# Transformer Cheat Sheet

·
A transformer is used to transform from one voltage to another.

·
The voltage ratio is proportional to the turns ration. Current
is inversely proportional to the turns ratio.

·
The power ratio of an ideal transformer is 1:1. In other words,
if the voltage goes up, the current must go down.

·
Transformers only work with AC, Alternating Current.

## Dots and phase

· The dots can be thought of as voltage
polarity.

· When current flows into the dot on
the primary side, an induced current flows out of the dot on the secondary.

· The voltage is in phase at the dots,
but the current is flowing in the opposite direction.

## Impedance Matching

## Common Mode Choke

When configured as a common mode choke, a transformer is
seen as a low impedance to differential current and high impedance to common
mode current.

·
When common mode current flows through the windings, the flux
adds, creating more inductance.

·
When differential current flows through the windings, the flux
cancels, reducing the inductance.

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