Transformers
Transformers use magnetic fields to change an AC voltage. A transformer consists of two electromagnet coils (called the primary coil and secondary coil) next to each other, so that the magnetic field of one goes through the other. Usually, a heavy iron core is used to enhance the effect. The primary coil is connected to the input voltage; the output is induced in the secondaty coil. When the AC current sent through the primary changes polarity, its magnetic field changes as well. This fluctuating magnetic field induces current in the secondary coil. The AC in the secondary will have the same frequency as the AC sent into the primary, but the voltage will be different, depending on which coil has more windings (loops of wire). If the secondary has more loops than the primary, the AC voltage output from the secondary will be higher than the voltage input to the primary. That would be a step-up transformer. Conversely, if the secondary has fewer loops than the primary, the output from the secondary will be lower voltage than the input voltage to the primary — a step-down transformer. If Nprimary and Nsecondary are the number of wire loops on the primary and secondary coils, respectively, then... |
Here is the schematic symbol for a transformer.
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Remember, transformers only work for AC!
Activities & Practice
to do as you read
Play with this magnetism simulation. Click the Transformer tab at the top, and experiment. The primary coil is on the left, and is labelled the "electromagnet" on the screen. The secondary coil is on the right, and is called the "pickup coil" on-screen.
(a) What happens when you turn on (or off) the battery?
(b)
If a steady DC current is flowing in the primary (i.e. you aren't flicking it on or off, it's just on) what voltage do you get out of the secondary?
(c) Now put AC into the primary. You can adjust the amplitude and frequency of the AC input. If you increase the amplitude of the input, what happens to the amplitude of the secondary output? If you increase the frequency of the input, what happens to the frequency of the secondary output?
You use transformers every day, but you might not be aware of that fact. Those blocky, somewhat heavy AC adapters that power or charge small electronic devices usually have a transformer in them. Some people call them "wall warts" or "power bricks". The transformer is the heaviest part, because of the iron core. The transformer steps down the voltage from 120 VAC to the voltage needed by the electronic device, typically 6 VAC or 12 VAC. Other components, namely diodes and capacitors, then convert the AC into DC. This will be discussed in the next notes page. Some electronic devices are happy being fed AC, though, so some adapters skip that last step.
Somewhere in your neighborhood there is a transformer that steps down the high voltage of the neighborhood power lines (14,400 volts is typical) to the 240 volts that goes to your house. This picture shows a common type. If you get close, you can usually hear these larger transformers humming. The magnetic fields created by the currents in the primary and secondary coils squeezes the coils themselves, causing them to shrink ever so slightly — a phenomenon called magnetostriction. Because the fields are constantly dropping to zero strength and then building up again, the coils vibrate, creating sound. Click here to hear it. Because the AC is switching direction 120 times per second (twice 60 Hz), the sound created is 120 Hz, plus higher harmonics. |
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Additional Activities & Practice
1. You need to design a transformer to produce 9 Volts AC from an input of 120 VAC.
(a) Do you need a step-up or step-down transformer?
(b) You already have a coil with 1000 loops of wire on it, and you decide to use that for the primary. How many loops should the secondary have?
2. You have a transformer with 400 loops on the primary and 200 on the secondary.
(a) Is this a step-up or step-down transformer?
(b) You connect a 9V battery to the primary. What is the output voltage of the secondary?
3. You take the transformer from problem 2 and reverse it. In other words, the "secondary" coil is now the primary and the "primary" coils is now the secondary.
(a) Is this now a step-up or step-down transformer?
(b) You connect the new primary to a wall outlet. What is the output voltage of the secondary?
4. You come across an unlabeled transformer in your junk parts box. You connect it to a wall outlet and with a multimeter measure the output as 50 volts AC.
(a) Is this a step-up or step-down transformer?
(b) List three possible pairs of numbers, (Np,Ns) that would give you this transformer.