Transformers
Introduction

The voltage of the domestic electric supply is 110 volts in North America and 230 volts in Europe. However, many electrical devices such as radios, computers, and clocks require only a few volts to operate. Inside all these devices is a transformer that changes (or steps down) the voltage to a lower value.
Double helix
A transformer is made of two separate coils of wire, both wrapped around the same piece of metal, usually iron.

Figure 1.   A low-power transformer from inside a portable radio.
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Transformers come in many different shapes and sizes.

Figure 2.   A high-power transformer used by electricity suppliers at a substation.
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The two coils in a
transformer
Transformers are electrical devices that can change the voltage of a signal. They only work with alternating current.
transformer
are called the primary and the secondary coils. These two coils are clearly drawn in the circuit symbol for a transformer, shown in Fig.3 below.

Figure 3.   Circuit symbol for a transformer.
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Click on the figure below to interact with the model.

 Figure 4.  A transformer in a 9 V d.c. circuit.



In Fig.4 a 9 V
direct current
Direct current (often abbreviated to d.c.) is when the flow of electric current is in a constant direction. Batteries are a source of direct current.
direct current
(
d.c.
The common abbreviation for direct current is d.c. This is when the flow of electric current is in a constant direction. Batteries are a source of direct current.
d.c.
) source and a 100 W resistor are connected to the primary coil of a transformer. A light bulb is connected to the secondary coil. Push the switch and then release it and see what happens.

What happens to the light bulb?
  • Are you sure?
  • Correct! The bulb flashes on when you press the switch and when you release it.
  • Look more carefully at the bulb.
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The primary coil behaves like a solenoid. When a current flows through it, a
magnetic field
Around every magnet there is an invisible magnetic field.
magnetic field
is generated in the metal core.

The secondary coil behaves like an induction coil. A current is only induced in the secondary coil when the magnetic field inside this coil changes.

The bulb only lights up when the current through the primary coil changes.


Click on the figure below to interact with the model.

 Figure 5.  A transformer with a 9 V alternating current supply.



The circuit in Fig.5 above uses an
alternating current
Alternating current (often abbreviated to a.c.) is when the flow of electric current changes direction periodically. The mains is a source of alternating current.
alternating current
(
a.c.
The common abbreviation for alternating current is a.c. This is when the flow of electric current changes direction periodically. The mains is a source of alternating current.
a.c.
) source.

Push and hold down the switch in the circuit and see what happens.

What happens to the light bulb now?
  • Correct! The bulb stays on when you hold the switch down (it may flicker a little).
  • Look more carefully at the bulb.
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Because the current supply is alternating, the current flowing through the primary coil is always changing. This means that the magnetic field generated by this coil also changes. As both coils share the same metal core, the changing field always induces a voltage across the secondary coil. This explains why the lamp is always on whenever the switch is pressed.

Transforming voltage
Transformers can be used to change (or transform) the voltage of an alternating electrical signal. The source in Fig.6 below generates a 12 V a.c. supply. The voltmeter on the left-hand side measures the voltage across the primary coil of the transformer. It displays a reading of 12.0 V.


Click on the figure below to interact with the model.

 Figure 6.  A transformer with a 12 V alternating current supply.



Click on the push button to send a current through the transformer.

The voltmeter on the right-hand side shows the voltage across the secondary coil.

What is the voltage produced across the secondary coil?
  •  V
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This type of transformer is called a step-down transformer because the voltage across the secondary coil is less than the voltage across the primary coil. The voltage has therefore been stepped down. A step-up transformer increases the voltage from the primary to the secondary coil.

The two numbers near to the transformer 3:1 mean that there are three times as many turns on the primary coil than on the secondary coil. The voltage on the primary coil is three times the voltage on the secondary coil.

Click on the number '3' and change it to '4'.

With the 4:1 setting, what is the voltage produced across the secondary coil?
  •  V
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Compare the voltage across the primary and the secondary coils. The voltage across the primary coil is …
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Calculating the voltage
The voltages across the primary and secondary coils are related to the number of turns on the coils. If you know the number of turns and the primary voltage, you can calculate the secondary voltage. The equation to use is shown below:


We can apply these equations to the previous experiments in Figs.5 and 6 above.

In the first case, the voltage across the primary coil (V1) was 12 volts.
We do not know the exact number of turns on each coil, but we do not need to. We do know the ratio of turns.
Now click on V2 in the equation above to rearrange the equation in terms of V2.
We can now use the rearranged equation to calculate V2.
     V2 = 4 V



In a similar way, we can calculate the secondary voltage (V2) when the ratio of turns is .

Assuming that V1 remains at 12 V, what will the new voltage across the secondary coil be when the ratio of turns is 6:1?
  •  V
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Set the turns ratio of the transformer in Fig.6 above to 6:1. Check whether your calculation in the question above was correct.

Summary


Transformers can change the voltage of a supply. Step-down transformers reduce the voltage and step-up transformers increase the voltage.

Transformers only work with alternating current. This is because a changing magnetic field is needed to induce a current.

The ratio of turns on the primary and secondary coils is equal to the ratio of the voltage across the primary and secondary coils.

Exercises
1. Which symbol below is the correct symbol for a transformer.
Figure 7.  
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2. The circuit is Fig.7 above is set up. What happens to the light bulb when the switch is closed?
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3. What happens to the light bulb in Fig.7 when the switch is opened.
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Figure 8.  
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4. What happens to the light bulb in Fig.8 above when the switch is closed.
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5. What happens to the light bulb in Fig.8 when the switch is open.
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Figure 9.  
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6. The diagram in Fig.9 above shows a transformer. The primary coil is connected to an a.c. supply. What do the letters a.c. stand for?
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7. Both the primary and secondary coils in Fig.9 are wrapped around the same core. What is the core made of?
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8. In Fig.9, what will be the value of the voltage on the secondary coil when you press the switch?
  • V
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9. When you press the switch, will the current flowing through the secondary coil in Fig.9 be a.c. or d.c.?
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10. Choose the correct values to complete the table below.

  • Primary voltage (V) Primary turns Secondary turns Secondary voltage (V)
    16 50 24
    16 50 150
    16 90 36
    30 18

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