Transformer Calculations

I'll strive to explain transformer calculations in such a way that you'll not need to memorize a list of formulas.

The concept:

Transformer Calculations can be thought of as ratios. It is all based on how many turns the wire makes on each side of the transformer. The ratio of the turns correlates with the ratio of the voltages and inversely correlates with the amperes.

Ok, so you want that in English? If you have 10000 turns on the primary and 1000 turns on the secondary, you have a 10 to 1 ratio expressed as 10:1. So if you have 120 volts on the primary then you'll have 12 volts on the secondary.

Amperes work in the reverse. A transformer that has a 10:1 ratio, and that has 2 amps flowing on the primary, will have 20 amps on the secondary.

Maybe think about it like a bicycle. Imagine that you have 100 teeth on the front gear, and 10 teeth on the back gear (10:1 ration). If you turn your pedels 1 time, then how many times will your wheel rotate?

10 times.

Just remember that the power, or Watts (Volts x Amps), will be the same on the secondary as it is on the primary.

For transformers with a primary and a secondary, there are sort of 2 types.
A step-up transformer will have more turns and higher voltage on the primary then on the secondary. A 5:1 ratio would be a step-up transformer.
A step-down transformer will have more turns and higher voltage on the secondary such as a 1:2 ratio.

Step 1:

Figure out the ratio. If the question gives you the amperage on both sides, or voltage on both sides, or number of turns on both sides then you can use them to figure out the ratio.

Remember, the side with the highest voltage has the most turns; and to keep the power (watts) the same, the side with most turns has the lowest amperage.

Step 2:

Use the ratio to figure out the missing number using multiplication or division. If a transformer has a 2:1 ratio and the secondary has 120 volts then the primary will have 2 times as much, ie 240 volts.

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