Lab 9: Diodes: Power Supplies
In this lab you will use build 4 different types DC power supplies. You will then use the final supply to power a neon lamp relaxation oscillator.
Caution: The circuits you are using in this lab have enough power to fry components. Use caution. Both you and your partner should double check each circuit before applying power to it.
In order to do this lab you need to understand the characteristics of diodes.
Ch 3.8, 4.1-2, 10.1, 11 + lecture notes.
(Ch 11 has detailed information on power supplies.)
Oscilloscope probes. Obtain a 10X scope probe and connect it to channel 1. Verify that CH1 is configured for the 10X probe. The voltages in this lab will be too high to measure with the 1X probe.
Scope set up. Set up the scope. In this lab the function generator is the power grid. Oscilloscopes allow you to easily synchronize the sweep to the 60 Hz power grid. Select line under trigger controls, to synchronize the sweep to the 60 Hz power grid. No additional connection is required because the oscilloscope is plugged into the same 60 Hz power grid as your transformer. Be sure to DC couple the oscilloscope channel!
The Transformer (ch 3.8). In previous labs we used the function generator as a source of AC sine waves. The function generator is very flexible, but it is not designed to supply power. For power supply applications we use a transformer. Typically the frequency will be fixed at the frequency of the local power grid, 60 Hz in the US. In this lab the transformer is rated to convert a 115 VAC 60 Hz to 24 VAC. This transformer has a center tap, a third output terminal connected to center of the secondary winding. Because the voltage induced in the secondary winding is proportional to the number of turns between the taps, the center tap allows us the also get 12 VAC from the same transformer. The center tap makes the transformer more flexible.
Grounding. When we used the function generator we found that we did not need to ground our oscilloscope probes because both the grounds of the function generator and of the oscilloscope are connected through the power supply ground. Now that we are using a transformer, this is no longer true! We will need to connect the scope probe to the point in the circuit we define as ground.
Sketches. In this lab you are asked to sketch the waveforms you observe. You must not only have the correct shape, but also measure the peak voltages. In a few cases you will observe a sine wave offset from ground. In that case you need to measure the maximum voltage at the top of the sine wave as well as the minimum voltage at the bottom.
Components: In the following circuits, diodes are 1N4148.
Measure the Output of Your Transformer. Use the oscilloscope with the 10X probe to measure the peak voltages from your transformer. Measure and sketch: Va to Vb, Va to VCT, and Vb to VCT .
Q1) 5pts. Sketch the three (3) waveforms and measure peaks. Be sure to label the peak voltages on your sketch.
Q2) 5pts. What is the maximum power you can get from your function generator? Consider that its Thevenin equivalent circuit is a 10 Vp-p AC sine wave source with a source resistance of 50 Ω. This is a question for later. (Hint: Please refer to the section of the Thevenin-Norton lecture notes handout Maximum Power Transfer.)
The Half-Wave Rectifier. The simplest DC supplies can be made using one rectifier. The disadvantage is that it only provides power for half of the cycle. Build the following circuit and sketch the output wave form.
Q3) 2pts. Sketch the waveform and measure peak voltage of the output and compare it to the peak voltage of the transformer.
Most circuits that require a steady source of DC power rather then the train of DC pulses produced by the previous circuit. Clearly the half wave rectifier cannot produce any power during the half cycle when the diode is reverse biased. What we need it a way to store energy during the half cycle when the diode is conducting and release it during the half cycle when it is not. This is achieved with a capacitor. The capacitor charges up to the peak voltage and then can discharge into the load during the next half cycle.
CAUTION: The capacitors you are using are polarized. Be sure that they are connected to your circuit in the correct with the correct polarity. The polarity is marked on the capacitor.
Q4) 2pts. Sketch the waveform and measure peak voltage of the output and compare it to the peak voltage of the transformer.
Q5) 2pts. In this circuit, how many diode drops Vf are between the transformer and the output?
The Two diode Full-Wave Rectifier. Two half-wave rectifiers can be connected so that the diodes conduct during alternating half cycles. This configuration requires a center tapped transformer. An advantage of this circuit is that only two rectifiers are required. The disadvantage is that a large secondary winding is required.
Q6) 2pts. Sketch the waveform and measure peak voltage from Vout and compare it to Va and to Vb of the transformer.
Add the capacitors shown in the schematic and repeat the above measurements.
Q7) 2pts. Sketch the waveform and measure peak voltage from Vout and compare it to Va and to Vb of the transformer.
Q8) 2pts. In this circuit, how many diode drops Vf are between the transformer and the output?
The Full-Wave Bridge Rectifier. The bridge rectifier is a full-wave rectifier constructed from 4 diodes. Check and double check the direction of your diodes. If one is reversed it can destroy the other diodes.
Q9) 2pts. Sketch the waveform and measure the peak voltage from Vout and compare it to Vin.
Add the capacitors shown in the schematic and repeat the above measurements.
Q10) 2pts. Sketch the waveform and measure the peak voltage from Vout and compare it to Vin.
Q11) 2pts. In this circuit, how many diode drops Vf are between the transformer and the output?
The Voltage Quadrupler. In this section you will build a charge-pump voltage quadrupler. Read ch 4.2.5 (p417) for an explanation of how it works.
Q12) 5pts. Sketch the waveforms and measure the peak voltages for all the points shown in the diagram. Compare the voltages to Vin.
Q13) 5pts. For each point you measured above, how many diode drops Vf are between that point and Vin?
Q14) 5pts. What is the Thevenin equivalent circuit? Use the scope for these measurements. Set the automatic measurement to Vrms, this will accurately include the contribution of 60 Hz ripple on the DC output of the quadrupler, which is very pronounced under load. You have measured the open circuit voltage VTH. You need to calculate the source resistance. It is usually a bad idea to short circuit a power supply because it could be damaged. In this case use a 10 K resistor as the load resistor and then measure the output voltage while it is connected to the load resistor. Calculate the equivalent source resistance from these two measurements. (Show your calculations.)
Using your Voltage Quadrupler to Power a Neon Lamp Relaxation Oscillator. Connect the output of the voltage quadrupler to a 10 μF capacitor with a 1M resistor. Place the neon lamp in parallel with the capacitor. The neon lamp will flash. Monitor the voltage across the capacitor using the scope. Set the time base to 1 s/div. You should see a saw-tooth waveform. To make a voltage measurement, you can freeze the display using the Run/Stop button, then use the cursors to make measurements from the waveform.
Q15) 2pts. What is the minimum and the maximum?
Q16) 1pts. What is the period (in seconds) of the flashing neon lamp?
Q17) 2pts. Based on the Thevenin equivalent circuit you found for the power supply, what is the RC time constant of this circuit? (Check your notes on RC transients. Hint: include both Rs and R.)
Q18) 2pts. What happens when you replace the 1 M resistor with a 330 K resistor? If the neon lamp does not flash (remains illuminated), try a 3.3 M resistor instead. What is the new period (in seconds) and the resistor you ultimately used?