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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.
Page 2 of 12 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.
Page 3 of 12 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
Design Planning Purchase of Materials PCB Etching
Component Placement Casing Assembly Finalizing Soldering Evaluation
Page 4 of 12 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
Page 5 of 12 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.
Page 6 of 12 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:
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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
Page 7 of 12 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,
Page 8 of 12 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.
Page 9 of 12 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.
Page 10 of 12 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
Page 11 of 12 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 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
Page 12 of 12 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.
