Section Index Owatonna Steele County Amateur Radio 20-Jul-2007


A transformer is a static electrical device that transfers energy from one electrical circuit to another by magnetic coupling. A simple single phase transformer consists of two electrical coils (or windings) in close proximity to each other. The primary coil is normally the input, where the energy is applied. The secondary is the output. The primary is fed with a varying (AC or pulsating DC) electric current which creates a varying magnetic field around the coil. The magnetic field also suurounds the secondary winding because of its proximity. If the ends of the secondary are connected complete an electrical circuit, current flows out of the secondary. Energy is transferred from the primary coil to the secondary coil through the magnetic field.

Most transformers are used to convert energy from one voltage to another. These are generally termed power transformers because they are designed to transfer energy, or power. Other applications for a transformer include:

  • Audio signal generation such as microphones.
  • Isolate and link different parts of radio receivers and audio amplifiers.
  • A special type of transformer called a balun is used in radio and audio circuits to convert between balanced circuits and unbalanced transmission lines such as antenna downleads.
  • Current transformers provide a current in its secondary which is accurately proportional to the current flowing in its primary. They are commonly used in electricity meters to facilitate the measurement of large currents which would be difficult to measure more directly.

As indicated above, the simple transformer consists of two coils in close proximity to each other. Another component of a transformer is the core, which the coils are wrapped around. The core can be:

  • Steel: Keeps the field more concentrated around the wires, so that the transformer is more compact.
  • Ferrite: Used at high (radio) frequencies.
  • Air: Eliminates some types of loss in low-power, high-frequency applications.

Continuing on with power transformers, there are three basic types:

  • Step-Up indicating the output voltage is higher than the input voltage.
  • Step-Down signifying the output voltage is lowerer than the input voltage.
  • Isolation meaning there is the output voltage is the same as the input voltage.

The type of transformer is mostly determined by the ratio of the numbers of coils between the primary and secondary. The low-voltage windings have fewer turns of wire than the high-voltage windings.

The role of a transformer is to transfer power. Neglecting any losses in the transformer, the power applied to the transformer should equal the power used on the secondary side. Power is calculated by multiplying the votage times the current ( P = E * I ). It should be recognized that if the voltage changes across a transformer, the current changes flowing on the primary and secondary are inversely proportional. For example, consider 1,150 Watts flowing across a 230 VAC to 115 VAC power transformer. If the power being applied primary is 230 VAC at 5 amps, the current flowing through the 115 VAC secondary is 10 amps.

Schematic symbols for transformers may indicate:

  • Number of windings.
  • Relative number of turns between windings.
  • Type of core material.

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