How do I know what a transformers VA is?

[caxxxxxx]

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.

Thanks

 

[nigelwright7557]

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.

 

[AndrewT]

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.

 

[Andrew Eckhardt]

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.

 

[AndrewT]

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.

 

[caxxxxxx]

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?

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.

 

[Andrew Eckhardt]

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.

 

[bob91343]

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.

 

[nigelwright7557]

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.

 

[event horizon]

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.

 

[Rembrant]

LOL, 20LBs? Surely you are joking. I have whole gain clones that don't weigh that much. If it really is 20Lbs then you could probably power about 5 of those amps. You might want to save it for a Home Theater project. It's great for testing too. It will be quite hard for the normal GC to drag the voltage down on that one.

Honestly, I would try to find a 150-200VA transformer in salvage or otherwise and use that for the permanent install. Seems like a waste otherwise. At least you have room to grow.

 

[Elvee]

Have a look at this thread:
http://www.diyaudio.com/forums/power-supplies/156030-trafo-unknown-power-rating.html

 

[caxxxxxx]

LOL, 20LBs? Surely you are joking. I have whole gain clones that don't weigh that much. If it really is 20Lbs then you could probably power about 5 of those amps. You might want to save it for a Home Theater project. It's great for testing too. It will be quite hard for the normal GC to drag the voltage down on that one.

Honestly, I would try to find a 150-200VA transformer in salvage or otherwise and use that for the permanent install. Seems like a waste otherwise. At least you have room to grow.

Yes, you are right. I weighed myself today without the transformer in hand, weighed 174 lbs. Then I grabbed the transformer and reweighed myself - 190 lbs. So it weighs 16 lbs, don't know where I got 20 lbs from.

Funny you mention, but I am actually planning a home theater project. But my point wasn't really whether or not it was enough for a gainclone, I was wondering if there was any reliable method to figure the max avg. available power of the trafo, since I have 2 other ones that I need to measure as well. Looks like from reading the other posts here that it is mostly guesswork, kinda bummer. Here's a pic of mine from the ebay auction

 

[TonyTecson]

if you are using that traffo to power up you gainclones you have nothing to worry about, you have it on hand, use it....that's good for a t least 5 gainclones, maybe more...

 

[Andy5112405]

As everyone is saying. The best bet is to measure the iron core and compare it with others at Farnell or RS Components. Transformers come in standard sizes so you should find it fairly easy to compare its dimensions with known values.

 

[AndrewT]

... is about 3.75 Inches tall and 5.75 inches in diameter, and is a toroid. ............. it weighs 16 lbs,

to measure the iron core and compare it with others at Farnell or RS Components. Transformers come in standard sizes so you should find it fairly easy to compare its dimensions with known values.

this is very tall relative to diameter toroid not normal toroid.
It is a special not a standard.
I suspect it is well over 1kVA, which usually weigh 6.5kg to 7kg (14.7lbs to 15.4lbs) possibly 1.2kVA to 1.5kVA.

 

[Andy5112405]

Just looking at the Farnell site should give you a good idea when comparing it with other known transformers. A 1000VA transformer would need to be approx 167mm x 71mm and would weigh around 7.5Kg. I would estimate that yours is somewhere around 800VA.

 

[caxxxxxx]

if you are using that traffo to power up you gainclones you have nothing to worry about, you have it on hand, use it....that's good for a t least 5 gainclones, maybe more...

this is very tall relative to diameter toroid not normal toroid.
It is a special not a standard.
I suspect it is well over 1kVA, which usually weigh 6.5kg to 7kg (14.7lbs to 15.4lbs) possibly 1.2kVA to 1.5kVA.

Wow, so this would be the only trafo I would need for a Home Theater project? Thats actually what I was jokingly thinking to myself when I first grabbed and almost dropped it from the UPS guy, lol. Looks like I may have been right.

What should I look for if I decide to power up 5 or more gainclones from this? Heat, power spikes, common sense (smoke is probably not a good thing right......???...)

 

[AndrewT]

What should I look for if I decide to power up 5 or more gainclones from this? Heat, power spikes, common sense

extreme difficulties in sorting the grounding to achieve low to zero hum and similarly buzzes which both become audible at the speakers.