Sensors: Loggers: An EASYSENSE capable of fast logging Physics Logging : 500 ms Teacher s notes 56 Diodes: A.C. diode rectification Read In investigation 55, students will have found out that diodes only conduct electricity in one direction. They can therefore be used to convert an alternating current onto a direct current. This process is known as rectification. In a car, the dynamo is an alternator. The car s circuits use d.c. It is therefore necessary to rectify the a.c. produced by the alternator. The mains supply is a.c. but transistors, integrated circuits, and many applications require d.c. and rectifier circuits depend upon diodes. Which raises the question, why is the mains a.c.? why do we supply a.c and then spend money and energy converting it to d.c? In this investigation, you will find out how diodes are used as rectifiers, and learn about the basic circuits in use in rectifier circuits. Students will investigate the rectifying effect of: 1. A single diode. 2. A single diode with a capacitor. 3. A four diodes in a bridge circuit. 4. A four diode bridge circuit with a capacitor. The investigation will be carried out in two parts: A) 56a A diode rectifier. B) 56b A bridge rectifier At the end of the investigation students should have learnt that, 1. The meanings of the terms Full and Half wave rectification. 2. A single diode can only achieve Half Wave rectification. 3. A bridge circuit can achieve Full Wave rectification. 4. Capacitors can be used to smooth the output. 5. The ripple voltage is less if the capacitor is larger. 6. Better smoothing circuits use another capacitor and an inductance. Apparatus 1. An EASYSENSE logger capable of fast logging. 2. 2 Smart Q sensors ±12 or 20 V. 3. 4 diodes (1N4000). The diodes should all be of the same type and value. 4. 1 kω resistor 5. Switch. 6. Capacitors 470 μf, 100 μf, 33 μf, 3.3 μf. 7. Signal generator or low voltage a.c. power supply. 8. Wires and crocodile clips. T56-1 (V2)
Set up of the software The logger needs to be set to a fast log for this experiment. A The diode rectifier 56a AC rectification (diode) Recording method Total recording Intersample Graph 500 ms 200 μs Start condition sensor (1) Trigger Units Pre-trigger Rises above 0.0 V At Start The use of the trigger means that all waves will be in phase with each other, analysis of the data is simplified. B The bridge rectifier 56b AC rectification - bridge Recording method Total recording Intersample Graph 500 ms 200 μs Start condition sensor (1) Trigger Units Pre-trigger Rises above 0.2 V At Start For the trigger to work with the fully rectified and smoothed output the data collection will need to have been started before the power is connected. The rise in the output voltage should then release the trigger. Notes 50 Hz is used because the experiment is designed to investigate power supplies from the mains. The signal generator has (may have) an appreciable internal resistance. This causes the supply voltages to be reduced when a significant current is flowing. 1N400 diodes are suggested as they are easy to source, carry a good current and have an easily identified polarity mark on them. Other diodes could be used. In the investigation into full wave rectification, it is not possible to have two sensors in the circuit at the same. There are 2 ground voltage rails created, the Smart Q sensors need to be connected to common ground. Students are asked to make one recording with the sensor in the position of sensor 1 in the circuit diagram and then repeat the log (with overlay selected) with the sensor in the position of sensor 2 in the circuit diagram. Using the trigger keeps the data in phase. Sample results and analysis T56-2 (V2)
The data as originally collected and displayed. Zoom used to show about 4 cycles, of the input voltage and the diode voltage. The Half Wave rectified output voltage only shown. The Half Wave output smoothed using a 100μF capacitor. T56-3 (V2)
Data for the Full wave rectification. The input voltage, the Full Wave rectified output un-smoothed, and the smoothed output voltage. The smoothing capacitor had a value of 470 μf. Learning summary Half Wave rectification The graph of the input voltage. It is a sine wave. The output voltage across the load resistor R when a diode has been put in series with it. The diode is only conducting when it is forward biased. It is only conducting during half of each cycle of the input voltage. The output voltage is Half Wave rectified. The current only flows for a maximum of half of each cycle of the input voltage. T56-4 (V2)
The effect of connecting a 100μF capacitor in parallel with the load resistor. The output voltage has been smoothed. There is still a ripple voltage. Using a larger capacitor will reduce the ripple voltage. Full Wave Rectification The input voltage being Full Wave Rectified. The current now flows during both halves of the output cycle, and in the same direction all the. It is therefore easier to smooth and there is less energy loss. The smoothed output shown here was achieved with a 470 μf capacitor. Smoothing circuits with two capacitors and an inductance are even more effective, and the output can be made very smooth. T56-5 (V2)