MM0704, Lesson 4
Task. The skills and knowledge taught in this lesson are common to all missile repairer tasks.
Objectives. When you have completed this lesson, you should be able to explain the purpose and function of
transformers and describe their use, as well as the use of reflected impedance matching and the losses and turns ratio.
Conditions. You will have this subcourse book and work without supervision.
Standards. You must score at least 70 on the end-of-subcourse examination that covers this lesson and lessons 1, 2,
and 3 (answer 23 of the 30 questions correctly).
One of the most common electrical devices used in radio and radar equipment is the transformer. This device is used to
up or step-down, current step-up or step-down, frequency selection, and isolation of circuits.
Transformers are also designed and used to accomplish specific jobs. Your understanding of the actions of a
transformer is of considerable importance.
Recall your study of electromagnetism and inductance, in Basic Electricity, Part I. You learned that a wire or coil in
which current is flowing has a magnetic field about it; you also learned that the amount of current determines the
relative strength of that magnetic field. Another factor is that if a magnetic field cuts through a conductor, a voltage
will be induced in the conductor. These important facts are the bases of the principles of transformer operation.
With two conductors parallel to each other as in figure 4-1, the magnetic field caused by the current flowing through
circuit P sweeps across circuit S inducing a voltage in it. You can apply the terminology used in discussing
transformers to explain this simple circuit. Call circuit P the primary since it is from this source that the magnetic field
is produced. Call circuit S the secondary because it is in this circuit that a voltage is being induced.
When the switch in the primary is closed, the current starts to flow and build up rapidly from zero to its maximum
value. This produces a rapidly expanding magnetic field around the primary. Remember that motion is necessary in
order to have an induced voltage. The motion in this case is the expanding field which cuts the secondary loop,
inducing a voltage in it. This