Current and voltage digital measurement tools application in the electrodynamics demonstration experiment

O.A. Povalyayev, S.V. Homenko"Rosuchpribor"

Three equipment enlistments designed to hold demonstration experiment on electrodynamics have been developed and produced. They got names "Electricity 1", "Electricity 2" and "Electricity 3". Current and voltage digital measurement tools are also bundled. All the listed packages belong to "L-micro" equipment series. The "Electricity 1" package is assigned for DC studies, as well as the "Electricity 2" package is assigned for semiconductor electrical properties demonstration and "Electricity 3" is assigned for AC experiments such as the capacitor charging and recharging, induction and self-induction effects. Each package contains 8-10 items to create electrical circuits. All the elements are made up as standard modules which are installed at vertical surface of a class board, and have size 11 cm x 11 cm. The face module surface contains the electrical circuit element or its sign and clamps necessary for integrating with the electrical circuit. Some elements can not be mounted at standard module due to their sizes (e.g. choke-coil) or due to specific character of usage (e.g. thermoresistor). In this case the module with an appropriate sign is connected with its element by flexible wire. Fig. 1 illustrates the elements of "Electricity 2" package. The light-sensitive semiconductor elements' design provides for their installation perpendicularly to the board surface. The photodiode and the photoelement might be enlightened by lamp or other source of light (Fig. 2). When configuring these modules on a board ordinarily, the characteristics of semiconductor devices are demonstrated in "dark" mode. The "Electricity 2" package is an add-on to the "Electricity 1" package as well as the "Electricity 3" package is an add-on to the "Electricity 2" package. The demonstration experiment holding can not do without the tools for measurement of electrical values. It has been a tradition to use demonstration amperemeter with galvanometer and demonstration voltmeter with galvanometer (AG/VG). The demonstration pointer-type tools have to meet the following requirements: first, they should have great scale division and, consequently, the space between scale divisions is rather wide; second, they should provide with measurement precision. The AG/VG tools were designed to provide with good visibility. The mistake the digital tools make is usually in the last unit; it has nothing common with the size of digits. Modern engineering requires precise values, and digital measurement tools have replaced the pointer-type ones. The pointer-type measurement tools are considered as indicators in which the pointer indicates not the values but colored parts of a scale, for example a zone of normal temperature or a zone of high temperature. The destinations of pointer-type and digital tools in studies experiment should be parted the same way. The experiments, which acquaint students with the electrical circuit, usage of two kinds of tools is necessary. However, the experiment of higher level with quantity analysis of the studied laws should rely on digital tools. The pointer-type tools should be considered when studying their construction. The developed L-micro series digital measurement tools are compact and look naturally within a demonstration experiment mount. They indicate one number only that is why students pay attention to the result. The tools have digits bright and contrasted enough for visual perception from quite long distance. The greatest advantages of the developed digital devices are realized with usage of L-micro demonstration equipment because they are compatible by technical characteristics and design.

Current and voltage digital measurement tools

The module principle (Fig. 3) is the basis for assembling digital amperemeters and voltmeters, i.e. two multi-purpose digital indicators can be installed on different platforms called measuring modules which provides with the current and voltage measurements in different ranges and modes. The digital measurement tool is assembled before holding the experiment. It is presented to students as the whole one. The digital measurement tools package is applied at the earliest stage of students' getting aquatinted with electricity. It is proved by necessity of creating correct conception of operating with electrical tools and their position in electrical circuit. That is why the voltmeter is itself and amperemeter is itself too in the package. The assembled measurement tools are inserted into the electrical circuit as its components. It leads to easier understanding of the considered phenomena. One has to note that usage of multi-purpose tools requires higher educational level of students. Current and voltage digital measurement tools contain digital indication blocks (2 pcs) and the following measurement modules (the face surface labels and limit values are referred n brackets):

