EE 43 100 INAL P ROJECT AN AUDIO AMPLIFIER EE 40FFinal Lab An Audio Ampli er Part 1 Power Supply Lab Guide In this lab we will build and analyze a simple AC to DC converter You will receive a breadboard and some parts that you may keep and take home Take care to build this circuit carefully so that it will last a while Treasure it forever 1 Analyzing The Transformer The transformer we will be using in lab is shown in Figure 1 The primary winding is connected directly to the wall outlet Place an oscilloscope probe across the secondary winding plug the transformer into the wall and sketch Vout DO NOT SHORT THE SECONDARY WINDING What is the maximum voltage you see at Vout What is the minimum How does the waveform di er from your expectations and why is it this way 2 Adding In The Bridge Recti er Now change your circuit to look like Figure 2 We ve taken the output of the transformer and fed it through a bridge recti er circuit Sketch Vout What is the maximum voltage you see at Vout What is the minimum What is the frequency of Vout Why 120 Vrms 60 Hz US Wall Outlet 10 1 Figure 1 Transformer circuit 1 Vout EE Final Project AudioAmpli er Amplifier Part I EE43 100 40 Final Lab An Audio 2 120 Vrms 60 Hz US Wall Outlet 10 1 Vout Figure 2 Adding in the bridge recti er 120 Vrms 60 Hz US Wall Outlet 10 1 Vout 2200 uF 2 2 k Figure 3 Loading the bridge recti er 3 Analyzing The Bridge Recti er Usually we place a capacitor on the output of the bridge recti er We then use this signal to power another circuit or electric device The other circuit we power is known as the load and can usually be approximated by placing a resistor on the output of the bridge recti er When we make these two changes to the circuit we end up with something like Figure 3 Together the R and C form a low pass lter Sketch Vout What is the average voltage seen at Vout 4 Bridge Recti er Ripple We like to power our circuits or electronic devices with an constant DC voltage However it s extremely di cult to get a truly constant DC voltage We can get an approximation to it however and often we like to compare DC power supplies based on how closely they can approximate this constant DC voltage A useful metric to measure the quality of an AC to DC converter is the output voltage ripple Vripple Often the output of a DC power supply will be approximately the DC value we want but it may rise above EE43 100 40 Final Lab An Audio EE Final Project Audio Ampli er Amplifier Part I 3 Linear Voltage Regulator Vin Vout Figure 4 A simple linear voltage regulator LM340T5 linear voltage regulator Figure 5 LM340T12 or fall below the DC value from time to time We say that the output voltage ripples We de ne Vripple as the peak to pick voltage of the ripple Use your oscilloscopes to measure Vripple for this very simple AC to DC converter What is the frequency of this ripple voltage 5 Linear Voltage Regulator One way to combat ripple voltage is to use a device called a linear voltage regulator v reg This device is illustrated in Figure 4 In the most simplistic view a v reg can be thought of as two variable resistors Vout is then obtained from Vin through a simple voltage divider circuit The v reg then adjusts the values of the two variable resistors until the output voltage is desirable If the input voltage changes it will dynamically adjust the two resistors so that the change is not seen on the output Using a v reg is a great way of cleaning up a power supply to generate a constant DC voltage LM340T5 The 12 5 su x means that this v reg is The voltage regulator we will use in this lab is an LM340T12 5 volts The pinout of this voltage regulator is shown in Figure 5 designed to output 12 Modify your circuit to look like Figure 6 Notice that the load resistance has been moved to the output of the v reg This is because we will power our circuit or electronic device from the v reg output Recall that the resistor approximates the load so it needs to connect to the v reg output just like another circuit or electronic device would Sketch Vout What is the average value DC component of Vout What is Vripple EE Final Project AudioAmpli er Amplifier Part I EE43 100 40 Final Lab An Audio 4 120 Vrms 60 Hz US Wall Outlet 10 1 in LM340T12 LM340T5 2200 uF out gnd Vout 220 uF 2 2 k Figure 6 Adding a linear voltage regulator 6 Response To A Changing Load Suppose the v reg is powering another circuit the load Then there will be some current owing from the v reg output to the load Suppose the load suddenly changes This will cause the current to change and because the v reg is just generating the voltage with a voltage divider Figure 4 the output voltage will change as well A good v reg will notice this change and quickly adjust the two variable resistors to compensate However this compensation takes some time Another way to measure the quality of an AC to DC converter is to measure how quickly it can respond to a changing load Modify your circuit so that it looks like Figure 7 Note that the pinout for the MOSFET is shown in Figure 8 Sketch Vout and IL on the same axes To get IL measure the voltage across R and use Ohm s law Approximately how long does the v reg take to stabilize the output voltage 7 E ciency We de ne the e ciency of a power supply to be the ratio of the output power to the input power It s interesting to nd the e ciency of this simple AC to DC converter Finding this e ciency does require a few circuit modi cations The modi ed circuit is shown in Figure 9 The output power is relatively easy to nd Simply measure the current going through the 50 load resistor and use P V 2 R Note the load resistance changed here to provide a slightly higher output power The input power is slightly harder to nd Insert a 1 shunt resistor as shown in Figure 9 Now if we measure the voltage across this resistor we will actually be measuring the current owing into the circuit Over one period make several meaurements of the input current IS and VS Use the fact that P IV to get the instantaneous power for each measurement Now average all your values to get the average power owing into the circuit After doing all this you should be able to get an e ciency for the circuit to the right …
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