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Multimeter Design Kit for Circuit Theory Education

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Multimeter Design Kit for Circuit Theory Education Multimeter Design Kit for Circuit Theory Education Seth W. Percy, Joshua E. Perry, Carlos L. Lück Electrical Engineering, University of Southern Maine Abstract— One of the most common tools faculty guidance throughout for the practicing electrical engineer is the implementation. multimeter. This paper describes a kit that was fabricated for use in laboratory Index Terms— Multimeter, Galvanometer, experiments to explore concepts of circuit D'Arsonval meter movement, Voltmeter, design and implementation of a simple Ammeter, AC, DC, PCB, 3-D printing, analog multimeter. The commercial Laboratory, Electromechanical, Multi-scale, multimeter is a combination of several Analog, Voltage, Current, Resistance, Needle individual instruments to measure voltage, Deflection, Electrical Engineering current, resistance, and capacitance. Common multimeter designs incorporate I. INTRODUCTION digital displays. Before the digital age, the ur multimeter laboratory kit was designed world relied on analog displays. Some argue O to combine the functionality of a DC that analog displays are outdated. However, voltmeter, DC ammeter, and an AC this overlooks one of the most important voltmeter into a single instructional design characteristics of analog displays, which is experiment with the intention of offering a an immediate visual and qualitative viable and memorable hands-on application of perception based on needle deflection. fundamental electrical engineering concepts to Analog displays are still used in many supplement the theory. The functionality of the applications such as aviation and multimeter is obtained by configuring resistor automotive, demonstrating their continued networks in series and parallel with the usefulness. The analog display is based on galvanometer. These networks are an electromechanical device called implemented on a PB-505 prototyping board. galvanometer. The galvanometer consists of The 12-position rotary switch is used to select an incrementally marked face and a needle its functionality to produce varying scales indicator. The needle deflection indicates between the three modes of operation. The the intensity of the flow of a small direct various scales provide the user with the ability current passing through the device. This kit to adjust the scale of the instrument. This consists of a galvanometer, a ribbon cable paper is organized as follows: this introduction with a breadboard connector, three external section is followed by sections on background, lead connectors, a 12-position switch and a design methodology, construction, results, fuse, all packaged in a 3D-printed enclosure laboratory experiment implementation, resembling a handheld commercial conclusion, acknowledgements, and multimeter. Students use the kit to design a references. multi-scale DC voltmeter, DC ammeter and AC voltmeter on a breadboard in the II. BACKGROUND circuits laboratory. A parts list and 3D The multimeter has been an essential tool for model files are publicly available online for the engineer in practice. As technology has other institutions and individuals to utilize. transitioned into the digital age, it has become This project was student-led with close prevalent within the modern industry to use Proceedings of the 2019 ASEE Zone 1 Conference, Niagara Falls, New York, April 11-13, 2019. 1 digital handheld units. This device made it convenient to obtain all measurements needed from any given circuit using only a single set of probes. Modern designs have become very compact to allow for the electrical engineer in practice to carry a portable one with them with great ease. However, as comfortable and convenient as it is to implement this tool, it is critical for the engineer in training to understand a number of aspects such as the core principles of its operation, the internal Figure 1. Conceptual representation of the components involved, and how the concepts Galvanometers functionality. [8] originated in its design. Understanding these aspects develops a deeper appreciation for its usefulness and effectiveness as a tool. Perhaps the first concept to address is the notion that this tool relies on an engineers visual perception. Modern commonly accepted standards for these devices generally produce a digital readout of the input, and because digital technology has made great strides in recent decades, a fair amount of trust is accepted with the use of this technology. This trust is based merely on countless hours of precision and Figure 2. Common example of an analog accuracy testing over a vast range of handheld multimeter (Left) [6] and a digital application spanning decades. However, handheld multimeter (Right). [1] before digital displays, people relied on analog ones to display some measure of an input As can be derived from Figure 1, when a stimulus visually. Figure 2 contains two direct current is applied to the coil, a magnetic pictures of common multimeters. On the left is field is generated that responds