Power Transformer Calculation

Power Transformer Calculation

Formula

N = Winding turn
E = Voltage
f = Frequency of the main voltage, measured in Hertz
B = Magnetic field strengh, measured in Tesla
Afe = Core cross section area (cm2)

Bobbin Dimension

By using wasteless EI#96, the bobbin dimension will be

T = 32mm
S = 40mm
H = 15mm
L = 46mm

Step 1: Calculate the core cross section area

Afe = T x S
Afe = 32 x 40 = 1280 mm2

Convert to cm2
= 1280 / 1000 000 = 0.00128 cm2

Frequency is 50Hz for my country, Malaysia.
B must not set too high, it should be around 0.9 to 1.2 for audio application

Step 2: Calculate the Turn per Volt (T/V) or (N/E)

T/V = 1/(4.44 x f x B x Afe)
T/V = 1 / (4.44 x 50Hz x 1.2 x 0.00128)
= 2.93

The TPV is calculated, else is just as simple as a click.

Calculate the winding turn

Winding (N) = voltage (E) x TPV (N/E)

Np for 120v ; 2.93 x 120 = 351 turns
Np for 240v ; 2.93 x 240 = 703 turns
Ns for 300v ; 2.93 x 300 = 879 turns
Ns for 6.3v ; 2.93 x 6.3 = 19 turns
Ns for 12.6v ; 2.93 x 12.6 = 37 turns
Ns for 300-0-300 ; 2.93 x 600 = 1758 turns,
center tap (CT) “at” 1758 / 2 = 879 turn

Example

If your transformer specification is 120v or 240 : 300v ; 6.3v ; 5v

Turn need to be winded for primary is 120v x 2.93 = 351 turns
Then continue with another 120v x 2.93 = 351 turn.
All together will serial up the winding to create 702 turns which is
240v x 2.93 = 703. It’s almost 240v, Let take it.

Turn need to be winded for 1st secondary is 300v x 2.93 = 879 turns
Turn need to be winded for 2nd secondary is 6.3v x 2.93 = 19 turns
Turn need to be winded for 3rd secondary is 5v x 2.93 = 15 turns

You have all the turns needed to wind a transformer.

Wire gauge

Select the correct wire gauge for the right job

Wire no. 22 will have 1.52A rating and wire no. 19 have 3.1A.

120v = 351 turn ; using 1.016mm wire (No.19)
240v = continue winding from 351 and add 351 turn ; using 0.711mm wire (No.22)
300v = 879 turn ; using 0.193mm wire (No.36)
6.3v = 19 turn ; using 0.813mm wire (No.21)
5v = 15 turn ; using 1.016mm wire (No.19)

Every thing is defined here. but you will notice one big problem. How to fit all the winding into the EI#96 bobbin with only 15mm height?

Winding Transformer

To solve the problem above, you can use o.7mm winding for both 120v then parallel up to get 3A, two winding of 0.4mm for 6.3v to get 1A…. etc. If this still not solve the problem, you can use a bigger core for it. So this is how we build an Audio Grade Transformer. Beside this, most people will reduce the VA to half(or 30%) by assuming that the transformer will not always work on full power. Others than this, recalculate it and set the magnetic strength to higher then 1.2 some way around 1.6. Adjusting this setting will reduce the winding but increase the core saturation when dc arrive and also increase the transformer noise. So this is how the economy transformer came from.

Winding Layer

Let use the example above as our winding diagram. Start with the primary layer from left to right until 315 turn for 120v, then continue with another 315 turn, serial it up to make 702 turn for 240v. After this, start a new winding for secondary, stop at 879 turn. Finally wind up the heater filament. If you need to parallel it you can do as what in the image.

Simple

This is how the winding for a linear transformer calculated. I written it in a most simple way which I can. They’re a lot of thing which is not written, likes transformer electrostatic shielding, transformer building material, magnetic phasing and winding geometry. These all have to be study and understand. The biggest or the best material used for a transformer will not tell you the whole story, knowledge will tell you the whole story.

You win perfect!