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Old 3rd May 2006, 11:55 PM   #1
vax9000 is offline vax9000  United States
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Default Estimate power transformer current rate?

I bought a Stancor transformer from ebay. It is labeled 6.3V 3A, 5V 3A, 2.5V 4A, and 400V-CT-400V. The high voltage output does not have a current rate labeled. How could I estimate the rating? It is potted, 12LB, dimension 5'', 5'', 4 1/2''. The resistance of 400V-CT is 46 Ohms, and CT-400V is 49 Ohms. Totally 95 Ohms. My multimeter is not good enough to measure the resistance of other outputs.

Could this transformer provide 200mA DC if I use a 5U4GB? Your experience is very useful. Thank you.
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Old 4th May 2006, 01:17 AM   #2
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At 200 mA DC, this transformer will have about 200 mA RMS in the secondary. This will result in 3.8W of heat in the secondary, about all you can manage to dissipate in a good-sized transformer winding. In a potted transformer, I'd derate that to about 150 mA. But if you don't load the heater windings to capacity, it should be OK.
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Old 4th May 2006, 01:59 AM   #3
vax9000 is offline vax9000  United States
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Thank you. I will not use the 6.3V or 2.5V windings. Only the 5V winding will be used for one 5U4GB rectifier. It seems I can try to use it at 200mA.

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Old 5th May 2006, 11:49 PM   #4
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One way of approaching this (leading to the same conclusion as Tom) is to use the common copper-loss figure for small transformers (yes, this is still small!) of about 2,5 - 3%.

That means in basic electricity that you take say 3% of 400V as design-allowed voltage drop in the wire resistance, which is 12V. Devided by the resistance, this gives you 250 mA - or about 200 mA for a 2,5% loss figure.

I have measured some of my known power transformers, and the rating for a secondary resistance of about 40 ohm (per side) was 190 - 240 mA, so .... as Tom said.

Finally, you can of course check the transformer's heat after an hour of operation, or how long it takes to become fairly hot.

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