opposing to the an example of a common center-justified field generated by the permanent magnet analog multimeter. On the right is a Fluke located in the center. This subsequently causes model and is a very typical example of a the coil to twist and cause measurable needle modern industry-trusted model. deflection based on its position with respect to The analog meter that we use is the the incremented face of the meter. [2] galvanometer, also called D’Arsonval meter A typical galvanometer contains an internal movement. It is a simple tack gauge that resistance in the order of kiloohms. A full- experiences deflection when current is passing scale deflection requires a current in the order through it. The intensity of the current of microamperes. Based on Ohm’s Law, the determines how much needle deflection is voltage across the galvanometer at full achieved. A representation of this can be seen deflection will be in the order of millivolts. in Figure 1. The fundamental mathematical models that govern the functionality of the instrument as part of a multimeter design are Ohm’s law, Kirchoff’s laws, voltage division, and current division. The mathematical model for Ohm’s Law Proceedings of the 2019 ASEE Zone 1 Conference, Niagara Falls, New York, April 11-13, 2019. 2 establishes that the current (I) through a understanding and design of the multimeter. component is directly proportional to the input Using this basic knowledge the student should voltage (V) applied across it, divided by its be able to construct a design with variable overall internal resistance (R). This law is modes of operation with each mode consisting symbolically represented in the following way of varying scales to safely measure a range of below in equation (1). input stimuli. A final conceptual requirement of the V designer is how this tool is used as a single I (1) R enclosed unit and how it is subsequently recognized in common schematic designs and As a consequence to these laws, all application. A medical analogy useful for resistances in series share the same current, distinguishing the difference in form between and all resistances in parallel share the same a voltmeter and an ammeter is that a voltmeter voltage. can be described as being a non-invasive Voltage division is a technique used to procedure conducted via a parallel connection determine the potential voltage drop across a with a single component or some subset of the selected resistor (Vx) within a given network. It circuit. This parallel requirement is essential is based on a simple ratio between the selected following Kirchhoff’s voltage law with resistance (Rx), the total resistance in the regards to sharing the same potential as adding network (Rt) obtained via series association, another branch to the circuit. The student and the applied voltage (Vs). It is worth noting should also understand that the voltmeter has that the total resistance (Rt) and the source minimal current passing through it and thus voltage (Vs) may also refer only to a particular conceptually has a virtually ideal infinite branch or subsection within a network. resistance. Voltage division can be symbolically The ammeter, on the other hand, can be represented in the following way below in thought of as an invasive procedure in which equation (2). one must break the physical circuit and place the unit in a series connection to obtain a R measurement. This series connection allows VV x (2) xSR the meter to share the same current passing t through it as the series components or nodes following Kirchhoff’s current law. By this Similarly, current division is used to understanding, unlike the voltmeter, the calculate the current in a particular branch (I ) x student should recognize that the ammeter is of a network based on the ratio of the total conceptually designed around a virtually ideal resistance (R ) obtained via parallel t zero resistance, and thus does not impede the association, to the resistance of the branch in flow of current passing through it. Because of question (R ), times the applied current (I ). x S the potential to damage an ammeter from Current division can be symbolically passing too much current through it, a fuse is represented in the following way below in placed in series with the Galvanometer. equation (3). Standard fuses for most handheld ammeters can withstand up to 0.5A before experiencing Rt IIxS (3) filament breakage resulting in an open circuit Rx typically described as a floating connection in design. These mathematical models and concepts provide the basis for the fundamental Proceedings of the 2019 ASEE Zone 1 Conference, Niagara Falls, New York, April 11-13, 2019. 3 The intent was to develop an understanding SolidworksTM 3D parametric modeling of the construction and implementation of a software produced by Dassault Systemes and multimeter and produce a cost-effective 3D printing with PLA for the material of strategy for teaching these aspects to students choice for its cost-efficiency and ability to be in a fashion that constitutes a memorable and safely printed in the confined space without rewarding hands-on experience.
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