I interpreted "desktop power supply" as "bench power supply" not "desktop computer power supply". So, @osha123 , how about a photo of this transformer, so we all know what you are talking about?

"We demand rigidly define areas of doubt and uncertainty" -- Hitchhikers Guide to the Galaxy.

Yes, bench power supply not desktop. My bad

 

i have broken 300w desktop power supply witch i took apart and it had this big transformer in it,
i have never worked with transformers and out of curiosity i measured resistance of coils i noticed that primary coil has only 2.6-ohms resistance.

Hi,
something wrong. If your power supply is SMPS , the primary of the ferrite transformer is much lower, a fractions of an ohm

 

Hi,
something wrong. If your power supply is SMPS , the primary of the ferrite transformer is much lower, a fractions of an ohm

It's a Iron core transformer, also it was bench power supply not desktop, i tought both are same

 

Are you sure that is a switched mode power supply?
Show us a photo of the entire circuitry.

 

Are you sure that is a switched mode power supply?
Show us a photo of the entire circuitry.

Im not sure, and now that i looked again at circuit i noticed theres no inductors

 

That is a linear power supply, not SMPS.

 

That is a linear power supply, not SMPS.

my bad then

 

Yes, bench power supply not desktop. My bad
View attachment 318732

That's a split bobbin transformer. The primary is on the top half of the bobbin and the secondary on the bottom.
The alternative is a concentrically wound transformer, which has a single section bobbin, where the primary is wound first, and then insulated with tape and the secondary wound on top.
Split-bobbin transformers have much more leakage inductance. Concentric transformers have better coupling between input and output.
Split bobbin transformers have better insulation between input and output.
Specifying a transformer is not a simple task. There are lots of possibilities to consider.

 

A "big" transformer in a computer power supply will weigh more than a pound. And I have run a desktop computer that utilized an external power supply off of a transformer type regulated supply for many weeks when a line transient fried the switcher. THAT is the motivation for switching off the line side of a supply at night.