Multitester Documentation

Experiment No. 1

Project Report on

Analog Multimeter (MIYAMA M-385)

Rommel Areola

BSEE 3-1

August 4, 2012

Project Summary

This project is a laboratory report on the principles of operation, construction, calibration, and

evaluation of an electrical analog multimeter or Voltmeter-Ohmmeter-Milliammeter(VOM). The

multimeter model used in this project is MIYAMA M-385. Parts of the design on the schematic

diagram, PCB layout and component placement are modified by the researcher, thus, it may vary

from the original overall design of MIYAMA M-385 multimeter.

Introduction

The multimeter is an essential tool for engineers, especially those specializing in the field of

electricity. Such testers may vary in design and purpose, and may include more than three

parameters of measurement. It is then a need for electrical engineering students to study how these

testers operate, how to repair and calibrate them, and how to use them properly. This paper deals

with these studies about multimeters.

Purpose

The project aims to explain the assembly and construction of a typical analog electric multimeter

and how it works. It may also serve as a reference for fellow researchers interested in making their

own multimeters.

Scope

The efficiency and general use of the project is confined to the nature of the multimeter circuit

design. And since the project is a measuring device, the use of accurate, less tolerant (resistors used

in this project are ±1% tolerant), and appropriate electronic components must be considered.

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Project Report on Analog Multimeter

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The model used for this project is MIYAMA M-385 that includes 4 ranges for AC Voltmeter, 6 ranges

for DC Voltmeter, 4 ranges for DC Milliammeter, and 4 ranges for Ohmmeter. The entire project and

research body is limited only to the mentioned parameters.

This paper will discuss the construction, design, and evaluation of the multitester, which is prepared

and built within a two-week period.

Discussion

Methodology

This section of the project paper discusses the step-by-step procedures followed in making the

MIYAMA multitester.

Electronic Components

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Equipments and Materials

2 1.5V AA Battery

1 9V Battery

Project Casing

Soldering Iron

Soldering Lead

Soldering Paste

General Procedures

The project is accomplished by following these steps. First, decide what specific design of multimeter

and measurement parameters you want to include. Then purchase all components and other

materials to be used. If the design needs PCB etching, consult Printed Circuit Boards-Design,

Fabrication, and Assembly by R.S. Khandpur. Next, place the components on the board. Be cautious

in reading the ratings of each component.

When all the components are placed on the board, cut all portruding component leads, leaving only

about 3mm (cutting the leads may also be done after soldering). Then solder each component

properly to the board. Avoid solder bridges and poor connection between board and component, for

these may result in a shorted circuit or malfunction. Check for any misplaced component before

assembling the board to the project casing.

Flow Chart of Procedures

For a better understanding of the procedures, the table below shows the chronological arrangement

of steps.

Design lanningDDDDFFF

Figure 1: Flow Chart of Procedures

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Design Planning Purchase of Materials PCB Etching

Component Placement

Casing Assembly

Finalizing Soldering

Evaluation


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Printed Circuit Board Layout

The tables below show the PCB layout design and component placement of the multitester. The

layout design shows how the PCB must look like after etching (ready-made etched PCB’s are also

available in market). While the component placement assists the researcher on how the

components are to be placed and positioned on the PCB.

Figure 2: PCB Design

Figure 3: Component Placement

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Project Schematic Diagram

The following table shows the MIYAMA M-385 multitester schematic diagram. Before evaluation,

the researcher must conform to the schematic diagram to prevent malfunctions.

Figure 4: Project Schematic Diagram

Test and Evaluation

This part presents the multitester evaluation procedures and basic statistical treatments used.

Initial Test

After the project was assembled, initial tests were conducted, like applying appropriate loads,

current, and voltage to each corresponding range to check whether the meter and each range

works. All three measurement parameters (Voltmeter, Milliameter, Ohmmeter) worked well with

this test.

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Accuracy Tests

Since the project is a measuring device, accuracy of the reading must be given due emphasis. It is in

this light that the researcher devised the following formula for getting the accuracy of the meter:

Accuracy=100−[( units of deviationunits applied )×100]The following tables show the reading accuracy of each of the ranges of the 3 measurement

parameters (ACV, DCV, DCmA, and Ohmmeter).

