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What is a Transformer?

A transformer is a device where two or more coils share a common magnetic field. Figure 2 is a drawing of a transformer. In this drawing you see two coils; a primary and a secondary coil. Both coils are wrapped around a ferro-magnetic core. The idea is that the primary coil takes in a time-varying voltage and creates a time-varying magnetic field. The ferro-magnetic core channels this magnetic field through the secondary coil. The secondary coil converts this time-varying magnetic field back into a time-varying voltage.

Figure 2: Transformer
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An ideal transformer obeys a simple relationship. The sinusoidal voltage on the secondary side, $v_s(t)$, will be proportional to the sinusoidal voltage on the primary side, $v_p(t)$. The constant of proportionality, $n$, is determined by the ratio of the number of turns on the secondary side to the number of turns on the primary side. This constant is commonly called the turns ratio. In equation form this means that

$\displaystyle v_s(t) = n v_p(t)$      

where $n$ is the turns ratio, $v_s(t)$ is the voltage waveform over the secondary coil, and $v_p(t)$ is the voltage waveform over the primary coil.

For our purposes, we will use a transformer that converts the 120 VAC available at the wall socket to a 12 VAC signal. You should have such a wall-transformer (see figure 3 in your kit. The transformer plugs directly into the wall socket and the wires coming out of the transformer terminate in a connector that you can plug into a special socket that is in your kit. The wall transformer looks like a very "fat" and "heavy" wall plug. It is fat and heavy because it has a fat and heavy transformer inside of it. The transformer has been designed to convert a 120 VAC input into a 12 VAC signal. So this means that your transformer has a 10:1 turns ratio.

Figure 3: Wall Transformer
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next up previous
Next: What is a Rectifier Up: lab8b Previous: What is an Alternating
Michael Lemmon 2009-02-01