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					Department of Electrical and Electronic Engineering Electrotechnique 114 Practical class 3
Procedure • • Each student should keep his/her own notes during practical classes, detailed enough for preparation for tests and examinations. The attendance of practical classes is mandatory and is a prerequisite to obtain admission to the examination. Students will be marked present after successful completion of the practical class.

Aim • • • To measure the characteristic line of a non-linear element. To use a load line to solve a circuit. To practically illustrate Thévenin equivalent circuits.

Components and apparatus 1 LED; resistors (1 × 680 Ω, 1 × 820 Ω en 1 × 1.2 kΩ; 1 × 1.5 kΩ; 1 × 1.8 kΩ; 1 × 3.3 kΩ; 1 × 3.9 kΩ); 1 terminal board. 1 power source and a digital multimeter are used for the measurements. 1. LED characteristic line

An LED is a non-linear element. This means for example that if the voltage across the anode and the cathode of the LED is doubled, the current will not necessarily double as well. The characteristic line of the LED can be found when drawing a graph of the LED current against the LED voltage. This characteristic line fully describes the element and it can be used to determine the voltages and currents of a circuit that contains that LED. Figure 1 shows a measuring arrangement that can be used to measure the characteristic line. This circuit is similar to the one that was used in practical 1. (In that practical we were however only interested in the brightness of the LED for different currents.)
1k2

I LED

E

+ VLED -

anode katode

Figure 1 • • Build the circuit of Figure 1 and make sure that the LED is connected correctly. The long leg is the anode and the short leg the cathode. Measure the current through the LED and the voltage across the LED by adjusting the power supply voltage E from 0 V to 15 V in steps of 1 V.

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There is only one multimeter per bench available, so the current and the voltage can therefore not be measured simultaneously. The easiest way is to measure the voltage of the power supply and the LED and to then calculate the current be using ILED = (E – VLED)/R. Take note: The voltage meter of the power supply is not accurate. Use the multimeter to measure the voltages accurately. • Now use the graph paper to plot the characteristic line. Draw ILED on the y-axis and VLED on the x-axis.

The characteristic line should look similar to that shown in Figure 2. (Each LED is unique and the characteristic lines may differ from component to component.)

Figure 2 2. Load line

The load line technique is a very handy method to determine the voltage and current of a nonlinear circuit. Take a look at the circuit in Figure 3. We have already drawn a characteristic line of the current against the voltage for the LED. ILED is therefore known (graphically) for each voltage VLED. The power supply and the resistance is linear elements and we can easily write down an equation for the relationship between the current and the voltage, namely ILED = (E – VLED)/R.
lineêre deel 1k2 nie-lineêre deel

I LED

10V

E

I LED + VLED -

anode katode

Figure 3

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The circuit can now be solved easily, because VLED and ILED is the same for the linear circuit and the LED. A graphical solution can be found by plotting the two curves for ILED on the same graph. The point of intersection of the two curves is then the solution. We have already plotted ILED against VLED in 1. Know we have to plot ILED = (E – VLED)/R on the same graph. • • • 3. Draw the equation ILED = (E – VLED)/R on the same graph as the LED characteristic line and read the LED voltage and current from the graph. Now build the circuit of Figure 3 and measure the LED voltage and current. (Make sure that the voltage of the power supply is accurately set by using the multimeter.) How do the practical and theoretical values compare? Thévenin equivalent circuits

R1 820

R2 680 1k5 R3

A

15.5V

E

Figure 4 • • • • • Build the circuit of Figure 4. Measure the voltage VA. Now connect the LED across point A and ground and measure the LED voltage and current. How does this measurement compare with the measurements of 1 and 2? Calculate the Thévenin equivalent circuit of the circuit of Figure 4 and draw it. How does this compare with the circuit in Figure 3?

PS.: This measurement should confirm that the circuits of Figure 3 and 4 are equivalent and that they deliver the same voltages and currents if the same load is connected to the circuits. 4. Star-Delta connection

RC 1k8

A 3k3 RA

15.5V

E

3k9 RB

Figure 5

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• • • •

Build the circuit of Figure 5. Connect the LED across point A and ground and measure the LED voltage and current. Show that the Thévenin equivalent of this circuit is the same is in the two abovegiven cases. Use the star-delta transformation to indicate that Figures 4 and 5 are equivalent.


				
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