EI Transformer Questions

[Bernie7]

I have an EI with secondary winding center-tapped 7v - 0 - 7v rated at 6VA. My load needs 0-7v and 6VA.

If I tap from one 0 - 7v secondary output, will I get the full 6VA out, or only 3VA?

Since the other secondary winding will be unused, will there be some loss of efficiency as I've read that an unloaded secondary generates max flux. Not sure how this plays out with only half the secondary winding in use. Will I be better off getting an EI with a single 0-7v secondary?

Thanks in advance.

[Roushon]

Usually VA ratings are given for each half. That is you will
get 6VA from each 7V line unless the manufacturer
of the transformer follows a different custom. If you are
worried about the unused line, then you can use it to
power an LED (with a diode and a series resistor of
course) which will also indicate that the power is ON.


Roushon.

[UrSv]

And it is just as common, if not more, to specify for the transformer as a whole so you really need to know. I can't really say that I've ever seen anything other than total transformer power specified for an article anywhere. To get that number you usually have to look at the transfomer itself where it sometimes is listed per secondary winding.

[mymindinside]

The 7-0-7 6VA transformer is usually referred to as a 14V CT (center tapped) 6VA transformer.
if you use only one half of the winding (0-7) you will need to halve the rated load i.e. 3VA. If you need 0-7V 6VA get another transformer
someone correct me if i am wrong.

cheers,
mymindinside

[discrete]

File:Fullwave.rectifier.en.svg - Wikipedia, the free encyclopedia

[AndrewT]

your 6VA is almost certainly the total maximum continuous output capability of all the secondary copper.

6VA and 7-0-7Vac gives a maximum continuous output current of 428mAac from each secondary tapping.

The maximum continuous DC current after feeding a capacitor input filter will be ~214mAdc
The recommended continuous DC current for cool running and long life is <=110mAdc

If any of this reply is confusing or double dutch then you better learn pretty damn quick before you start mucking about with a mains powered PSU !

[infinia]

'discrete' has the correct answer, assuming DC conversion the intent. You will see close to full VA, b/c each secondary current is less than 50% duty cycle.

[Roushon]

If there was no center tap then the manufacturer would
have mentioned 12VA. This is the custom I know.
Whenever I get a transformer made it is explicitely
mentioned the Amp for each line out.

So it is best to read the data-sheet.

Roushon.

[infinia]

You are probably overbuilding ie making them put too much copper in the secondary. XFMRs are designed for a heat rise, given the insulation rating at the specified output VA. core loss + copper loss using RMS currents.