1. MULTIMETER

In electronics, the most frequently measured electrical parameters are the voltage, the

resistance, and the current. These provide the basic information about the circuit.

A tool specifically intended to measure all these parameters is the multitester, multimeter,

or simply referred to as the VOM (Voltmeter, Ohmmeter, Milliammeter).

A multimeter or tester is as indispensable for technician as firearms for a soldier. The

interpretation of meter readings gives technicians clues to troubleshooting. Learning how to

use a tester and to interpret its readings is an essential part of the training for all would-be

technicians.

Figure 1.1 Multimeter

The multitester is a versatile tool and it is not just limited to measuring voltage, current, and

resistance. It could be used to check variety of electronic components or circuits. In this

way, defective components, lead polarity or pins, or continuity may be verified or checked.

1.1 Types of Multimeter

Two general types of VOMs are available, the analog and the digital types.

The analog multitester has a moving coil assembly, which is characterized by a needle

pointer. This multitester is based primarily on the d ' Arsonval meter, which works on the

principle of magnetic attraction and repulsion. It has two major parts; the stator is made up

of a permanent magnet, the soft iron pole pieces, and the cylindrical core. This magnetic

assembly has a uniform air gap between the core and pole pieces and has been designed

to produced a magnetic field between the air gaps that is constant in magnitude, uniform,

even, and radial in direction. The scale is calibrated according to the internal resistance of

the multitester and the parameter it is designed to measure.

The moving coil assembly, on the other hand, is made up of several turns of very fine wire

wound on a rectangular aluminum frame called bobbin. The vertical axis of this bobbin,

connected to a shaft, where the pointer is also attached. Both ends of the shaft are

supported by jeweled bearings so that it could freely rotate back and forth in the air gap.

The hairspring, located at the top and bottom of the bobbin, connects the coil to the

external circuit and also holds the pointer in the zero position when no current flows through

the coil. The hairsprings are wound in opposite directions to compensate for the effects of

temperature changes. The stationary ends of the hairsprings can also be adjusted to

reposition the reposition the moving coil and the pointer setting whenever necessary.

When current flows through the coil, a corresponding torque is generated causing the coil,

and hence, the pointer to rotate in the air gap. This rotation is limited both by the tension of

the hairspring and the electrical damping produced in the bobbin and the other by the

counter electromotive force (EMF) within the coil itself. This damping results to braking

action, which slows down the movement of the coil. Eventually, the pointer would rest on

particular position on the scale corresponding to the torque and hence to the current flowing

through the coil.

1.1.2 Digital Multitester

A digital multitester, on the other hand, is very different. As its name suggests, it is purely

electronic circuit, without any moving element or coil. Figure 1.2 shows a pocket size digital

multitester, the AC500D.

Just like an ordinary electronic circuit, when you open up a digital multitester, you'll find only

a printed circuit board with all the components as well as the liquid crystal display (LCD)

assembly mounted on the PCB. The unit actually resembles an ordinary calculator except

for the absence of a keypad. Instead two jacks for the pair of probes are provided.

Figure 1.2 Equivalent Functional block diagram of the AAC500D Pocket-Size

Digital Multimeter.

The functional block diagram of a digital multitester, the AC500D in particular, is shown in

Figure 1.2. The unknown voltage, current or resistance is initially passed through the

Protection Circuit formed by a 0.5 - Ampere fuses for DC currents. From here, the signal

to a specific block, which can either be a Voltage Divider when measuring AC and DC

voltages, Current Shunt when measuring current or Ohm Converter when measuring

resistances. These functional blocks are calibrated, through the Range Control, which is

an internal part of the IC, and also with the Range Resistors, in such a way that a "full scale

deflection" results with 2.000 Volt DC input to the IC.

The Analog-to-Digital Converter converts the analog input into a discrete number of steps

that can easily be counted and presented in decimal form on the LCD. For the 7106, the

steps correspond to time and in turn, correspond to an accurately known voltage. The

time is determined by comparing it with the equivalent time of the Reference Voltage, Vref.

The ratio of time of the Input Voltage, Vx, and the Reference voltage, Vref, is the counted.

The Digital Section of the 7106 IC displays the digital interpretation of the input signal

measured. A decoder ensures the transformation of the binary output of the memory into a

code suitable for the display. This LCD displays a digital readout corresponding to the

value of the input signal being measured.

Advantages of a Digital Multitester

Digital multitesters have a far greater advantage than analog multitesters mainly due to

ease of operation. These can be summarized as follows:

1. Direct reading hence no errors due to possible wrong interpretation committed in

analog-type multitester.

2. Automatic polarity indication, whenever the probes are improperly connected to

a circuit with respect to polarity. A " - " sign would be indicated by the multitester

readout.

3. Automatic decimal point positioning, automatic ranging, and zeroing.

4. Over/under range indication.

With all these advantages, a digital multitester is highly recommended.

1.2 Instructions in using Multitester (Analog)

1.2.1 Preliminary Procedures

Whatever type of multitester you have to use, before making any measurement you have

perform some basic procedures.

Connect the test probe to the appropriate jacks.

For analog multitesters:

The red test probe to the + jack and the black test probe to the - COM jack.

Note: Some analog multitesters also have some special jacks. Consult your multitester

manual for the specific application.

See: If the pointer rest exactly at the 0 (zero) position situated at the left side of the scale.

