# transformer question

Discussion in 'General Electronics Chat' started by jonisonvespa, Aug 9, 2012.

1. ### jonisonvespa Thread Starter Member

Mar 25, 2012
49
0
hi,
output windings are, 2 * 5v each winding rated @ 3kv, how do i work out the max current i can draw from it, im going to be using them in series.
thank you

2. ### #12 Expert

Nov 30, 2010
18,093
9,683
The 3kv means nothing about the output ability. Trying to find the current of a transformer without a label is almost impossible. The best you can do is look it up with a part number or guess what it can do, based on how much it weighs.

I just thought of another method. If you can give it exactly the labeled input voltage, you can load the output until the voltage drops to 5.00 volts RMS.

What? No label? Still stuck!

PackratKing likes this.
3. ### ifixit AAC Fanatic!

Nov 20, 2008
650
118
Did you mean 3kw (3 killowatt) per winding?

If so, then you could run a maximum of 600 Amps (3kw/5vrms) through each winding. Connected in series, that would be 600A X 10V = 6000 Watts. That's a BIG transformer!

Can you post a picture of it?

Regards,
Ifixit

Mar 25, 2012
49
0
5. ### #12 Expert

Nov 30, 2010
18,093
9,683
3VA means "volt-amps" (which is a lot like watts).

So...if the volts are 5, the amps are .6

6. ### jonisonvespa Thread Starter Member

Mar 25, 2012
49
0

hay thanks

is that .6a total when the two 5v windings are in series, ie 10v

and each winding 0.3a on their own?

thanks

7. ### #12 Expert

Nov 30, 2010
18,093
9,683
Slap forehead...think....two 5v windings in series makes 10 volts...true.
10 volts times .6 amps makes 3 Volt-Amps...false.
10 volts times .3 amps PER WINDING makes 3 Volt-Amps...I don't think so.

The problem is in the phrasing and meaning of "per winding".
I've never seen a transformer labeled that way.
I believe the proper answer always ends in "3 Volt-Amps" sum total for the whole transformer.

But then, after confusing KV with VA, (read my signature line).

Last edited: Aug 9, 2012
8. ### jonisonvespa Thread Starter Member

Mar 25, 2012
49
0
slapped,

ok if 1, 3va winding @ 5V =0.6A

2 windings in series 10v total current = 0.6 + 0.6 is this correct?

how was the 0.6a concluded can someone show me the equation please

thank you

9. ### #12 Expert

Nov 30, 2010
18,093
9,683
I guess it's time to stop joking with you and get down to serious.

If you put both the 5 volt windings in parallel, with the correct polarity to be in parallel, you could get .6 amps from them. 5Volts x .6Amps = 3Volt-Amps

If you put the 5 volt windings in series so that the voltage adds together, you will have 10 volts (RMS).

3VA/10Volts = .3 amps

Then, you consider a rectifier to make some DC voltage and it changes. You can't get what you expect from a transformer when you rectify the output (most of the time). Here's a chart.

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10. ### jonisonvespa Thread Starter Member

Mar 25, 2012
49
0
hay thanks number 12 i got it now i think, thanks for taking the time

11. ### #12 Expert

Nov 30, 2010
18,093
9,683
2 way street.
It's always a matter of discovering what you have to work with, and that takes time.
Patience is required from both ends.

12. ### ifixit AAC Fanatic!

Nov 20, 2008
650
118
Number Twelve,

I don't think it is a 3VA transformer.

According to the spec sheet in post #4 it is a 6VA transformer (3VA per output winding). So: 5V+5V=10V and 6VA / 10V = .6 Amp total through the two windings in series.

Regards,
Ifixit

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13. ### #12 Expert

Nov 30, 2010
18,093
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The first line on the page clears this up! It's a 6VA transformer.
I wish I had noticed that he posted a datasheet
instead of getting focused on the fact that transformers aren't labeled, "per winding".
I just believed that he saw, "3VA" somewhere and assumed it was, "per winding".
He just changed the way it was worded and got me focused on the wrong issue.

At least he has the math now.

Last edited: Aug 10, 2012
14. ### vk6zgo Active Member

Jul 21, 2012
677
85
Yes,but also remember that 3VA per winding is 3VA/5V ,again giving 0.6A max through each winding.
So it doesn't matter how you connect them,in every case,the max rated current for each winding is 0.6A

15. ### #12 Expert

Nov 30, 2010
18,093
9,683
If only they were labeled, "per winding" I wouldn't have gotten it so wrong.

16. ### jonisonvespa Thread Starter Member

Mar 25, 2012
49
0
the transformer in question, im thinking its 3va per winding is that correct?

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Last edited: Aug 10, 2012
17. ### #12 Expert

Nov 30, 2010
18,093
9,683
Ah! You're back!

The transformer in question is a 6VA transformer. Each of the two output windings can deliver 3 volt-amps.

So glad you have the opportunity to plug the numbers in and get answers for the transformer you really have. I hate it when I make a mistake like that!