I have an switch-mode power supply for HDD. That means +12V +5v.
How can I measure the transformer? In primary has 4 pins and with ohmmetre is making beep sound 2 by 2 pins. And in secondary i have 3 pins ( i think 1 common and 1 for 12v and 1 for 5v?) but all 3 have continuity on them. Is that ok?

In primary is reaching 300V but nothing in secondary...

 

There could be as many as 5 seperate coils on that transformer.

switching supplies have many many many different topologies and methods. Without more experience on your part, perhaps a picture of the supply and board might help someone recognize the type of supply it is.

Can't tell you much more about those transformer like devices used in swithcing supplies without lots more info.

You can find out by deconstruction, but it may not be reconstructable by you when you are done.

 

Hmm I just need to understand how is the secondary part of transformer working. How can I measure with ohmmetre that part. Need to have continuity or not on all output pins?

 

From what you have said so far, I think it would be dangerous for you to continue with this project. If you do not already know how to measure a transformer, then most likely you are also not aware of other things that may affect your safety, or that of other people.

You have mentioned 300V input. This is high enough to bring a danger of death by electrocution. Please leave this thing alone - mains powered equipment is not safe for beginners. Switched mode power supplies in particular are not simple things to understand.