How to Build a Current Transformer

Written by paul dohrman
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A transformer changes the voltage of an alternating current. Higher voltages are easier to generate and lose less energy during long transmissions. A transformer then converts the high voltage to low voltage in residential settings. A current transformer, a specialised type of transformer used to measure the current through high-voltage lines without damaging sensitive ammeter equipment, consists of a magnetisable ring, or loop, that circles around the larger power line. Thinner wires wrapped around the ring connect to an ammeter. Use a few common items to build a current transformer of your own.

Skill level:
Moderately Challenging

Things you need

  • Electrical tape
  • Coated copper wire
  • Circular iron ring
  • Ammeter

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  1. 1

    Select a magnetisable ring, or loop, large enough for the heavy power line to slide through (like a ring fits on a finger) when you initially install the line. The ring can't become a permanent magnet; if it attracts a kitchen magnet but not silverware, then you've selected the right sort of material. Appropriate materials include iron and steel.

  2. 2

    Wrap the ring with several (perhaps hundreds of) loops of thin, coated copper wire all the way around the circumference of the loop. Cut the wire and leave the two ends long so you can attach the ammeter some distance from the power line, for safety reasons.

  3. 3

    Scrape the tips of the two wire ends free of coating with anything sharp so you can attach your ammeter.

  4. 4

    Calibrate the transformer (which consists of the ring plus the coiled wire around it) by attaching and ammeter to the two bare ends of the wire, then inserting a line of known current through the ring. The ratio between the ammeter reading and the known current will serve as your conversion factor. Then when you attach the transformer to a large power line, you can easily convert an ammeter reading to find the power line's current using a simple multiplication.

  5. 5

    Slide the ring onto the power line once you've installed the line. If the you've already installed the line and you won't be able to slide the ring on, make the ring out of four bars of iron or steel made into an enclosed D-shaped square ring. Just use common sense and wrap just three of the four bars with wire before wrapping the D-ring around the power line and attaching the fourth bar to close it off.

  6. 6

    Attach the bare wire ends of the transformer to an ammeter. Leave the ring permanently attached, only attaching the ammeter when you need to test the current.

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