• DC voltmeter (V, 99.9V)
• DC amperemeter (A, 10.0 A)
• DC millivoltmeter (mV, 999 mV)
• DC milliamperemeter (mA, 999mA)
• AC voltmeter (~V, 99.9 V)
• AC milliamperemeter (~mA, 999 mA)

More than two parameters are not required to be measured during an educational experiment. That is why two digital indicators in package are enough to have. The measurement module has clamps on upper surface which are designed for connecting it to electrical circuit. The digital measurement block cutoff point is also present. The magnets designed to mount digital measurement tool at vertical metal surface are installed at bottom module surface. Each measuring module contains a board which controls mode of digital indication block (AC/DC). It also contains a board which adjusts the measured parameters with operating range of digital indication block, i.e. potential dividers or shunts. The digital indication block contains a three-digit LED indicator. It is fed by 220 V electrical supply network via network adaptor. The install and digital module plug-in cutoff point is mounted in the indication block case. The digital measurement tools usage instead of pointer-type ones does not change the traditional list of experiments which are held during the electrodynamics studying. However, many experiments meet the ability to study physical phenomena deeply. The brighter imagination of the phenomena observed also comes. The most significant difference between digital and pointer-type tools is high precision of digital ones. The "Electricity 1" package demonstrates it in the first experiments.

Direct current

When the students get acquainted with the current measurement they should use pointer-type and digital amperemeters both. The electrical circuit is assembled; it has a lamp, a jumper and a resistor. The lamp is plugged to DC source (Fig. 4). The pointer-type and digital amperemeters are utilized sequentially when measuring a current in different parts of circuit. It is concluded due to the amperemeters' values that the current is permanent in any flat subcircuit. Next the pointer-type and digital amperemeters are plugged into the circuit simultaneously. The students might be asked about measurement precision in each case. It is demonstrated during simultaneous current measurements that although the pointer-type and digital amperemeters give out similar values, due to their precision, they response differently to small current changes in circuit. The pointer-type amperemeter has lower precision than the digital one because its pointer remains immovable when the circuit changes slightly, while the digital tool reveals such changes. The same electrical circuit (Fig. 4) is utilized for the voltage measurement demonstration, via voltmeter, and comparison of pointer-type and digital voltmeters. When two measurement tools are plugged to resistor, they indicate the same values due to tools' precision (like the amperemeters do), but the digital voltmeter has wider range of the signals measured and higher precision. The measurement precision in experiments, which demonstrate current dependence on subcircuit resistance and subcircuit voltage, is 0.1 A and 0.1 V, respectively. Direct proportionality of current depending on voltage can be clearly shown (Ohm's law) if very simple values to operate with are utilized. If one considers a 2 Ohm resistor, then the following values of circuit and voltage can be found: (0.5 A, 1.0 V), (1.0 A, 2.0 V) etc. High precision resistors guarantee the success of an experiment. Such precision is provided with the "Electricity 1" package. The entire mentioned above can be referred to experiments on parallel and serial conductors linking as well as the experiments which demonstrate power on current sink.

Current in semiconductors

Digital millivoltmeter has 1 mV sensitivity and digital amperemeter has 1 mA sensitivity. It is found to be useful when studying physical properties of semiconductors and demonstrating the characteristic of semiconductor devices.

Temperature and illumination influence on semiconductor's conductivity

The semiconductor's conductivity rises when it is heated or illuminated by external source of light. To study temperature dependence on semiconductor's conductivity one has to assemble an electrical circuit (Fig. 5). To avoid the resistor's self-heating, the current should raise over 10 mA. Real semiconductor's resistance dependence on temperature comes in this way only. Thus, the experiment performance can not do without demonstration tool which measures the current at several milliamperes precision. The photoresistance experiment meets the same requirements. It demonstrates semiconductor's conductivity dependence on illumination.

