Vol. DIY Electronics Projects
Chapter 4 AC Circuit Projects

AC Lab - Build a Transformer

In this hands-on AC electronics experiment, build a simple transformer and learn some important transformer fundamentals including voltage step-up and step-down.

Project Overview

Building your own transformer, as illustrated in Figure 1, is an excellent way to understand both how they are constructed and how they function. 


Illustration of a DIY transformer.

Figure 1. Illustration of a DIY transformer.


One nice thing about building this transformer using side-by-side coils, as shown in Figure 1, is that it helps us to visualize the meaning of the transformer schematic symbol illustrated in Figure 2.


he schematic symbol for a transformer.

Figure 2. The schematic symbol for a transformer.


Parts and Materials

  • Steel flat bar, 4 pieces
  • Miscellaneous bolts, nuts, washers
  • 28 gauge magnet wire
  • Low-voltage AC power supply

Magnet wire is small-gauge wire insulated with a thin enamel coating. It is intended to be used to make electromagnets because many turns of wire may be wrapped in a relatively small-diameter coil. Any gauge of wire will work, however, a thin 28 gauge is recommended so as to make a coil with as many turns as possible in a small diameter.


Learning Objectives



Step 1: If your flat steel bars do not already have holes in them to accept the bolts, as illustrated in Figure 1 above, drill bolt holes through the bars before proceeding with the following steps.

Step 2: As shown in Figure 1, wrap two equal-length bars of steel with a thin layer of electrically-insulating tape. You want to wrap several hundred turns of magnet wire around these two bars. You may make these windings with an equal or unequal number of turns, depending on whether or not you want the transformer to be able to step voltage up or down. I recommend equal turns to begin with, then experiment later with coils of unequal turn count.

Step 3: Join those bars together in a rectangle with two other, shorter bars of steel (Figure 1). Use bolts to secure the bars together.

Step 4: Using an ohmmeter, check for shorted windings after you’re finished wrapping them. Place one probe on the exposed wire ends and the other on the steel bar. There should be no continuity (infinite or very high resistance) between the winding and the steel bar.

Step 5: Check for continuity between winding ends to ensure that the wire isn’t broken open somewhere within the coil. Connect one probe at one end of one of the wires and the other probe to the opposite end of the same wire. The resistance should be very low. Repeat for the second wire.

Step 6: If either resistance measurements from steps 4 or 5 indicate a problem, the winding(s) must be re-made.

Step 7: Power the primary winding of the transformer (you can pick either one to be your primary) with the low-voltage output of the power supply described in the previous project, Using a Transformer to Build a 12 VAC Power Supply.

WARNING: Do NOT power your transformer directly from wall-socket voltage (120 volts), as your homemade windings really aren’t rated to handle any significant voltage!

Step 8: Measure the output voltage from the secondary winding of your transformer with an AC voltmeter.

Step 9: Connect a load of some kind (i.e., a light bulb) to the secondary winding and re-measure voltage. Resistors can also work as your load if they are rated for enough power. Note the degree of voltage sag at the secondary winding as the load current is increased.

Step 10: Loosen or remove the connecting bolts from one of the short bar pieces, thus increasing the reluctance (analogous to resistance) of the magnetic coupling between the two windings together. Note the effect on the output voltage and voltage sag under load.

Step 11: Retighten the bolts.

Step 12: If you’ve made your transformer with unequal-turn windings, try powering the opposite winding. This will change the transformer between step-up and step-down modes. If you built your transformer using an equal number of turns, unwind some of the turns on one side to make a step-up/step-down transformer. Repeat steps 7 through 9.


Related Content

Learn more about the fundamentals behind this project in the resources below.





Published under the terms and conditions of the Design Science License
  • hrrshal February 02, 2021

    How many times I should wound around the iron bolt. Minimum no.

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  • R
    Rijndael June 21, 2023

    I’m measuring a winding short with respect to the steel flat bar, but I’ve followed the instructions as written up until that point. What are my troubleshooting options? Add more electrical tape? Not sure what could be causing the short.

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