transformer ratings... pls help
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 10th May 2003, 05:07 PM #1 JojoD818   diyAudio Moderator     Join Date: Mar 2003 Location: searching... transformer ratings... pls help Hi, This may sound stupid to others but I have been thinking of this for the p[ast 3 days. How do I compute a transformers VA value? Example: 1. If I have a 48-0-48/10A transformer, how do I express this in VA? 2. If I have a 48-0-48/1000VA transformer, is there a way to know it's current rating? Is there a formula for this? Thanks in advance, Jojo
 10th May 2003, 05:22 PM #2 moamps   diyAudio Member   Join Date: Oct 2002 Location: Croatia Hi, P=V*I, for V=2*48V, I=10A, then P=96*10=960VA or if P=1000VA, V=2*48V, then I=1000/(2*48)=10,42A Regards
 11th May 2003, 10:34 AM #3 Circlotron   diyAudio Member     Join Date: Jun 2002 Location: Melbourne, Australia The answer to the next logical question (maybe) If you feed your 1000VA transformer into a bridge rectifier and then straight into a big capacitor like you would in most amplifier power supplies, you will be able to get about 600 watts dc with the transformer fully loaded. __________________ Best-ever T/S parameter spreadsheet. http://www.diyaudio.com/forums/multi...tml#post353269
 12th May 2003, 02:08 PM #4 JojoD818   diyAudio Moderator     Join Date: Mar 2003 Location: searching... Thanks! Thanks for the formula! Is "VA" value same as the wattage? If a transformer is rated 1000VA, is it also considered 1000watts? Sorry for this questions but I'm so confused. Jojo
 12th May 2003, 02:10 PM #5 trigon diyAudio Member     Join Date: May 2002 Location: Canada Yes it is. And you may say as well 1 kW Trigon
dhaen
diyAudio Moderator Emeritus

Join Date: Jun 2002
Location: U.K.
Re: Thanks!

Quote:
 Originally posted by JojoD818 Thanks for the formula! Is "VA" value same as the wattage? If a transformer is rated 1000VA, is it also considered 1000watts? Sorry for this questions but I'm so confused. Jojo
VA =P (Watts) when the power factor is unity. That is, when the voltage and current are in phase.
For most things we attempt in DIY the power factor will be unity.
There are other losses, primarily to do with rectification, which cause our useable wattage to be less than the rated wattage or VA.

Cheers,

 12th May 2003, 02:22 PM #7 JojoD818   diyAudio Moderator     Join Date: Mar 2003 Location: searching... Wheew! Trigon, Thanks! Now things are getting clear. I ask this questions because here in the Philippines I can only order transformers using their secondary voltage ratings and current rating. However, almost all projects I find in the net (especially amps) give the transformers' ratings in VA. Now I know how to convert them ratings. Jojo
peranders
Electrons are yellow and more is better!
diyAudio Member

Join Date: Apr 2002
Location: Göteborg, Sweden
Re: Thanks!

Quote:
 Originally posted by JojoD818 If a transformer is rated 1000VA, is it also considered 1000watts?
Yes, but only if the load is purely resistive!

A heavy power supply will reduce the VA rating to half. A "normal" power supply reduce less but this is continuously power. If we talk music power I think you could answer yes to your question.

A rectifier bridge and caps creates reactive power which warms the transformer.

EDIT: dhaen was faster.....
__________________
/Per-Anders (my first name) or P-A as my friends call me

moamps
diyAudio Member

Join Date: Oct 2002
Location: Croatia
Re: Thanks!

Quote:
 Originally posted by JojoD818 Thanks for the formula! Is "VA" value same as the wattage? If a transformer is rated 1000VA, is it also considered 1000watts? Sorry for this questions but I'm so confused. Jojo

Hi,

1000VA is 1000W only when your transformer is loaded with resistive load. Current and voltage are then in phase (fi is zero, cosinus fi is 1), and you can get 1KW from transformer.
If you have bridges and caps (complex load), then you can get smaller power (Watts) from your transformer.

Regards

P.S. Circlotron, what is next Q&A ?

Edit; I'm slooow.

 12th May 2003, 03:26 PM #10 JojoD818   diyAudio Moderator     Join Date: Mar 2003 Location: searching... Hey guys thanks for the info. My test transformer is a 44-0-44/4A. I used 4 6A/200V diodes in bridge mode and 6,800uF per rail. I left it plugged without any load, when I came back it was very warm, not hot but my fingers can't stand the temp. Why? If it heats up without load, what more if I load it? Jojo

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