Transformer – power supply

Chapter 4.2 Transformer – power supply

Lessons In Electric Circuits Volume VI – Experiments Book
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Summary of Contents

Lessons In Electric Circuits Volume VI – Experiments Book

  • 4.2. TRANSFORMER – POWER SUPPLY1474.2Transformer – power supplyPARTS AND MATERIALS• Power transformer, 120VAC step-down to 12VAC, with center-tapped secondary winding(Radio Shack catalog # 273-1365, 273-1352, or 273-1511).• Terminal strip with at least three terminals.• Household wall-socket power plug and cord.• Line cord switch.• Box (optional).• Fuse and fuse holder (optional).Power transformers may be obtained from old radios, which can usually be obtained froma thrift store for a few dollars (or less!). The radio would also provide the power cord andplug necessary for this project. Line cord switches may be obtained from a hardware store. Ifyou want to be absolutely sure what kind of transformer you’re getting, though, you shouldpurchase one from an electronics supply store.If you decide to equip your power supply with a fuse, be sure to get a slow-acting, or slow-blowfuse. Transformers may draw high ”surge” currents when initially connected to an ACsource, and these transient currents will blow a fast-acting fuse. Determine the proper cur-rent rating of the fuse by dividing the transformer’s ”VA” rating by 120 volts: in other words,calculate the full allowable primary winding current and size the fuse accordingly.CROSS-REFERENCESLessons In Electric Circuits, Volume 2, chapter 1: ”Basic AC Theory”Lessons In Electric Circuits, Volume 2, chapter 9: ”Transformers”LEARNING OBJECTIVES• Transformer voltage step-down behavior.• Purpose of tapped windings.• Safe wiring techniques for power cords.SCHEMATIC DIAGRAM