FOUNDATION EXP 6 – RECTIFIERS

EXPERIMENT 6

RECTIFIERS

1.0INTRODUCTION

Diodes are widely used in switching circuits to clamp or clip input voltages to ensure that they do not go negative. Diodes used for this purpose are known as small signal diodes and are designed to switch small currents at high speed. Diodes are also used extensively in power supply circuits where they are used to rectify the alternating mains supply to produce a DC output.

2.0PROCEDURES

2.1 HALF WAVE RECTIFIER

For this part of the investigation, you will require one conventional diode and a 10kΩ resistor for R1. You will also need to use the Function Generator and oscilloscope for all the inclusive tests.

Figure 2.1

Connect up the half-wave rectifier circuit as shown in Figure 2.1. Switch on the scope, and prior to making any connections, use the AUTO SET button to initialise both channels.

Set up VS using the Function Generator, and use the appropriate controls and a BNC-BNC cable to obtain a sine wave signal with a frequency of 50Hz and a peak to peak voltage 4.0V, using the 50Ω output. Ensure that the DC OFFSET button on the frequency generator control panel has been pushed in. Use the SEC/DIV and VOLTS/DIV dials to obtain a suitable display on the screen, so that you can adequately see two cycles. Note down the corresponding settings.

Once you have set up VS, draw this signal in your lab book, noting down the waveform’s dimensions and scope settings.

Apply this signal to your circuit, by connecting a BNC-croc clip cable from the Frequency Generator to points A (red) and C (black – this is the earth/common point), and use another lead to observe this signal on CH1 of the scope.On Channel 2, observe the waveform at B (red) with respect to earth at C (black). Use the same VOLTS/DIV as you used when setting up VS. Draw VBC in your lab book, noting down the dimensions (peak-to-peak voltage, period) and scope settings. Note how VBC is periodic (i.e. repetitive at equal time intervals). Based on your observations, can you explain why this is called half wave rectification? You should have observed that only the positive half cycle of the sine wave appears across R1, hence the circuit being known as a half wave rectifier.

2.2 FULL WAVE RECTIFIER

A full wave rectifier may be produced by using a second diode, as shown in Figure 2.2.

Figure 2.2

For this circuit, you will need to set up VSby using the function generator so that it produces a sine wave from its 50Ω output, with a frequency of 50Hz, and a peak to peak voltage of 12V. Use the appropriate controls and leads to set up and test the required waveform, which should be displayed on Channel 1. Draw the sine wave in your lab book, noting down its dimensions and the corresponding scope settings.

Because the earth of both the oscilloscope and Function Generator are not isolated, scope measurements for the circuits of Figures 2.2 and 2.3 are not possible using the known conventional techniques. Thus, the scope has to be set up to function in differential mode. A consequence of this set up is that you will only be able to use the scope to measure one waveform, which in this case will be the output of the circuits. Therefore make sure you have set up your input waveform correctly before making the following adjustments to the way the scope operates.

To set the scope up in differential mode, follow these steps:

1. Set both Channel 1 and Channel 2 to DC coupling

2. Enable the red trace, i.e. press the red MATH MENU button

3. Set the scope to display CH1 – CH2

4. Connect BNC-croc clip cables from both channels

5. Connect the black ends of the cables together

6. Set both channels to 2V/DIV

7. Connect the red cable from Channel 1 to point D

8. Connect the red cable of Channel 2 to point C (earth point).

If you are unsure about the above procedure ask a member of staff for help.

Construct the circuit of Figure 2.2 on your breadboard, where RA = RB = 560Ω and are resistors that provide a mid-point reference node at C. R1 = 10kΩ, and C is the earth point. Connect VS to your circuit, but do not use the scope to measure it, as the scope’s earth will effect what is displayed on the output, so it is only necessary to measure the output, using the procedure outlined above.

To measure the output, use steps 7 and 8 as mentioned above. Draw the waveform into your lab book, noting the dimensions and scope settings. Based on your observations, can you explain why this circuit performs full wave rectification?

