Facebook
Google
GitHub
LinkedinDiscrete Semiconductor Devices and Circuits
Design Project: AC-DC Power Supply
5 questions By Tony R. Kuphaldt
-
Question 4 of 5
When you measure the low-voltage AC output of your transformer and compare that measurement to the rectified and filtered DC output voltage, you will notice something very counter-intuitive. The DC output voltage reading is substantially greater than the unrectified AC voltage from the transformer’s secondary winding!
Explain why this is, and then show mathematical calculations that relate the two voltage measurements together with reasonable accuracy.
Reveal answerThis is something that catches almost all students by surprise when they first measure the voltages. I’ll give you a big hint why the rectified and filtered (DC) output voltage is so much greater than the unrectified (AC) secondary voltage: the AC voltage measurement you make with your voltmeter is most likely an RMS measurement, not a peak measurement.
Notes:Let students figure out this mystery on their own: it is one of those phenomena that really reveals the nature of RMS measurements in contrast to peak measurements.
-
Question 5 of 5
One thing you will want to do with your completed power supply is subject it to a full-current (1 amp) load, and test the output voltage. To do this, you will need a load that draws close to 1 amp without overheating or causing other problems.
A resistor will work fine for this task, but which resistor should you use? Identify the two parameters you must be concerned about when selecting a load resistor for the task, and explain exactly how those parameters will be calculated.
Reveal answerThe two parameters are: resistance (how many ohms), and power rating (how many watts). I will leave it to you to show how to calculate each parameter for your particular power supply.
Notes:Even though this is nothing more than an application of Ohm’s Law, do not be surprised if students approach you dumbfounded by this question. There is a large cognitive difference between calculating current and power for a resistor of known value and a voltage source of known voltage, and selecting a resistor based on known current and voltage for a practical test of a power supply. While studying Ohm’s Law in a theoretical context, students become comfortable making calculations on paper, but may not realize just how to apply that same math to a real-world situation. Or, they may express apprehension when faced with having to make calculations that carry real consequences (such as damaging their power supply!).
