Any ideas how to estimate VA or mA of Power Tranny - diyAudio
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Old 20th February 2009, 05:23 PM   #1
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Question Any ideas how to estimate VA or mA of Power Tranny

I have an old power tranny presumably from a large tube jukebox. It is massive and about 750-760 center tapped. It had only markings as H.I. Company no other markings.

Any ideas on how I can estimate the VA or mA's available?

Ammeter and LARGE resitive load?
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Old 20th February 2009, 07:26 PM   #2
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This article describes how to size unknown transformers based on a function of the secondary's voltage and DC resistance.

http://diytube.com/phpBB2/viewtopic.php?t=1465&start=7
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Old 20th February 2009, 07:30 PM   #3
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AWESOME THANKS!!!!!!!!!!!!
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Old 20th February 2009, 07:32 PM   #4
Klimon is offline Klimon  Belgium
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Identify the windings (primary, secondary); measure DC resistance

Plug the trafo in the outlet and measure voltage on primary + secondary

Insert these values in PSUDII (duncanamps.com): rightklick the transformer symbol; in window 'edit transformer properties' click on 'source res' + insert your measured values, write down the calculated value of 'source res' is; last step: click 'values' and take a guess at the nominal output; this will change the 'source res' in the previous window: play with inserting a nominal value untill 'source res' equals what you've written down.

Simpler than it sounds
Simon
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Old 20th February 2009, 07:53 PM   #5
rdf is offline rdf  Canada
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Jukeboxes are an interesting case. My Rowe similarly has a massive PS transformer yet the original circuit ran near Class-B outputs with very low static bias current. Guessing, a manufacturer's financial success depended heavily on avoiding field service calls. These boxes idled most of the time and the Rowe circuit appeared optimized to minimize component wear under those conditions. Conversely, an over-rated power TX would also enhance reliability, so it's an odd combination of big parts serving a (usually) low requirement circuit.
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Old 20th February 2009, 09:03 PM   #6
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Question ????

Ok,

So following the two different methods I get TWO COMPLETELY Different answers.

Using the chart

I get an offload voltage of 356.4/half or 712.8 total with a total DCR of 89.2Ohms. This is .125 Ohm/Volt so the chart says for choke input about 240mA's

Using PSUD method. I come to the conclusion it is about 500mA's. Could this be because PSUD used half the transformer so it is doubling the mA's?

What typically is the efficiency (watts in versus watts out) of a transformer? According to the chart for the primary DCR I get a max allowable current of about 1.3Amps so 115V x 1.3Amps which is 143 Watts Power out cannot be great than power in so a guesstimate at 100% efficiency is 143 watts/712V = 200mA's Now I have a third number that doesn't match either the other two (accept that it kind of invalidates the number from PSUD)

The other thing is the transformer was feeding a 5U4GB rectifier. The heater current @ 5V is 3A so there goes another 12 watts.
Then the 6.3V winding has to be at least 2A or so, so there goes another 12 watts.

I just cannot believe that this thing is only about 180mA?
But, it makes sense. It is pretty big and beefy so I am going to impose my own spec at about 200mA's and try some measurements under load with it in a circuit with a draw about that much.

Or does anyone have a 500 Ohm 200WATT Resistor I could borrow?
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Old 20th February 2009, 09:11 PM   #7
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Default Re: ????

Quote:
Originally posted by coldcathode
I am going to impose my own spec at about 200mA's and try some measurements under load with it in a circuit with a draw about that much.
Your logic is sensible. I think you're on the right track. How many pounds does this monster weigh? To put things in perspective, a Hammond 374BX is rated for 175mA @ 750VCT, plus 3A @ 5V and 6A at 6.3V. That's 185 watts total output. It weighs eight pounds.
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Old 20th February 2009, 09:32 PM   #8
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I usually allow 2W copper loss per winding on a small transformer. a bit more on larger ones. With 89 Ohms and 200 mA, I^2*R gives 3.56W, which is about as high as I would go. With choke input, about 260 mA would be the limit. You can simulate it in PSU designer and find the RMS current in the secondary for a closer estimate.

With 6.3V heater windings, I use a 6AS7 as a load (2.5A) and check voltage drop - you can extrapolate the current for a 5% drop.
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Old 20th February 2009, 09:59 PM   #9
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Thanks all. I guess it to weight about 8 or 9 pounds.

My overall feeling is that this tranny is built pretty well. One more thing I wasn't considering is AC amps and DC amps as Tom said with the RMS secondary I.

Well all seem pretty well conviced that it should be good for about 180mA DC @ 350v or so.

Since it had a 5U4GB right on the PS chassis I am sure it has the requisite 3A on the 5V.

I would also think I can get about 2-3 Amps on the 6.3.

Thanks again to all!
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