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Old 4th April 2010, 05:12 PM   #1
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Default How do I know what a transformers VA is?

Hello everyone, I'm new here and fairly new to electronics in general. I recently purchased a transformer from ebay that is rated @20-0-20 AC volts, but I have no idea how many amps it can safely supply nor did the original owner. It is rather large and I'm sure I will have no problems with it powering my LM3886 Gainclone kit, but is there a way I can test it to know what it's max average power output is? I'm sorry if this is a noobie question or if it has been asked before but I can't seem to google and get the answer I'm looking for.

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Old 4th April 2010, 05:15 PM   #2
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You could try loading it up with resistors and see how well it copes.
If the output drops say 10% then that is the max loading current.
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Old 4th April 2010, 05:17 PM   #3
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a bit of detective work will help.
Measure the wire diameter in the primary.
Measure the wire diameter in the secondary.
Measure the weight of the transformer.
Measure the size of the transformer.
Confirm if a toroid or EI or other type.
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Old 4th April 2010, 05:18 PM   #4
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It boils down to temperature rise. If you connect the transfomer to the line with no load at all you'll notice that it warms up. If it rises maybe another 20 degrees C more than that at full output it's probably plenty large.
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Old 4th April 2010, 05:18 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by nigelwright7557 View Post
You could try loading it up with resistors and see how well it copes.
If the output drops say 10% then that is the max loading current.
and if it was a 4% regulation transformer, your test has overloaded it by +150%.
Many transformers are VA rated by the maximum internal temperature they can safely endure.
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Old 4th April 2010, 05:25 PM   #6
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Quote:
Originally Posted by nigelwright7557 View Post
You could try loading it up with resistors and see how well it copes.
If the output drops say 10% then that is the max loading current.
What power rating of resistors would you recommend?

Quote:
Originally Posted by AndrewT View Post
a bit of detective work will help.
Measure the wire diameter in the primary.
Measure the wire diameter in the secondary.
Measure the weight of the transformer.
Measure the size of the transformer.
Confirm if a toroid or EI or other type.
At this moment without taking it out of the case I can tell you that it weighs nearly 20 lbs, is about 3.75 Inches tall and 5.75 inches in diameter, and is a toroid. The wires are all encased in a silver plastic wrap (like most other toroids), so I cannot tell you the wire diameter.
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Old 4th April 2010, 05:29 PM   #7
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That's plenty large enough for 2 3886's loaded at 8 ohms, maybe even 2, so long as you ain't clipping nothing. It's probably more than you need.
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Old 4th April 2010, 05:29 PM   #8
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I have found that the best indicator is weight. Compare it to other transformers of the same general construction. If you don't have enough transformers laying around for a comparison, look in transformer catalogs and see where yours fits in. Even distributor catalogs like Mouser and Digi-Key have some good listings for comparison.
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Old 4th April 2010, 05:30 PM   #9
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Quote:
Originally Posted by caxxxxxx View Post

What power rating of resistors would you recommend?



At this moment without taking it out of the case I can tell you that it weighs nearly 20 lbs, is about 3.75 Inches tall and 5.75 inches in diameter, and is a toroid. The wires are all encased in a silver plastic wrap (like most other toroids), so I cannot tell you the wire diameter.
Sounds like about 500VA just going by size.
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Old 4th April 2010, 05:33 PM   #10
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Quote:
Originally Posted by caxxxxxx View Post
At this moment without taking it out of the case I can tell you that it weighs nearly 20 lbs, is about 3.75 Inches tall and 5.75 inches in diameter, and is a toroid. The wires are all encased in a silver plastic wrap (like most other toroids), so I cannot tell you the wire diameter.
Sounds like at least 500VA by the size of it, if not 750VA or greater

Lets just say that it'll certainly be more than adequate

Here is a link to Airlink Transformers site for toroidal transformers. If you look at 750VA & 1KVA transformers you'll find stuff of a similar kind of size & weight. Looks like you have a rather OTT transformer for a few chip amps lol.
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Last edited by event horizon; 4th April 2010 at 05:51 PM. Reason: Added link
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