Demonstration of p-n barrier properties

The studying of p-n barrier properties is staged in two parts. First, the lamp and the amperemeter connected in series are used to demonstrate the current presence or absence due to voltage polarity of a p-n barrier. When the diode is plugged in reverse conclusion, no current is indicated even by milliamperemeter. When the p-n barrier is getting opened, the forward voltage is rising. It can be demonstrated at the example of a LED when the current rising is followed by light emitting. Fig. 6 illustrates a scheme of the circuit mentioned above. In the beginning, when the voltage is rising the milliamperemeter registers no current and LED is not emitting. As soon as the LED voltage is reaching a certain quantity (about 1.5 V), the LED is beginning to emit light. The current is les than 0.1 mA but after the voltage is risen at several 0.1 V, the milliamperemeter indicate a current which significantly rises versus the voltage rising at a value about 0.1 V. The brightness is rising due to current getting raised. When experimental results are discussed, one has to pay attention to absence of light emitting at low voltage (up to 1.5 V); it is caused by potential barrier at the p- and n- regions.

Transistor intensive mode

We will consider the experiment on electric signal intension. Such experiment can not do without measuring tools, i.e. voltmeter and amperemeter. Fig. 7 illustrates the circuit necessary to hold the experiment. The lamp is plugged into collector branch. Collector's current is measured by digital amperemeter. The resistor, which limits the current of p-n emitter-base, is plugged into transistor base. The transistor is prevented from destroying. Base's current is measured by digital milliamperemeter. Variable resistor is a potential divider. It is designed to control the transistor's operating mode. Variable resistor sets the voltage applied to emitter-base barrier. The experiment begins at zero voltage applied to base barrier. The transistor remains locked and the lamp does not shine. Digital measurement tools both show zero current values. Next, the base-emitter voltage is rising and base's current is reaching (2-3) mA value. Transistor is opening and digital amperemeter indicates the presence of base's current. After the base's current has risen at (2-3) mA and the collector's current has changed, one can conclude that small change in base's current leads to significant change in collector's current. When the base's current raises up to 15 mA the collector's current raises linearly. This correlation is known as current multiplication factor. This is very important characteristic. The lamp shines if the current is over 0.8 A. The transistor is not already operating in linear mode. In other words, the collector's current in mentioned range rises more slowly than it would do in case of linear dependence. Linear dependence is true in case of small current values.

Photo-cell studies

The studying of photo-cell properties should be staged in two parts. Photo-cell is semiconductor device which transforms light energy into electric one. First, the presence of p-n barrier is demonstrated. Second, electrical characteristics are studied. It should also be demonstrated that photo-cell can be utilized as the source of electric power. Fig. 8 illustrates electrical circuit necessary to demonstrate the p-n barrier presence. The lamp of 3.5 V voltage is plugged in series with photo-cell. Photo-cell module is installed so that light does not reach its surface. Positive clamp of photo-cell is plugged to positive current source clamp. The lamp shines when the jumper is locked, as it is shown in Fig. 8. If the polarity changes, the lamp does not shine after plugging in. Thus, the photo-cell's polarity inversion coupled with absence of light on its surface leads to the same result as it would be in case of diode's polarity inversion. One can study features of the current behavior in semiconductor the same way as it was done during studies of LED characteristics (Fig. 6). If the voltmeter with high internal resistance is plugged to source clamps and the circuit has no external load then the measured voltage is very similar to source's EMF. Internal resistance of digital voltmeter is about 1 MOhm. That explains why digital millivoltmeter can be utilized to measure the photo-cell's EMF. EMF of photocell in darkness is zero. Photo-cell's surface enlightening leads to voltage rise. P-n barrier determines a certain potential barrier. Maximal value of open-circuit voltage is 0.5 V which is caused by value of potential barrier. Fig. 9 illustrates electrical circuit necessary to demonstrate photo-cell features. Digital voltmeter is plugged directly to photo-cell's cutoff points. Milliamperemeter measures current in variable resistor which is a load. The experiment demonstrates load voltage dependence on circuit's current when the photo-cell is enlightened at most. One can calculate the transmission capacity of a photo-cell using experimental data. The voltage value, which refers to a half of EMS, should be considered for calculation and current value is an appropriate one. Finally, students should be told that photo-cells are utilized in many different applications such as calculators and space stations. Although photo-cell has low voltage and power, solar battery which has required voltage and power characteristics is the combination of photocells.

