How does a Transformer Work?
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A transformer is an electromagnet (with an iron core in the center) that transfers current from one side to the other via magnetism, changing the voltage from a higher to lower level or vice versa. In this case, we have a step-down transformer that takes 120v, 208v, or 240v power on the primary and steps it down to fixed 24v power on the secondary. The common wire (white) goes on the primary, the red/black/orange wire will also be used to match the incoming voltage on the primary. The primary and secondary don’t actually touch; they interact magnetically.
The number of copper wire wraps on the primary vs the secondary is what dictates whether a transformer steps up voltage or steps down voltage. Those wraps are set up as a ratio, and they are what drive the electromagnet. This transformer is rated for both 50 and 60 hertz, meaning that it can work in the USA and Europe, and it’s a 40VA universal transformer. There is no fuse on the secondary, meaning that we’ll have to put one in.
When we cut into the transformer, we realize that the secondary has a much larger-gauge wire than the primary. That is due to the higher amperage on the secondary; when we step down the voltage, Watt’s law states that the amps will increase to make up for the reduced amperage and keep the same amount of power. The primary and secondary are also physically isolated from each other.
If you come across a failed transformer, you can ohm it out to determine which side is open. The primary side usually fails when there are issues from the power source, and the secondary side usually fails when there are issues on the low-voltage side.