2.3 BRIDGE RECTIFIER

In practice it is more common to use a four-diode bridge rectifier for power supply applications. Such a circuit is shown in Figure 2.3, where R1 is a 10kΩ load resistor and point C is the earth point. Prior to carrying out the tests, ensure that both channels of the scope have been centred, and that DC coupling has been selected for both channels. Once this has been done, carefully construct the circuit of Figure 2.3 on your breadboard. For this part of the investigation, you do not need to include the capacitor.

Figure 2.3

For the same scope set up as described in the previous section, observe the output and the diode (BC) voltage waveforms VDC and VBC on the oscilloscope. Use the scope controls to obtain clear waveforms, and draw both waveforms in your lab books, not forgetting to note the dimensions and settings.

2.4 BRIDGE RECTIFIER WITH SMOOTHING CAPACITOR

The output from the circuit in Figure 2.3 (without the capacitor) would not be much use for power supply purposes, where a smooth DC output would normally be required.

To smooth the output, a capacitor is frequently employed as it can be connected so that it charges up quickly from the supply and then slowly (depending on the CR time constant) releases charge to the load. The time constant (remember Experiment 4) delays the change in the output, resulting in a change from the input. If CR is made large enough, the voltage across the load will not have fallen by very much before the next half cycle of the input arrives to quickly restore the charge on the capacitor back to its peak value.

(i)Switch off the function generator (i.e. disengage VS), and use anelectrolytic capacitor, 47μF or 100μF, (long leg is positive, short is negative) so that it is connected across points D-C of your bridge rectifier circuit, as in Figure 2.3.

(ii)Take care to ensure that the positive end of the capacitor is connected to point D, and the negative end to point C.

You must take particular care with the set up and recording for the following tests, as the traces will be needed for measurement purposes.

(iii)Firstly, using the same VS as in Section 2.3, turn on the function generator, and observe the voltage across D-C using the oscilloscope in differential mode. Draw this waveform in your lab book as accurately as possible, noting scope settings and dimensions.

(iv)Try to improve the resolution of the scope by adjusting the VOLTS/DIV for both channels (make sure they are the same settings for both channels). Observe and sketch the waveform across D-C again, recording your VOLTS/DIV and TIME/DIV settings and the waveform’s dimensions.

(v)Observe and draw the diode voltage waveform VBC.

(vi)Turn off the power supply, and replace the 10kΩ resistor with a 680Ω resistor. Repeat step (iv) again, and drawing the resulting waveform.

3.0COMMENTS AND CONCLUSIONS

1. Explain the operation of the circuits by identifying the times during which the diode(s) are ‘forward’ or ‘reversed’ biased, in particular indicating where evidence of the forward drop of 0.7V can be observed on your waveforms.

2. Use your waveforms derived for the half wave rectifier (Section 2.1) to determine the following:

a) The period and the frequency of the output signal.

b) The maximum value of the load current.

c) The value VD at the time instants when VS is a positive maximum, and when VS is a negative maximum. You will need to use Kirchoff’s Voltage Law (KVL) for Figure 2.1 (VS = VD + VL) and take measurements from the waveforms drawn in Section 2.1 (take care over the ± signs).

d) The % of the period for which VS is positive and the % for which it is negative. It should be 50%:50%.

e) The % of the period for which VL is positive and the % for which it is zero. Explain how the 0.7V diode drop affects this change from a 50:50 ratio.

3. For the bridge rectifier, and by taking into consideration the circuit when the 10kΩ resistor and capacitor (Section 2.4) are connected, determine the following:

a) Apply Ohm’s Law to the DC coupled waveform for the 10kΩ resistor load with the capacitor connected to estimate the average (or DC) load current.

b) Using the waveforms drawn in Section 2.4, part (v), determine the gradient of the waveform as the voltage is reducing (volts-per-second).

c) Check that the current and gradient values are consistent with the equation for a capacitor, stated below

4. Finally, investigate some possible uses of the different types of rectifiers studied above.

Dr. Daniel Nankoo1 of 5