Design Project: AC-DC Power Supply

As a general policy, I inspect each and every one of my students’ power supplies before energizing for the first time. This does not mean, though, that I am the one making the meter measurements! Each student must prove to me that their grounding is adequate by making the meter measurement under my direct supervision. This way, they learn how to do it themselves while safety is still personally ensured by me.

I also have the students tug on the wire connections while making this test, to ensure that it is not a random low-resistance measurement we are seeing with the ohmmeter. Loose wire connections are best tested by resistance measurements under mechanical stress, not by visual inspection alone.

Ampacity figures may not be readily available for wires of the gauge most electronics students are accustomed to dealing with. However, ampacity for small-gauge wire may be roughly calculated by plotting data points of published wire ampacities for wire of several larger gauges, and then extrapolating downward on the gauge scale. If nothing else, this would be a great example to students of how to use simple statistical techniques (regression, curve fitting, plotting) to make practical estimations.

Care should be taken if the transformer in question is a complete unknown. It may be impossible to tell what the respective voltage ratings of the windings are if one does not even know whether the transformer was designed for line power (120 volts, 60 Hz AC in the United States) or not. Small power transformers are easy enough to obtain from electronics parts suppliers and from scrap consumer electronic devices (stereos, computer accessories, etc.) that no one should have to take chances with a completely unknown transformer.