Laboratory 5: Half- and Full-wave Rectifier Circuits

Transcription

1 Laboratory 5: Half- and Full-wave Rectifier Circuits Laboratory 4 Laboratory 5 Laboratory 6 Aims and Objectives The aim of this laboratory is to investigate the effect of a non-linear device on ac signals. By the end this laboratory you will: -know how half and full-wave rectifier circuits work; -know how to extract relevant information from a diode datasheet and use them to select suitable rectifier diodes; -be familiar with the MATHS function of the oscilloscope. Guide to progress You have two half-day sessions to complete this laboratory. Theory related to this laboratory will be covered in lecture in week 4. You should attempt to reach the end of section 1 during the first half-day session. If you are stuck on a problem for more than a few minutes or if you feel you are falling behind, then seek guidance from a demonstrator or a member of academic staff member. 9

2 1. Half-wave rectifier circuit In this section you will investigate the effect of a single diode on a sinusoidal voltage waveform. Figure 1 shows a circuit diagram of a half-wave rectifier circuit. This circuit can be realised on the PCB provided to you. Secondary coil D1 1N4001 RL1 10 k Figure 1. Half-wave rectifier circuit connected to the secondary coil of the transformer. (a) Briefly explain the operation of the circuit shown in Figure 1. Sketch the voltage waveform you would expect at the output (across RL1) for a sinusoidal input to the rectifier circuit. (b) Remove all components and links placed on the PSU Design Exercise PCB from the previous laboratory session before you start this laboratory. (c) Connect the step-down transformer to the PCB and switch on the transformer. (d) Use both channels of the oscilloscope to observe the waveforms at two outer-taps of the transformer making the centre-tap as the reference voltage level (ground). i.e. connect CH1 probe to TP1 and CH2 probe to TP3 and one of the grounds (croc clip) of oscilloscope probes to TP2. Note down your observation(s) of the two waveforms. TIPs: use x10 probes, select the same scales for the vertical axis for both channels, and adjust the vertical position of both waveforms to observe the two waveforms clearly. (e) Measure the following quantities of one of the waveforms (e.g. CH1) i. peak-to-peak voltage ii. mean voltage (f) By placing components on the component holder posts in the PCB set up the halfwave rectifier circuit shown in figure 1 across one of the outer-taps (first yellow block connector) and the centre-tap (black) of the transformer. Always use the centre-tap 10

3 as the reference terminal (Ground). Place the links on headers J1, J4 and J5 appropriately to complete the circuit. (g) Use CH2 of the oscilloscope to observe the voltage waveform at the output of the rectifier circuit while observing the input to the rectifier circuit with CH1. Test points TP4 or TP9 can be used to observe the output of the rectifier. (h) Comment whether any part of the output signal is actually of sinusoidal form. (i) Sketch both waveforms on a graph paper and mark all important quantities on the sketch. (j) Measure the following quantities of the waveform at the output i. Peak-to-peak voltage ii. Mean voltage v. peak reverse voltage across the diode TIPs: You may need to use MATHS function in the oscilloscope to find this. Select the same vertical scale for both channels To switch off the MATHS menu press and hold the MATHS button for a few seconds. (k) Briefly explain the reason for the reduction in rms value in the rectified output signal compared to input signal. (l) As an engineer it is important to interpret information given in datasheets of electronic components. The datasheet for the diode is being used in this exercise, 1N4001, is given in the appendix at the end of this script. From the datasheet find the peak reverse voltage that the diode can withstand and thus comment on the suitability of the diode for the rectifier circuit you have just built. (m) Search for a suitable replacement diode for the above application with the price. Briefly explain reason(s) for your choice supported by relevant parts of its datasheet. To complete this part you may need to know some component suppliers in the UK and web addresses of two such companies are given here for you to make a start. But you are welcome to search the component from any supplier known to you. RS components - Farnell - DO NOT dismantle the circuit as you will build upon what you have competed so far in Section 2. If time permits you could start section 2. 11

4 2. Full-wave rectifier circuit In the previous part you have observed that when an ac signal is applied to a diode a considerable portion of the signal is lost. Therefore the half-wave rectifier circuit does not appear to be an effective way of delivering maximum possible power to a load. In this section you will build a full-wave rectifier circuit that produce a signal with a greater rms value. (a) Add another identical half-wave rectifier to the PCB as shown in Figure 2. Use CH1 and CH2 of the oscilloscope to monitor the outputs of the two rectifier circuits. Note: You may need to place links on headers J2 and J3 to complete the circuit. Secondary coil D1 RL1 10 k RL2 10 k D2 Figure 2. Two half-wave rectifier circuits. (b) Sketch the two waveforms and note down the following quantities of the output waveform of the second rectifier. i. peak-to-peak voltage ii. mean voltage The above values should be close to the measurements you obtained in Section 1 part (i). (c) Based on your observations of the two waveforms suggest a suitable modification(s) to your circuit to form a full-wave rectifier circuit with a single load resistor (Take RL1 as the load resistor). Draw the circuit diagram of suggested full-wave rectifier circuit and briefly explain the operation of your circuit. (d) Implement your design on the PCB. (e) Use CH1 and CH2 of the oscilloscope to observe the input and out of the full-wave rectifier circuit. Sketch both waveforms. (f) Measure the following quantities of the waveform at the output i. peak-to-peak voltage 12

5 ii. mean voltage v. peak reverse voltage across a diode (TIP: you may need to use MATHS function in the oscilloscope to find this) (g) Comment whether the selected diode is still suitable for the application based on measurements you took in (f) and datasheet of the component. Now you can tabulate the following quantities from sections 1 and 2: i. Peak-to-peak voltage ii. Mean voltage iv. Frequency for the input sinusoidal signal (between Yellow and Black), and outputs of the half- and fullwave rectifier circuits. See whether the values are sensible. Store the circuit to use in the next laboratory exercise. RNG/JKAE 01/13 13

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