20000 ohm/Volt||50 microA FSD||50 kohms|
|100000 ohm/Volt||10 uA FSD||250 kohm|
The movement of most modern multimeters, are protected by semiconductor diodes.
Zero adjust-screw setting as follows:
- Place the multimeter flat on a bench, face up.
- Set the range switch to OFF.
- Short two test probes together.
- Note if the meter pointer indicates exactly 0 at extreme left end of the black scale.
- If it does not read 0 turn the screw on the meter movement slowly until the proper 0 reading is obtained.
CHECK THE INTERNAL BATTERIES AS FOLLOWS:
- Short the test probes together.
- Set the range switch to R*1.
- Turn the 0 ohm adjust control, until the pointer reaches 0 ohms at the right end of the ohm scale. If the pointer cannot be brought to the 0 mark, then replace the 1.5v AA cell, or appropriate cells.
POINTS TO REMEMBER:
Before operating the multimeter, read the PRECAUTIONS. Failure to do so, may result in shock to the user or in damage to the equipment. Never connect the test leads directly across any voltage when the tester is used as a current meter. This could damage the instruments. Always connect the meter in series with a load, when current is measured.
MEASUREMENT OF EXTRA HIGH TENSION (E.H.T) VOLTAGE: (In excess of 1000 volts)
- An electrostatic voltmeter having a range suitable for the measurement to be taken or
- Use a multimeter switched to a dc voltage range and fitted with a special adopter called a E.H.T MULTIPLIER.
- Disconnect the equipment under lest from the supply.
- Discharge components such as capacitors and cathode ray tubes. These can hold a charge for sometime after the equipment is switched off.
- Connect the voltmeter to the circuit. Connect the lest leads firmly, connect the meter with the right polarity.
- Without touching the meter multiplier of test leads, Switch on the equipment and take the readings.
- Disconnect the equipment under test.
- Before disconnecting the meter, ensure that its reading has fallen to zero.