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Exploring Concepts of Current and Voltage

Jon M Pearce
Institute of Education
The University of Melbourne
Parkville, Victoria, Australia

Introduction

Much research has been carried out during the last few years into students' understanding of the basic concepts of current and voltage. Given a circuit containing a battery, for example, students will generally make certain assumptions about its behaviour. One such assumption is that batteries act as ideal voltage sources and hence maintain a steady voltage regardless of the changing load of a circuit. This is a reasonable assumption since (a) they usually do behave that way, and (b) most of our real world examples of power supplies are constant voltage sources (mains voltage applied to circuits and appliances, for example).

However, there are some circuits for which many students reason assuming a constant current source (even though they have generally not been introduced to one formally). This was noticed with some undergraduate Physics students who were presented with the circuit shown below.

They were asked to describe the change in brightness of globe A if globe B were removed. Many answered that the globe's brightness would increase significantly since all the current now flows through the remaining globe.

Figure 1. Two globes in paralle

(As an interesting aside, it would be instructive to investigate how frequently students making this response use a water analogy as part of their reasoning strategy. Often water flow does behave in a constant current manner - turn on a second garden hose and the first one's flow rate decreases, etc. It is ironic that we are often wary of using the water analogy of electricity because many students do not have a good intuitive understanding of fluid flows. Maybe in this case their understanding is correct, but applied to the wrong situation!)

It was as a result of these observations that this piece of demonstration/experimentation equipment was designed and constructed. It is being used to investigate students' understanding in this area as well as to help them resolve their own misunderstandings.

The Purpose of the Equipment

The demonstration board consists of a power source that can be switched from a constant current source to a constant voltage source. The sources can also be switched to power either of two sets of three light globes, arranged in series and parallel respectively. The globes may be loosened from their sockets, and, in the case of the series globes, shorted to ground by an attached wire (not shown in the photo).

Two meters measure the total current in the circuit and the voltage presented to it, respectively.

Students are given the task of

(i) determining which setting of the power supply switch supplies a constant current and which a constant voltage,

and then

(ii) making predictions as to what will happen to the brightness of the globes and readings on the meters as various changes are made to the circuit (such as removing globes). These predictions are then checked by experiment.

Layout of the Board

The board was constructed using particle board mounted on a small wooden frame (to allow room for components to protrude out the back and wiring to be done on the back). The electronic circuit was mounted in a small plastic box with a three-way switch performing the function of turning the power to the lights on and off, as well as selecting constant current or constant voltage supply (see figure 2 where A and B indicate constant current and constant voltage sources, respectively).

Figure 2. View of top of demonstration board.

A banana socket was mounted adjacent to each series globe (G4 to G6) so that each could be shorted to the ground rail using a lead connect to the lowest socket. The globes chosen were rated at 6 volt, 100 mA. This choice was made with some care. It was required that the globes show some light when presented with as little as one-third their nominal voltage and again when presented with one-third their nominal current. Unfortunately this means that the external supply voltage must be about 22 to 25 volts to enable the voltage regulator to supply adequate voltage to deliver full current to the three globes in series. (Lower voltage-rated globes would fail to light at a voltage much below their rating).

The meters were `universal panel meters' supplied from Dick Smith Electronics (Q-2045, 100 uA movement). One was given a shunt resistor (1.3 [[Omega]], made from two 2.7 [[Omega]] in parallel) to make it into an ammeter reading about 316 mA full-scale-deflection. A scale was drawn calibrated in 100 mA divisions (i.e. one, two or three 'globes worth').

The other was made into a voltmeter by using a 180 k series resistor. This produced a range of approximately 0 to 18 volts (again calibrated in 6 volt divisions to make it easy to read as one, two or three globes worth).

In each case, some fiddling of the meter resistors was necessary to make the meters read reasonably accurately as globes were added or removed. It was thought that this was worthwhile so that students could think in terms of units of current or voltage corresponding to the globe ratings, rather than be distracted by more accurate readings in mA or volts.

The Circuit

The circuit was based around a voltage regulator integrated circuit (LM317) which is switched to supply either a constant 6 volts or a constant 100 mA (figure 3). With the options of three globes in either series or parallel, this means that it could be supplying up to 300 mA or 18 volts, depending on the arrangement of globes. This IC tended to get rather hot, so a heat sink (small strip of metal) was attached to it, protruding from the back of the board.

Figure 3. Constant current and constant voltage circuit.

The circuit was constructed on a piece of 'vero-board' and mounted inside a small plastic box. A double-pole-double-throw switch (shown as two separate components on the diagram below) was used to isolate the globes from the supply, or to connect then to either of the power sources. Switch position A provides constant current while switch position B supplies constant voltage. Note that when the switch is 'off' (centre position) power is still being supplied to the circuit by the external power supply.

All resistors were 1%, 0.25 Watt, except for the 12 [[Omega]] resistor which was rated at 1 Watt. The voltage regulator IC requires a heatsink (a metal strip protruding from the back of the box is adequate).

The capacitor was a 10uF, 35 V tantalum. Its function, together with the 15k resistor, was to smooth out voltage transients that caused globes to burn out when the switch was changed.

Should you wish to alter the voltage or current output values, they are derived as follows:

output voltage = 1.25 x (1 + ) volts,

thus the 2k trim pot enables the voltage to be varied from 4.7 volts to 11.5 volts.

output current = = 100 mA

Conclusion

The circuit described serves as a useful demonstration tool for introducing the concepts of constant current and constant voltage sources. It is also useful as a piece of laboratory equipment through which students can make predictions about these concepts and then test their application.

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These pages are maintained by Jon Pearce ( jonmp@unimelb.edu.au), Department of Information Systems. The opinoins on them do not necessarily reflect those of the University of Melbourne. Tel: (613) 8344 1495 Fax: (613) 9349 4596. Last update: September 16, 2003 .