# The Transformer

If you start designing your amplifier, one thing is sure: you will need a power supply. At least you have to calculate what voltage and current your transformer will have to deliver

### Determine the power of your Transformer.

As a rule of thumb, you can apply following calculations:
Take the size of the total thickness of the lamination core and the width of the middle leg of the transformer core in centimeter.  Multiply these values and square the outcome. Multiply the outcome with 0,64. This value will be the approximate power rating in VA that can be transformed by the core.

It is often difficult to measure the middle leg of the core as the winding will cover that part. Alternatively the width of a side leg can be multiplied by two.

An example:
Lamination thickness = 5 cm
Middle leg  = 3 cm ( side legs = 1,5 cm).

The power that can be transformed by this core:  (5 x 3) x (5 x 3)x 0,64 = 144 VA.

#### Max current of the winding’s

Next, find out if the copper wire can handle the current that we need. Therefor you measure the thickness of the individual winding’s.  You can do this with a slide gauge.
Measure the diameter and calculate the Area in  mm² with the formula Area = (π/4) × D2.
As a rule of thumb (again) you can calculate 3.5 A per mm²

If measuring the wire thickness of the high voltage secondary side is problematic, all lower voltage secondary wires can be measured. For each winding, the max power must be calculated in VA . Then you add all these calculated values and subtract them from the total power that is allowed for this core. The remaining power is left for the high tension.

Example for the above transformer of 144 VA:
Assume we have 2 x 400 V high voltage, and 2 winding’s of 6,3V  (one for 7A and one for 3A)
The power occupied by both heating winding’s:  6,3 * 10= 63VA. Therefor 144 – 63 = 81 VA is left for the high voltage winding. At 400V we can  have 81/400 = 200 mA

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