AC Voltmeter

Table 1: AC Voltmeter Accuracy Test

Range Voltage Applied Reading Accuracy

1000V 230V 240V 95.65%

250V 230V 246V 93.04%

The above results were gathered by applying 230V household line into the first two ranges of the

ACV. Voltage sources lower than 230V may be used to test the lower ranges. The above test shows

that the ACV is in good and accurate condition.

DC Voltmeter

Table 2: DV Voltmeter Accuracy Test

Range Voltage Applied Reading Accuracy

2.5V 1.5V 1.7V 80%

10V 9V 9.3V 96.86%

50V 24V 25V 95.83%

250V 24V 24V 100%

In this test, a variable DC power supply ranging from 3-24V is used to obtain the results above. It is

important to note that power supplies supply a bit higher voltage than its rated voltage. And

accuracy of the meter depends also on the compatibility of the range and the applied unit. For

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example, an exact amount of 24V is hardly discernible in the 0-250V scale. The researcher may want

to use more compatible voltage source with that of a specific range.

DC Milliameter

Table 3: DC Milliammeter Accuracy Test

Range Current Applied Reading Accuracy

250mA 230mA 238mA 96.53%

25mA 25mA 25mA 100%

This test used small DC cells with the rated amperage as indicated in the table. The other DCmA

ranges were not tested because current sources of those ranges are not available.

Ohmmeter

Table 4: Ohmmeter Accuracy Test

Range Load Applied Reading Accuracy

10,000 44kΩ 4.4x10,000=44kΩ 100%

1,000 44kΩ 43x1,000=43kΩ 97.73%

100 1kΩ 10.2x100=1020Ω 98%

10 1kΩ 102x10=1020Ω 98%

1 200Ω 188x1=188Ω 94%

The test above was conducted by measuring different values of color-coded carbon resistors (44kΩ,

1kΩ and 200Ω). All used resistors are rated at ±1% tolerance. Considering this, all of the achieved

readings fall in the rated range of the resistors. Which means the project’s ohmmeter functions well

and measures accurately.

Evaluation and Recommendations

The figures presented above makes conclusive marks that the multimeter is in good electrical

condition and measures in high accuracy. All went well within the project, only except the

ohmmeter. The researcher compels the readers and researchers to enhance and improve the

multimeter circuitry, like adding an extra parameter of measurement (e.g. hFe tests, decibelmeter,

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frequency meter, etc.), or a buzzer circuit for continuity tests. The mathematical treatments used in

this paper may also be improved to achieve better results.

References

Buchsbaum, Walter. Buchsbaum’s Complete Handbook of Practical Electronics Reference Data.

Englewood Cliffs, NJ: Prentice Hall, Inc., 1975.

Ganic, Ejup, and Hicks, Tyler. McGraw-Hill Handbook of Essential Engineering information and Data.

USA: McGraw-Hill Companies, Inc., 1991.

Khandpur, R.S. Printed Circuit Boards: Design, Fabrication and Assembly. USA: McGraw-Hill

Companies, Inc., 2006.

Saul Marconi and Pagarigan. Basic Electronics, Solid-State Servicing, 1986.

http://www.mcmelectronics.com/content/ProductData/Manuals/80-5060.pdf

http://en.wikipedia.org/wiki/Multimeter

Appendices

Some significant parts of the study were not included in the research body. This section will discuss

additional information for easy understanding of the project, aided by tables and figures.

Resistor Color Coding

There are many types of resistors, both fixed and variable. The most common type for electronics

use is the carbon resistor. They are made in different physical sizes with power dissipation limits

commonly from 1 watt down to 1/8 watt. The resistance value and tolerance can be determined

from the standard resistor color code.

Figure 5: Color-coded Resistor

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A variation on the color code is used for precision resistors which may have five colored bands. In

that case the first three bands indicate the first three digits of the resistance value and the fourth

band indicates the number of zeros. In the five band code the fifth band is gold for 1% resistors and

silver for 2%.

The following table shows the comprehensive color-coding system used in resistors.

Table 5: Resistor Color-coding Reference

There is another scheme for resistors which have the values stamped on them. Since a decimal point

is easy to miss, this code uses R instead of a decimal point. For values over 100 W four numbers are

used. The tolerance is indicated by a letter. F for 1%, G for 2%, J for 5%, K for 10%, and M for 20%.

