The core area in square centimeters or inches must be determined and matched to the total power in watts or volt-amperes. The greater the size of the core, the more power it can manage. Once the core size is determined, the number of turns for the primary is determined. This should be equal to the number of volts required for the desired output current through the secondary.

For example, if you want 10 amps from a transformer with a voltage rating of **120 volts**, then the primary should have 10 volts across it. Since 120 volts is divided by 0.01 (one percent) approximately 12 feet of wire will be needed as the primary. The secondary can be any length up to but not including 12 feet; however, it is best if it is about half that length or less. For example, if you use 6 feet of wire for the secondary, that means the primary needs to be run at least 3 feet away from it.

Transformer cores are usually made out of iron or steel and may be solid or split. A split core has **two parts** that fit together like a sandwich. The outside surfaces of the core pieces would be covered with cotton or silk cloth and wrapped with enameled wire to prevent corrosion. A solid core does not have this treatment and therefore requires **some sort** of insulation around it to keep it free from damage. This is usually done by wrapping the core with paper containing glass fibers.

Using the transformer's voltage ratio formula, voltage ratio = 330/10.29 = 32.1. In a transformer, the voltage ratio and turn ratio are equal. As a result, N = 32. So now we have all of the information needed to compute **the secondary turns** of the ferrite core transformer. It has 1 primary (pin) lead with **32 secondary leads**.

The best way to identify any type of transformer is by using its voltage ratio. If you know the voltage ratio of a transformer, you can determine many things about it. For example, you can see that this transformer has 10 volts output per **300 volts** input. This means that the transformer has a voltage ratio of 3:1 or 30%.

Another way to identify a transformer is by looking at its physical size in relation to the size of its windings. A transformer designed for use with 110-volt power will usually have two sizes of iron cores: one for 220 volts and one for 110 volts. The winding on a transformer works based on the number of magnetic poles it contains. A single-pole winding only produces electricity when there is a current flowing through it; a double-pole winding produces a magnetic field in one direction and then the opposite direction again. A triple-pole winding produces a magnetic field in three different directions at once. Most low-voltage transformers have three separate windings on one bobbin; high-voltage transformers often have four or more separate windings on one bobbin.

Core area: 1.152 x (output voltage x output current) sq cm is typically given to the best feasible cross-sectional component of the core. In the case of transformers with many secondary windings, the total of the output volt-amp product of each winding must be considered. A simple rule of thumb is that you should use a core **one third** the size of the overall footprint of the primary and secondary coils.

The output voltage of a transformer determines how large its core needs to be. For example, if the output voltage is to be 100 volts, then the core diameter should be 20 times the distance between **the center points** of the two terminals (or 10 cm). If the output current is to be 12 amps, then the wire used to connect the two terminals together must have sufficient cross-section to carry this load. The output current of a transformer determines how thick the conductor needs to be. For example, if the output current is to be 12 amps, then the wire used to connect the two terminals together must be at least as thick as **an insulated copper wire** capable of carrying such a load.

The choice of material for the core affects the price and the performance of the transformer. Generally, ferritic materials are more resistant to corrosion from power surges and heat than iron cores. However, aluminum cores are becoming increasingly popular due to their relative lightness and low cost.