Alternative current

Digital measurement tools provide with numeric indication of basic regularities of AC circuits. They also demonstrate how inductive and reactance impedances depend on frequency and, respectively, inductivity and capacity. Voltage distribution in complete circuit can be considered too. The experimental technique will be presented below.

Reactance impedance

The experiment which demonstrates how reactance impedance depends on frequency and capacity should be started at demonstrating the ability of capacity to conduct alternative current. Condenser and lamp are connected in series in a circuit. Sinusoid generator feeds this circuit and the following ones which refer to AC studies. Generator's frequency is set to a certain value which provides with reactance impedance of 1 Ohm or even less. The output signal raises its amplitude until the lamp lights bright enough. More detailed consideration of the effect studied can be done after demonstrating the presence of alternative current in circuit. Fig. 10 illustrates the circuit scheme. The capacitor is connected directly to AC source. Current is measured by digital milliamperemeter. Voltage is measured by digital voltmeter. Speaker is intended for detecting voltage frequency change by ear. Variable resistor is connected in series with the speaker; it is intended for volume control. The experiment begins with the frequency value of 20 Hz. The students are offered to calculate reactance impedance at that frequency due to data of digital tools. Next, the frequency of generator's output signal should be raised. Thus, the capacitor's current rises while its voltage remains the same. When the current value of the whole circuit reaches its maximal value due to generator, frequency raise should be stopped. The students have to be aware of the frequency value. The students have to calculate once again the reactance impedance due to current and voltage. It must be concluded that reactance impedance depends on frequency via inverse proportionality. To demonstrate such dependence, the students are shown the operating mode of circuit once again. Next the capacitor is changed to another one having 1/4 primary capacity (that is how the capacitors of "Electricity 3" package are sorted). The current becomes less in ratio of 4. It means that the reactance impedance has risen in ratio of for meanwhile the voltage has remained the same. It must be concluded that reactance impedance depends on its capacity via inverse proportionality.

Inductive impedance

An experiment which demonstrates AC properties in coil is similar to previous one. Fig. 11 illustrates the circuit scheme. Digital voltmeter and milliamperemeter measure appropriate values at choke-coil. The voltage of choke-coil remains the same. That is why the only reason of current change is the change in resistance of choke-coil. The AC choke-coil impedance has two components: inductive and active ones. Inductive impedance only depends on frequency and active impedance is constant. Generator's frequency raise leads to current reduction in choke-coil while its voltage is almost the same. Thus, it can be concluded that AC frequency raise leads to inductive impedance raise. Moreover, the inductive impedance is much more than the active one if the frequency is over 100 Hz. Linear character of the phenomena studied is shown clearly. To demonstrate the link between inductive impedance and inductivity, one should pull out slowly the ferromagnetic core out of coil. Thus, inductivity reduces. The current rises which leads to a conclusion that inductive impedance raises due to raise of inductivity.

AC series circuit

After the summary voltages measured by digital voltmeter at serial resistor, capacitor and coil are compared with the voltage of the whole circuit, one can conclude that the rules of AC voltage summing should differ from DC ones. The experiment demonstrates voltage distribution on elements of AC circuit. Such experiment is based on comparison of AC voltages distribution in active elements only versus coil and capacity. Fig. 12 illustrates how two different resistors R1 and R2 are connected to the sinusoid 50 Hz generator. Digital voltmeter measures summary voltage of R1 and R2 resistors. Summary voltage measured at resistors is compared with general voltage. It is concluded that if the circuit has active elements only then summary voltage equals general voltage. Then the R2 resistor is replaced with the C capacitor. The students have to note that general voltage of a circuit is less than a sum of respective voltages. The same phenomenon is observed in the circuit containing resistor and choke-coil. Finally, the circuit with choke-coil, resistor and capacity connected in series is assembled. The result in conclusion that algebraic sum of voltages does not equal the voltage applied to the whole circuit. The students are explained causes of the phenomenon observed. The rule of AC voltage summation is formulated. Next, digital milliamperemeter measures current and the measured current value is compared with calculated value due to Ohm's law applied for AC circuit.

