being a restricted amp voltage source is analogous to a high impedance output meaning a amp surge would create such a voltage drop that its output is reduced to zero. .

transformers are impedance transformation devices.. .it translates to the primary the impedance seeing in the secondary side...

To design a transformer you start with current rating than start all the calculation

when i had that problem with my probe power supply i made a nice description how to engineer a transformer for iron cores...

the resistance of the coils is what restrict the amps to obtain regulation.

FOR exx

my load needs 9v 300ma so

i design the secondary to output 10v if the secondary has 3ohm as it will have a 1v drop under load..

the primary voltage drop also must be calculated... for example its feed with 120v and the primary have to allow 10v x 0,3a so it will absorb this 3w plus the energy that will be dissipated on the primary resistance.. lets say the primary has a thinner wire and longer so its resistance is maybe 100 ohm

i dont really know how to calculate exactly but it goes in approximations

for example you start with the transformer factor for 120vrms to 10vrms already compensating for the secondary resistance... 12x

so the current in the primary is 0,3/12 so 25ma than it will cause a voltage drop of 2,5 volts

so the stepdown should now be a little smaller to get exact 10vrms at the output under no load and to have 9v under load 3,33ohm

the correct transformation factor is than 120-2,5 =117,5v/10v so 11,75 should be the transformation factor

the core size will tell how much power it will handle... the more laminations you add the less turns is needed for a given working voltage input...

to obtain maximum power of the core it must work close to the saturation level such it can use max the flux swing and this can be achieved using known equations like Vrms/4,44/f/A/Bmax where area is in meters, frequency in hz and Bmax in tesla ... this will give the primary number of turns required for at that voltage to obtain that bmax hz times per second...

http://ludens.cl/Electron/Magnet.htmlhttp://ludens.cl/Electron/trafos/trafos.htmlthe transformer work by creating a magnetic field on the primary and the secondary when loaded creates a contrary magnetic field a field that cancel the field created by the primary so the power output of a transformer is related also to the coupling and resistance of the coils as the resistance will directly limit the power output by creating a voltage drop.

if we short the circuit of the secondary the current output will lead to a burn...

if we take a 120v input transformer and connect to 5 volt instead we could use a frequency 24 times smaller than the frequency it was rated for.... so it would allow 2,5 hz square wave