Brandeis University Physics 29a Fall 2018 Kevan Hashemi

# Lab 5: Transformers and Rectification

Overview: A rectifier is a circuit that turns an alternating current into a direct current. You are going to build the following power supply circuit in stages. It consists of a transformer (T1), a four-amp fuse (F1), a bridge rectifier (D1-D4), a smoothing capacitor (C1), and a bleed resistor (R1). If you blow the fuse, get another one. You will apply a 60-Ω, 20-W power resistor to the circuit as a load. The resistor and the diodes of teh bridge rectifier will get hot.

Part 1: Connect the 60-Ω power resistor directly into the output of the transformer and fuse. Do not insert D1-D4, C1, or R1. Plug the input of the transformer into AC power. Measure the amplitude and frequency of the transformer output. Given that AC power is 110 V rms (root mean square), what is the turn ratio of your transformer?

Part 2: Remove the 60-Ω power resistor. Insert diodes D1-D4 and R1, but do not insert C1. Look at the output from the rectifier. What is the peak voltage? What is the frequency of the peaks? What is the rms voltage?

Part 3: Insert C1. Note that it has positive and negative terminals. Measure the average output voltage. What is the average current in R1? The periodic variation in the output voltage is the ripple. What is the peak-to-peak amplitude of the ripple? What is its frequency? What is the current flowing into the capacitor when it charges up? What is the current flowing out of the capacitor when it discharges?

Part 4: The V and 0V terminals in the circuit diagram above are the output of your power supply. Connect a 60-Ω power resistor the output of your power supply. Measure the average output voltage. How much power is dissipated in the 60-Ω resistor? Calculate what you expect the ripple amplitude to be for this 60-Ω load. Measure the ripple amplitude. What perfect voltage source in series with what resistance would make a good model for your power supply?