Resonance studies

Resonance phenomena demonstration using digital amperemeters and voltmeters includes not only registering of current's dependence on frequency of voltage but even measuring the voltages distributed in circuit elements. Fig. 13 illustrates an appropriate circuit. The circuit includes the L choke-coil, the C capacitor and lamp. Digital milliamperemeter measures current. Lamp is intended to replace a resistor. It also represents the changes of current. The circuit is fed by sinusoid generator. Generator must keep constant voltage. Digital voltmeter monitors the voltage of circuit. Speaker is intended to determine changes in signal, by ear. Variable resistor which is connected in series with speaker is utilized for setting sound volume. Student should pay attention to changes in data values and speaker's tone. While the frequency is rising, the current is rising too but then it is reaching its maximal value and falling next. One should not that the lamp lights in very narrow frequency band near resonance. It does not light if current is less than 100 mA. Generator's frequency is measured in both spectral domains again and again to ensure that lamp light the most brightly and, respectively, current reaches its maximal value with certain sound tone. The students are announced of approximate resonance frequency. That frequency is determined due to generator's frequency scale. Resonance is coupled with the coil's and capacitor's voltage overgrowth in comparison with source's voltage. It must be concluded that "capacitor's" voltage equals "coil's" voltage and voltage of active element equals generator's voltage. However, choke-coil's impedance ((20-30) Ohm) is much higher than lamp's resistance ((1-12) Ohm due to circuit's current). Choke-coil's resistance distorts voltage distribution on circuit elements. That explains why choke-coil's voltage exceeds capacitor's voltage and lamp's voltage is lower than source's one. All the voltages are measured by digital voltmeter. The reasons of difference between measured and theoretical voltages are given. The next experiment demonstrates how resonance frequency depends on parameters of oscillatory circuit. Electrical circuit is fed by generator with operating frequency equaling resonance one ((300-330) Hz). The students observe lamp lighting and remember speaker's tone. The core is removed from choke-coil next which leads to inductivity reducing at a ratio about 5. The lamp fades and milliamperemeter detects current's reducing. The students are announced of a new generator's resonance frequency. Thus, they discover inverse proportionality between resonance frequency and inductivity.

Transformer studies

Fig. 14 illustrates an appropriate circuit necessary to study transformer and its properties. The L1 choke-coil is installed onto horizontal surface in front of stand or metal class board. Ferrite core is inserted into coil. The L2 coil is mounted to prominent part of a core. LED and photoresistor are connected with L2 coil in series. The circuit is fed by rectifier (* marks the cutoff point) or by 42 V AC voltage of classroom's electrical control unit. First, the transformer's functional properties are demonstrated. LED lights if the choke-coil's circuit is completed. Electrical power which is essential for LED feeding is transferred to LED while it is not connected with the source. An experiment on electromagnetic induction should be utilized to explain the phenomenon observed. Magnetic field is changing periodically in coil. The change causes EMF observed in coil. Digital AC voltmeter measures voltages of choke-coil (L1) and coil (L2). Turn ratio is calculated due to the voltages measured. The experiment should be held at two different voltages to demonstrate that the turn ratio does no depend on operating voltage values. To demonstrate how number of turns in coil influences the turn ratio, one should put an additional coil on the choke-coil (L1). These coils are connected in series. The secondary voltage rises doubly and it is proven so that output voltage depends on its number of coils.

The packages described above do meet modern requirements to educational experiment. These packages demonstrate as full as possible the statements represented in courses of physics. The Package provides with fast and obvious experiment. It helps student get the right conception of physics and develop skills.

Materials
1. www.l-micro.ru