How 2 determin Unknown Xfrmr VA Rating? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 20th September 2004, 06:30 AM   #1
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
Default How 2 determin Unknown Xfrmr VA Rating?

I have several "pulled" transformers that i need a reliable way to rate the secondary current for. How do transformer Mfg's do this?

I have heard that it is a temperature rise Vs time measurment? is this correct. where can i find info on this?


Thanks
  Reply With Quote
Old 20th September 2004, 08:52 AM   #2
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
There are so many variables the best we can do working backwards is quite rough.

Keeping this in mind:

Get some data on E&I transformers and then toroids, try Signal Transformer for their catalog and maybe Toroid Corp of Maryland.

Get a rough idea of the VA by weight.

Look up the regulation spec for that VA size, load the transformer until the voltage drops by that amount.

There is your answer.
  Reply With Quote
Old 20th September 2004, 09:10 AM   #3
HV-MAN is offline HV-MAN  Australia
diyAudio Member
 
HV-MAN's Avatar
 
Join Date: Jan 2004
Location: Brisbane
So no identification marks at all ....bummer.....

Manufactures design a transformer to a particular specification/rating etc. and then test to prove that rating etc.

Without any information as mentioned above it would be extremely difficult if not impossiable to determine without.
  Reply With Quote
Old 20th September 2004, 09:15 AM   #4
Banned
 
Join Date: Feb 2002
Location: As far from the NOSsers as possible
You can estimate the VA rating by measuring the cross sectional area of the laminations. I put a graph of this here a year or two ago.......

Current rating............700 circular mils/amp is the usual value. Higher quality stuff may be designed as high as 1000 circular mils/amp.

You can also get handle on "quality" by measuring the magnetising current, IOW, the current through the priamry with no load. The lower the current, the more likely it will be a good transformer for audio use.

Jocko
  Reply With Quote
Old 20th September 2004, 06:06 PM   #5
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
Some of these have markings but as of yet have proved no value. some are quite large. and at least the primaries and secondaries are labeled.

I know one transformer is 120/240 with dual 120V secondaries. I want to run this transformer at half power. IE connect 120V to the 240V primary to produce 60-0-60 on the secondary side. But i dont know is what the current rating for the transformer is. It is quite large, about 6" overall and 2.5" thick so i am guessing 500Va or better but. there has to be a way to test these and get a reasonably accurate idea?

I just figured there had to be some such formula out there that views current vs. voltage drop vs. temp rise vs. time. or something.


Thanks


Zero Cool
  Reply With Quote
Old 21st September 2004, 01:48 PM   #6
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
"I know one transformer is 120/240 with dual 120V secondaries. I want to run this transformer at half power. IE connect 120V to the 240V primary to produce 60-0-60 on the secondary side. "

I've done this with good results. Leach suggests it in his 'Build a double barreled amplifier" article.

The transformers I used were a North Electric model with E&I construction and weighed in at about 11 lbs. and rated at 480VA.

With 10% regulation I was able to get 800VA out, at 100% duty cycle this would overheat.

My no load voltages were in the range of 93V, and with a single pair of 10,000F caps I got 600W/4R drawing 8A off the 120V line.

I used a 4A slow blow fuse to protect the transformer.
  Reply With Quote
Old 21st September 2004, 05:37 PM   #7
diyAudio Member
 
Join Date: Dec 2003
Location: Orange County, CA
Default VA Rating

There is an article on this subject at www.schematicsforfree.mattsoft.net

I think it is in the misc section.
__________________
Dan Fraser
  Reply With Quote
Old 21st September 2004, 07:23 PM   #8
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
Thanks, I found 3 articles there but all deal with how to identify primary and secondary etc etc. only one mentioned anything about how to determin current rating and all it said was if you can touch it and count to 3 its within ratings.....

Many thanks for the pointer...But i am still looking for something more solid.


Zero Cool
  Reply With Quote
Old 22nd September 2004, 11:24 AM   #9
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
"Many thanks for the pointer...But i am still looking for something more solid."

What part of my post didn't you understand?

Core losses in a toroid can be ignored, especially when running 120V into a 240V primary.

The loss is now all I^2R, the same amount of current produces the same amount of heat.

Since the voltages are half, the VA will be half.

Weigh it.

Estimate VA based on weight.

This is close enough.

You could double check this by % of regulation if you have the equipment.

It doesn't sound like you do.

Remember to de-rate by 50% for 120V on 240V primary.
  Reply With Quote
Old 22nd September 2004, 04:58 PM   #10
diyAudio Member
 
Join Date: Dec 2003
Location: Orange County, CA
Default VA

There are also formulas you can use based on the cross sectional area of the core. I have only seen these for "EI" lamination transformers. These, however, only tell you the total VA of the entore transformer and not of individual winding because the formulas are meant to work the other way around. WHere you determone the VA you want then size the core accordingly. The Transformer design program on the schematicsforfree web site uses there formulas. I wrote the program in the 80s and I have since forgoten the formulas. I got them from a Sams book called "Practical Transformer Design". I think the source code is in the program as I wrote it in GW Basic.
__________________
Dan Fraser
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
What Paramaters do I need to determin box size skooter Car Audio 7 19th December 2007 06:57 PM
6V6 PP power xfrmr cornman Tubes / Valves 2 10th May 2007 07:48 AM
KT88 SE with Edcor xfrmr Mr. Triatic Tubes / Valves 7 19th June 2006 09:20 PM
Will this xfrmr work for LM3875 gainclone? ransom peek Chip Amps 1 28th November 2004 05:12 AM
Unknown transformer rating?? bigparsnip Parts 4 4th June 2003 10:26 AM


New To Site? Need Help?

All times are GMT. The time now is 05:38 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2