How to Mount a Toroidal Transformer

Written by david sandoval
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How to Mount a Toroidal Transformer
Toroidal transformers are the round copper-coloured devices on a circuit board. (Printed Circuit Board image by Rob Hill from

Transformers have two primary uses: to isolate DC electrical energy from one side of an electrical circuit, and to increase or decrease the voltage of an AC signal. One type of transformer --- the toroidal transformer --- is commonly used in tight spaces or when high transformer efficiency is required. A toroidal transformer is a special type of transformer that has two or more wires wrapped around a circular core. The way these wires are wrapped around a toroidal transformer's core reduces "stray coupling," or loss of energy due to unwanted changes in the transformer magnetic field.

Connecting a toroidal transformer into a circuit --- also known as "mounting" the transformer --- requires connecting the wires from the transformer in the proper order to ensure that the circuit works properly.

Skill level:

Things you need

  • AC power supply; adjustable
  • Resistor; 1 kiloohm
  • Needle-nose electrical pliers
  • Digital multimeter

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  1. 1

    Check the transformer owner's manual to obtain the transformer's turns ratio (see Tips). Connect one of the transformer's primary lead wires to one of the terminals on the AC power supply. Connect the other primary lead wire from the transformer to the remaining terminal on the AC power supply.

  2. 2

    Twist one of the resistor's leads together with one of the secondary lead wires from the transformer. Twist the other resistor lead together with the remaining secondary lead wire.

  3. 3

    Turn on the multimeter, and set the scale to "Volts AC." Connect the red multimeter probe to one of the primary leads on the transformer. Connect the black multimeter probe to the remaining transformer primary lead. Turn on the power supply; the voltage across the primary leads will equal the power supply output voltage.

  4. 4

    Disconnect the multimeter probes from the transformer primary leads. Attach the red multimeter probe to one of the secondary leads from the transformer. Attach the black multimeter probe to the remaining lead from the transformer secondary. Observe the display; the voltage across the secondary leads will be equal to the primary voltage multiplied by the reciprocal of the turns ratio. For example, if the power supply voltage is 2 volts AC and the transformer has a 1:2 turns ratio, the voltage across the secondary will be 4 volts AC.

Tips and warnings

  • The turns ratio represents the proportion of wire turns around the transformer's core. The primary of a transformer with a 1:2 turns ratio has half as many wire turns around the core as the secondary has.
  • Toroidal transformers do not have a universal colour code for the wire leads attached to the transformer. Consult the owner's manual or transformer documentation to determine which wires lead to the primary and secondary terminals.

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