How to Read Multimeters

If you're using a multimeter for the first time, one of the things you'll need to learn is how to read it,

which isn't that obvious. This article explains in detail how to read the current and voltage using the

multiple scales on an analog multimeter or multitester (the kind with a pointer), not how to use a

multimeter. This article does not apply to resistance or dB measurements.

Determine the voltage at full scale. It depends on the setting of the range switch. The meter is

designed to give full scale when the voltage you're measuring matches the switch setting. Thus, if

the switch is set to the 30 volt range, this picture shows 30 volts is applied across the inputs.

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Similarly, if the switch is set to the 0.1 amp range, you have 0.1 amps of current running through the

meter.

Remember that the meter is linear. So at half scale (shown here), you can just divide the range

setting by 2. Here it is 150 volts on the 300V range, or 50mA on the 100mA range.

Expect variations in scales. In this example, there are two scales, 0 to 1 and 0 to 3. Not all meters are

like this. Some have 0 to 5, or 0 to 2, but scales are provided to match each setting of the range

switch. Just find the one that matches the switch, and then move the decimal point mentally.

This example shows 7.2 volts on the 10V range, or 216mA on the 300mA range.

Here it's 36.5mV on the 100mV range, or 11A on the 30A range.

Tips:

1. If the needle points below zero, then you've connected the "+" lead to a negative DC voltage

(compared to the "-" lead). Note this, and reverse the connections to take the measurement.

2. For DC (Direct Current)(Amps) measurements, the conventional current is flowing in to the

"+" lead and out of the "-" lead when the pointer operates properly.

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3. If the pointer goes above full scale, the reading is meaningless. Always select a high enough

range that the pointer stays at or below full scale.

Multimeter Theory of Operation

A multimeter is a combination of a multirange DC voltmeter, multirange AC voltmeter, multirange

ammeter, and multirange ohmmeter. An un-amplified analog multimeter combines a meter

movement, range resistors and switches.

For an analog meter movement, DC voltage is measured with a series resistor connected between

the meter movement and the circuit under test. A set of switches allows greater resistance to be

inserted for higher voltage ranges. The product of the basic full-scale deflection current of the

movement, and the sum of the series resistance and the movement's own resistance, gives the full-

scale voltage of the range. As an example, a meter movement that required 1 milliamp for full scale

deflection, with an internal resistance of 500 ohms, would, on a 10-volt range of the multimeter,

have 9,500 ohms of series resistance.

For analog current ranges, low-resistance shunts are connected in parallel with the meter movement

to divert most of the current around the coil. Again for the case of a hypothetical 1 mA, 500 ohm

movement on a 1 Ampere range, the shunt resistance would be just over 0.5 ohms.

Moving coil instruments respond only to the average value of the current through them. To measure

alternating current, a rectifier diode is inserted in the circuit so that the average value of current is

non-zero. Since the average value and the root-mean-square value of a waveform need not be the

same, simple rectifier-type circuits may only be accurate for sinusoidal waveforms. Other wave

shapes require a different calibration factor to relate RMS and average value. Since practical

rectifiers have non-zero voltage drop, accuracy and sensitivity is poor at low values.

To measure resistance, a small dry cell within the instrument passes a current through the device

under test and the meter coil. Since the current available depends on the state of charge of the dry

cell, a multimeter usually has an adjustment for the ohms scale to zero it. In the usual circuit found

in analog multimeters, the meter deflection is inversely proportional to the resistance; so full-scale is

0 ohms, and high resistance corresponds to smaller deflections. The ohms scale is compressed, so

resolution is better at lower resistance values.

Amplified instruments simplify the design of the series and shunt resistor networks. The internal

resistance of the coil is decoupled from the selection of the series and shunt range resistors; the

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series network becomes a voltage divider. Where AC measurements are required, the rectifier can

be placed after the amplifier stage, improving precision at low range.

Digital instruments, which necessarily incorporate amplifiers, use the same principles as analog

instruments for range resistors. For resistance measurements, usually a small constant current is

passed through the device under test and the digital multimeter reads the resultant voltage drop;

this eliminates the scale compression found in analog meters, but requires a source of significant

current. An autoranging digital multimeter can automatically adjust the scaling network so that the

measurement uses the full precision of the A/D converter.

In all types of multimeters, the quality of the switching elements is critical to stable and accurate

measurements. Stability of the resistors is a limiting factor in the long-term accuracy and precision of

the instrument.

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