how u find the core number,is it printed on core like EI-33, E-42
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NO. As i salvage them from computer psu,TV,VCR,Monitors,printers and any other dead equipment with Smps based power supply inside.
I never found a core with part number printed on the core itself.AS they are always salvaged and being never able to find a new one in our country,it becomes a tricky (actually way too tricky)business to work with these cores.
Some time the manufacturer of the ferrite transformer puts a certain code on the tape wrapped around the core which may have initials for the core geometry and physical size.Some time they just mix it with the code even some time no such initials are given and the part number is customary to the manufacture.
Getting the physical dimension of the core it easy,you can use just a ruler and measure the dimensions(it is to be measured for one half of the core).
Then you have to open the core.There are more than one way to open it.They are prone to break with small force and broken core is useless no matter how you can fix it it will never work properly or may not work at all.
After opening the the core you have to see the center leg if it is square or round or spherical in shape then you need to measure it and count the cross sectional area.There is a separate formula to count for each shape of center leg and it is an important part of design.
When you are done with measurements you can search the net for equivalent part data sheet.But this data sheet may not be of any use other than you can just confirm the dimensions you measured and part number.
The core material is most important factor and is different from one manufacturer to the other and there is no way to measure it until it is known to you already.
Further,for same dimensions there may be different types.
i hve only experience in EI core 50 Hz
EL 50hz is steel sheet strips core for 50-60hz mains AC operated transformer.
Ferrite cores or not for mains AC operated core. There general way of working is same as for 50hz cores(conversion) but they are entirely different. These are DC operated high frequency cores (most of the times) designed to work from few Khz to several hundred Khz.
You can't just put them to DC source they need to be operated with driver circuitry either single transistor or dedicated chips.Most common way is pulse width modulation and many more.
I myself am still learning and has a long way to go yet for perfection.