If and only if the pointer is indeed off position at the time measurement would be

made, carefully adjust the zero corrector with a lightweight screwdriver.

Figure 1.3

Check the probes if they are okay. Rotate the multitester selector switch towards the x 1

Ohm (Resistance range). Short the two probes together.

Figure 1.4

The pointer must deflect towards the rightmost position or "0 position". This indicates that

the probes are good.

If you'll be measuring resistance, the pointer must rest exactly at the "0 position".

This may be adjusted through the 0-ohm Adj.

If the pointer did not deflect at all, check the probes, there's a possibility that one is broken

or open at some point.

If the pointer could not rest exactly at the "0" ohm position at the right hand end of the scale

no matter how the circuit 0 ohm Adj is adjusted, replace the batteries of the multitester.

Now, proceed to the appropriate measurement desired:

1.2.1 Voltage Measurements

When measuring voltages, AC or DC, the meter is placed parallel with the component or

circuit as illustrated in Figure 1.5.

Figure 1.5

Procedure:

Set the function selector knob or the range selector to the proper scale range. The

chosen scale range must be higher than that of the anticipated voltage to be measured.

If in doubt or you don't have any idea, choose the higher voltage range of the multitester

initially and gradually move down to the lower scale range that will give a good mid

scale reading.

The proper scale range: AC, DC, or Output should be differentiated. The scale should

be:

AC if it's an AC VOLTAGE

DC if it's a DC VOLTAGE

OUTPUT if it's a purely AC VOLTAGE

1.2.1.1 Measurement of DC Voltages

1. Always observe the correct probe polarity. Connect first the "-COM" probe, the

black one to the ground side of the circuit as shown in the illustration.

2. If the meter needle deflects to the left at the 0 position, the voltage polarity is

opposite to that anticipated. Interchange the connection of the probes to the

circuit.

3. If the pointer swing beyond the scale limit on the right as shown in Figure 1-6,

immediately disconnected the probe. The setting of the Range Selector Switch

is insufficient. Move the switch to a higher range scale.

Figure 1-6

4. Even if tester is a set at the DC Voltage Function, an analog voltmeter would

respond also to an AC voltage at the same time as that of the DC only if the AC

frequency is within the meter frequency response and the AC voltage value is

greater than the DC value.

Example, if the multitester frequency response is 30 Hz to 30 kHz, the multitester would

measure the 220 Volts, 60 Hz AC line even if it's in DC range. The reading, however,

would be erroneous.

1.2.1.2 Measurement of AC Voltages

1. The analog voltmeter responds to both AC and DC voltages even if the function

switch is set at the AC VOLTAGE range or function. The voltmeter cannot

distinguish the AC from the DC voltage, hence an erroneous reading would be

registered when both types of voltages are present.

Figure 1-7

2. To measure AC voltage only, a 0.1 to 0.05 µF capacitor may be connected to

one of the probes as shown in Figure 1-7. The capacitor blocks the DC

component and allows the AC component of the voltage to pass through.

Note: Some multitesters such as the AC360 has an OUTPUT jack. This has an internal

0.05 µF capacitor that links the probe with the + jack. Hence, eliminating the need

for an external capacitor.

3. AC voltage readings on the multitesters is the RMS value of the voltage, or

simply the average value. This is not the peak value of a voltage.

4. When measuring the secondary voltage of an unloaded transformer, the AC

voltage reading is usually about 10% higher than its rated value.

1.2.2 Resistance Measurements (ohmmeter function)

To measure resistance, the multitester is connected across the particular component.

Procedures:

1. Select the desired resistance range by moving the selector switches to ohm

range: x1, x10, x100, x1K, or x10K. Again, select the range, which would cause

the pointer to deflect to no more than 2/3 of the full scale.

2. Set the tester to zero. (This is the preliminary procedure stated above. This is

repeated only to avoid confusion.) Clip the two probes together. Adjust the

Zero Ohm Adj. until the pointer rest exactly on the "0" (zero) position on the

right hand end of the Ohm scale.

3. Place the probes across the component whose resistance will be measured.

4. Multiply the reading obtained with the multiplier factor of the selector switch:

Notes in Measuring Resistance:

1. Always set the Zero Ohm Adj. whenever to the Ohm range or from one Ohm

rang scale to another.

2. When making resistance measurements, the component whose resistance

would be measured should not be parallel with another component or circuit.

3. Switch off power to the circuit where resistances would be measured. Failure to

switch off power would not only result to a wrong reading but also damage the

meter movement.

4. If the meter does not read 0 (zero) when the test probes are shorted, no matter

how much the zero Ohm Adj. is adjusted, replace the multitester batteries.

Examples:

The readings are:

30 on the x1: 30 x 1 = 30 Ohms

45 on the x10: 45 x 10 = 450 Ohms

22 on the x 1K: 22 x 1K = 22 Kilohms

Figure 1.8 Resistance Measurement

1.2.3 DC Current Measurements

When current is to be measured, the meter must be connected IN SERIES with the circuit

or electrical load and the power source.

The + or red probe should be connected to the positive side of the power source and the -

COM to the circuit side.

Initially set the selector switch of the multitester to the highest DcmA range and

progressively move the selector switch to a lower range where the pointer would deflect

between half 2/3 of the full scale. This region gives a relatively accurate reading.

If the pointer deflects to the left of the position for DcmA, interchange the probes.

Figure 1.9 Current measurement