How to make a school transformer

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This article explains how to make a demonstration transformer. A transformers is a device that converts one alternating voltage to another. A detailed description of how they work is given under transformer. The purpose of this page is to give details of how a demonstration transformer can be made very simply in a classroom in order to teach the basic principles. Demonstrations similar to this are usually performed in schools by pupils at KS4 level, under the supervision of a qualified science teacher.

You will need

  • Some pieces of soft iron, preferably in a U shape but a cylinder will do.
  • Plastic-coated wire
  • Some 6 V bulbs (torch/flashlight bulbs) in a suitable bulb holder
  • Connecting wire with 4 mm plugs
  • Alligator clips
  • A 4 V AC power supply that is fitted with a 5 A trip switch (IMPORTANT!) Under no circumstances should this homemade transformer be plugged into home current.


Transformer setup
Coils, supply, and utilization devices

Strip the insulation off the end of a long piece of copper wire and wrap it 30 times around a cylinder of soft iron, to make a solenoid. (If you can use a U-shaped piece of soft iron then so much the better. You could also use two more short pieces of iron attached to the two rod ends to help complete the magnetic circuit.) Connect the ends of the wire into the power supply using crocodile clips or other suitable connectors. This is called a primary coil. Connect a 6 V bulb in parallel with the primary coil.

Now take another long piece of wire. Strip the ends, and either: wrap it around the same soft iron core 60 times or (even better) if you have U-shaped soft iron cores; use a separate core to put the 60 turns on. This is called the secondary coil.

You should now have two pieces of bare ends from the second piece of wire. Connect them to a 6 V bulb so that you have a loop that is independent of the power supply.

Turn on the power supply and (if applicable) join the two u-shaped soft iron cores together. Be careful, the primary coil will now be a powerful electromagnet.

Compare the brightness of the two bulbs. The secondary will be much brighter.

Repeat with 15 turns on the secondary coil. This time the secondary will be much dimmer.

This is a quick and dirty demo suitable to teach the basics of transformers. Other experiments using professionally wound coils and Oscilloscopes are more suitable for more advanced or older students.

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