Transformer VA rating

[Buzzy]

Hi ! I have a toroidal transformer that i took out from an old amp. I want to reuse this to build a DIY amp.


I have measured the secondary voltages. However the I dunno whats the VA rating of the transformer. Is there a way to measure it ?

Thankz.

[sreten]

Hi, size/weight is a very good indicator, measure and compare, /sreten.

[Rodeodave]

Hi, I personally don't know of any direct way to measure the VA rating of a transformer, but there are a few things that might probably give you a hint.

What's the primary/secondary voltage? How much does it weight? What amp was it taken from and/or do you have it's schematic? The rating of the amps fuse could be a reference.

[Elvee]

Hi,

The primary DC resistance is also a good indicator. And you only need a multimeter, which is an electronician tool.
LV

[wg_ski]

Load test it. Most toroids are rated full VA at 3 to 5% regulation. 3% or so for units about 1kVA, 5% for smaller units of 200VA or so. Load it down with light bulbs (resistive load only) on the secondary until it drops about 5% from no load value. You can overload by about 10x for a few seconds at a time or about 4x for a few minutes with no ill effects. Measure the current to get approximate VA rating. Let it run this way for a while and monitor the temperature - if it gets too hot to touch then back off some on the continuous load.

[SRMcGee]

Buzzy:

Perhaps this is cheating, but why not simply contact the manufacturer's technical service department and ask?

Regards,
Scott

[Geek]

Originally posted by wg_ski
Load test it. ...

This has been my method and seems accurate so far

(though with EI cores, you also have to take into account manufacturer. Tek's for example are built like a tank with overrated wire will be at their maximum core VA with a 2% drop)

[bananaslug86]

Originally posted by wg_ski
Load test it. Most toroids are rated full VA at 3 to 5% regulation. 3% or so for units about 1kVA, 5% for smaller units of 200VA or so. Load it down with light bulbs (resistive load only) on the secondary until it drops about 5% from no load value. You can overload by about 10x for a few seconds at a time or about 4x for a few minutes with no ill effects. Measure the current to get approximate VA rating. Let it run this way for a while and monitor the temperature - if it gets too hot to touch then back off some on the continuous load.

Being a newbie with several toroids to test, let me see if I understand what you are suggesting. Do I set up a test rig with several light fixtures in parallel and a bunch of light bulbs? Then add one at a time until the voltage load drops by 3-5%? I.E. in the US from 115 volts to 111.5-109.25 volts? The VA rating would then be the total wattage of the bulbs used?

bananaslug86

[wg_ski]

Yes, a bunch of light bulbs. Or water heater elements for higher power. I have an old DJ lighting system that I can screw in PAR lamps until I hit a desired load and let a transformer or power supply cook as long as I need it to. You just need a bunch of light sockets and a way to mount them. They'll be runing at secondary voltage - the load will be less than rated wattage so you need to measure secondary current. Monitor the secondary voltage drop, and keep an eye on the trafo's temperature rise. A 5% drop will indicate at least the short term capacity, but to determine long term you need to run it a while and see how hot it gets. In most home audio amp applications, a short-term rating for the trafo is sufficient. For sustained load like battery chargers, inverters, etc. the long term rating would need to be determined and that would be based on temperature rise and that may take hours at load to properly determine.

[bananaslug86]

Originally posted by wg_ski
Yes, a bunch of light bulbs. Or water heater elements for higher power. I have an old DJ lighting system that I can screw in PAR lamps until I hit a desired load and let a transformer or power supply cook as long as I need it to. You just need a bunch of light sockets and a way to mount them. They'll be runing at secondary voltage - the load will be less than rated wattage so you need to measure secondary current. Monitor the secondary voltage drop, and keep an eye on the trafo's temperature rise. A 5% drop will indicate at least the short term capacity, but to determine long term you need to run it a while and see how hot it gets. In most home audio amp applications, a short-term rating for the trafo is sufficient. For sustained load like battery chargers, inverters, etc. the long term rating would need to be determined and that would be based on temperature rise and that may take hours at load to properly determine.

Thanks. Now to assemble my mad scientist light bar.

Bananaslug86