# Design Project: AC-DC Power Supply

## Discrete Semiconductor Devices and Circuits

• #### Question 1

Safety is very important when designing and/or building a device powered by potentially lethal electrical sources such as residential AC line power. Explain how you can prove that the metal case of your power supply is indeed “grounded” for safety, such that an internal fault from one of the “hot” conductors to the metal case will result in a short-circuit that will trip the power receptacle’s fuse or breaker rather than shock the individual touching the case?

Hint: a visual inspection is not good enough, and we don’t want to actually create a ground fault situation to test the grounding.

• #### Question 2

When connecting components together to build your power supply, it is important that you use the proper type(s) of wire. Identify what characteristics are required for the wires you use in this project, for each of the following parameters:

Wire gauge:
Insulation type:
Stranding (solid or stranded):
• #### Question 3

How do we tell which winding of the step-down power transformer is the primary, and which is the secondary, without actually powering it up with AC line power? This is often an issue when students purchase cheap transformers that are unmarked and undocumented.

• #### Question 4

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.

• #### Question 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.