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Old 17th September 2006, 08:07 AM   #1
jarthel is offline jarthel  Australia
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Default help with transformer secondary voltage reading

with transformer secondaries, is the voltage reading higher than rated voltage when there is no load?

I measured voltage reading a 12V transformer is around 14.7V without any load. when I connected it to the power supply circuit (linear regulated and again without any load after the regulator), the voltage reading on the 1st cap is 17+V.

thank you for the help.
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Old 17th September 2006, 09:17 AM   #2
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Hi,

Yes, that is normal. For a true reading, load the secondary down with a power resistor for the rated load. So, if the transformer is rated 2 amps, load the secondary with a 25W, 6 ohm resistor and remeasure.
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Old 17th September 2006, 06:51 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
the difference between loaded voltage and open circuit voltage is specified as the regulation of the transformer.

Small transformers can give over 30% high on open circuit measurement.
Larger transformers get down to near 4%.

Your one measures out at 18%, if your input voltage was set accurately.
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regards Andrew T.
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Old 17th September 2006, 07:26 PM   #4
poobah is offline poobah  United States
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The "why" portion...

Both the primary and secondary windings have resistance... so the voltage drops (or appears to) as the current increases. Some of the voltage is being "burned" inside the transformer... half the reason they get warm. The tranny people take this into account... all very normal... Andrew's numbers, as always, are quite correct.



I am designing a transformer today... hate it, hate it, hate it!

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Old 17th September 2006, 11:18 PM   #5
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Hi Poobah,

Quote:
Originally posted by poobah
I am designing a transformer today... hate it, hate it, hate it!


Would this help?

http://www.dissident-audio.com/OPT_da/Page.html
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Old 18th September 2006, 12:38 AM   #6
jarthel is offline jarthel  Australia
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thank you for the help
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Old 18th September 2006, 12:48 AM   #7
poobah is offline poobah  United States
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Thanks Geek,

But no... I am doing a 60 Hz power transformer. I am trying to learn McLyman's method... which is supoosed to be simple and direct (BS).



I know how to do it the hard way... iterate, iterate, iterate. I am not having such good success.

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