i´m investigating how to design a transformer properly.. every time i try it i discover something new..

this weekend i decided to design a transformer to be the power source of one of my equipment for audio and it must be able to output 9v at 1.3 amps without saturating , having a great regulation, and not getting hot if not operating..

i tried the following and made some mistakes so i could learn from it

I first found the number of turns i should do at the primary by considering the voltage drop caused by the resistance of the primary and secondary coil by the current at full load.. .

first to do so i had to estimate the number of turns and wire size for a particular core...

i used an old E core that was around here and at first i didnt care about the power handling of the core....

to know this i later found that we should consider that the crossection area in inches must be calculated from the square root of the power divided by 5.58 this considering 1tesla saturation

here i have mistake double because i also considered the saturation to be used at 1.5 tesla

so i used the old formula turns = V/4.44/f/A/Bmax where A is the area in m2

i estimated the mean turn length to estimate the length of wire so i could calculate the resistance

but also i decided to calculate the required resistance to give an specific efficiency first dividing equally the power dissipation on both coils L1 and L2

RESULTS

my core seem to be saturating since the output waveform is not the perfect sine wave

my core and windings are getting hot because:

the L1 wire was not designed to hold the 300ma when the circuit is open on the secondary side

at my calculation it would only need to handle 110ma

because i didnt considered the magnetizing current

which was probably that high as 300ma because the core its getting close to saturation...

I wound the primary first than the secondary over it... this made a small gap between the L1 and the core because the primary is on the way 2mm thick ( two layers of 18 wire plus insulations) and and also on the other side because the core was not complete filled

I believe this may have reduced the leakage inductance but it may have decreased the magnetizing inductance which is important to remain as high as possible...

the most increadible result however is the following

with the transformer open circuit the primary current is 300ma

with the secondary having a load of 8.4ohms the current at the primary side reduce to 265ma the output voltage drops 0,5v

this lead me to discover the relation of the primary resistance to secondary voltage drop also exist... a voltage drop at the primary will appear at the load too but respecting the transformation factor...