About power supply

rulezzz · 5th January 2004, 04:40 PM

Hi there i'm new in electronics and i dont find in books or in internet answer on my question.The question is:i bougth transformer 24-0-24 with 1.2 A rating ,putted 2x4700 uF capacitors and then checked the voltage on the output it was +/-31 volt ,and i quickly found the answer in the internet that it multiplies by about 40%.Ok but then i checked the Amper rating it was 9A i couldn't believe from 1.2A after capacitors 9A.In the internet everybody write that if i need to supply 50w amplifier i need 50w or more transformer but i bought another 15-0-15 transformer with 0.35A and after capacitors it could hold 50 w of output power.I couldn't belive that ,because it is like 0.35A*(22v+22v)=15.4W - it is transformer watt rating and it hold 50 w.Ok after that i checked the Amper rating after capacitors it was 3A icouldn't believe again so it mean 3A*(22v+22v)=132 W !!!!????????Can uyou explain me what rating is real transformer or after capacitors ,and if i use like 15-0-15 3A so will i get around 25 Amp after capacitors and will the microschem hold or blow after 25A......?

dhaen · 5th January 2004, 05:13 PM

You can only measure current (amps) by putting the ammeter in SERIES with the load.

Zombie · 5th January 2004, 05:28 PM

Hello!
Isn't it possible to measure the voltage over a 1ohm resistor (added to the circuit) as the voltage = current?
Cheers
Tom

sam9 · 5th January 2004, 05:45 PM

The problem with the current rating of a transformer is the same as the problem with the VA rating -- you can't measure it very well. The reason is that it is possible to draw quite a bit more than the rating from the secondaries -- until the transformer fails! The only way tio be totally sure is test to failure. It may be possible to get somewhat close by tracking the temperature, but that will be the external temperature and it is the internal one that is most closely tied to the failure point. As a practical matter you pretty much have to take the manufacturer's rating on faith.

In any case, there is no simple way to put a meter on a transformer secondary and say "Ah, the VA rating is X."

rulezzz · 5th January 2004, 06:36 PM

(here should be image amps.bmp)
SAFE OPERATING AREA it is tda1514a so if i use +/- 25 volt i would need to supply ~0.75A which transformer do i need?:+/-18v 0.75A or =/-18v and ~0.25 and can i use +/-18v and 2A??
P.S. help newbie how to post pics not by attachment?

5th January 2004, 04:40 PM	#1
rulezzz diyAudio Member Join Date: Jan 2004 Location: not nessesary	About power supply Hi there i'm new in electronics and i dont find in books or in internet answer on my question.The question is:i bougth transformer 24-0-24 with 1.2 A rating ,putted 2x4700 uF capacitors and then checked the voltage on the output it was +/-31 volt ,and i quickly found the answer in the internet that it multiplies by about 40%.Ok but then i checked the Amper rating it was 9A i couldn't believe from 1.2A after capacitors 9A.In the internet everybody write that if i need to supply 50w amplifier i need 50w or more transformer but i bought another 15-0-15 transformer with 0.35A and after capacitors it could hold 50 w of output power.I couldn't belive that ,because it is like 0.35A(22v+22v)=15.4W - it is transformer watt rating and it hold 50 w.Ok after that i checked the Amper rating after capacitors it was 3A icouldn't believe again so it mean 3A(22v+22v)=132 W !!!!????????Can uyou explain me what rating is real transformer or after capacitors ,and if i use like 15-0-15 3A so will i get around 25 Amp after capacitors and will the microschem hold or blow after 25A......?

5th January 2004, 05:13 PM	#2
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	Measurement error You can only measure current (amps) by putting the ammeter in SERIES with the load.

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5th January 2004, 05:28 PM	#3
Zombie diyAudio Member Join Date: Feb 2003 Location: At home	Hello! Isn't it possible to measure the voltage over a 1ohm resistor (added to the circuit) as the voltage = current? Cheers Tom

5th January 2004, 05:45 PM	#4
sam9 diyAudio Member Join Date: Jun 2002 Location: Left Coast	The problem with the current rating of a transformer is the same as the problem with the VA rating -- you can't measure it very well. The reason is that it is possible to draw quite a bit more than the rating from the secondaries -- until the transformer fails! The only way tio be totally sure is test to failure. It may be possible to get somewhat close by tracking the temperature, but that will be the external temperature and it is the internal one that is most closely tied to the failure point. As a practical matter you pretty much have to take the manufacturer's rating on faith. In any case, there is no simple way to put a meter on a transformer secondary and say "Ah, the VA rating is X."

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