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Transformer primary rating question
Transformer primary rating question
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Old 26th June 2008, 05:34 AM   #1
gary h is offline gary h  United States
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Location: bay area, california
Default Transformer primary rating question

Hi all,

I'm trying to build a preamp PS and checking transformers. I see a lot of them with primary ratings of 115 or 117 VAC.

The AC coming out of my wall varies between 119-121. Does this mean that the current output will just be a little higher than stated or will this extra voltage add heat to the transformer it was not designed for?


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Old 26th June 2008, 05:55 AM   #2
Klimon is offline Klimon  Belgium
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Location: Leuven
You'll just get some extra voltage; no problem at all for the transformer (keep in mind that in many places mains voltage normally varies throughout the day alot more than the few volts you quoted). I've got quite a few old transformers having primaries for 220 and 240V; measured mains voltage here is 230V which means that chosing either of both primaries lets me fine-tune output voltage.

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Old 26th June 2008, 06:24 PM   #3
BudP is offline BudP  United States
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Transformer primary rating question
gary h,

In the modern world, circa 1970, power transformers are supposed to tolerate 110 % of their rated input voltage. This is determined by the turns in the primary and the square area of the center leg of the core, assuming a fixed mains frequency.

Modern day transformers are a minimalist design due to cost constraints in the industrial market. One we must service to stay in business. This means that the least expensive power transformer is likely to be the least capable of withstanding out of or near to out of spec parameters.

To avoid any difficulties with over voltage, buy a transformer rated for 50 / 60 Hz duty at 115 / 230 or 120 / 240 input voltage. It would be a good thing to also buy one that has a VA rating about 150 % of what you need, just to get the no load to full load rating of the transformer to less than 5 %

Almost any power transformer will do, really. But you are not faced with sourcing for hundreds of units here, so spend the extra cash. Most especially since I have never seen a project come in at the expected power draw, in 35 years of design work in transformers. So, get the extra capacity up front. It can't hurt.

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