Brandeis University Physics 29a
Spring 2017 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. The periodic variation in the output voltage is the ripple. What is the amplitude of the ripple? What is its frequency? Use the capacitor equation to calculate what you expect the ripple amplitude to be, given the average output voltage and the values of C1 and R1.

Part 4: Connect the 60-Ω power resistor to the output of your power supply. Measure the average output voltage. How much power is the 60-Ω resistor dissipating? What is the equivalent output resistance of your power supply? Measure the ripple amplitude. Calculate what you expect the ripple amplitude to be for this 60-Ω load.