help with transformer secondary voltage reading

jarthel · 17th September 2006, 07:07 AM

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.

Geek · 17th September 2006, 08:17 AM

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.

AndrewT · 17th September 2006, 05:51 PM

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.

poobah · 17th September 2006, 06:26 PM

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!

Geek · 17th September 2006, 10:18 PM

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

jarthel · 17th September 2006, 11:38 PM

thank you for the help

poobah · 17th September 2006, 11:48 PM

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.

17th September 2006, 07:07 AM	#1
jarthel diyAudio Member Join Date: Jan 2002 Location: somewhere in Australia	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.

17th September 2006, 05:51 PM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. __________________ regards Andrew T.

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17th September 2006, 08:17 AM	#2
Geek diyAudio Member Join Date: Sep 2004	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.

17th September 2006, 06:26 PM	#4
poobah diyAudio Member Join Date: Nov 2005	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!

17th September 2006, 11:38 PM	#6
jarthel diyAudio Member Join Date: Jan 2002 Location: somewhere in Australia	thank you for the help

17th September 2006, 11:48 PM	#7
poobah diyAudio Member Join Date: Nov 2005	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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