Transformer 25VA

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#1
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Guys I have a transformer named as 25VA.It has two 12V tappings on secondary. See the attachment how I have configured.

 

I want to know how much current I can get from this transformer? Is it 25/24 = 1.04Amps?

 

If I need 2V,5V,12V (by adding regulators) do I still can get 1.04 Amps?

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Last Edited: Sun. Mar 19, 2017 - 03:01 AM
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That is 1 amp, AT THE TRANSFORMER. If they are linear regulators, then the sum of all load currents must be less than 1A. If it is a switcher, then you can get more current out, because they are (approximately) constant power devices: input power = output power (independently, for each switcher).

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

Last Edited: Sun. Mar 19, 2017 - 03:15 AM
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The peak voltage after the rectifier will be near:

 

octave:1> (12*2)*sqrt(2) - 2*0.6
ans =  32.741

Edit: Rephrase for [AwNeil]

When drawing 1A from the rectified 32.7V you clearly have more than 25VA and the transformer will be overloaded.

You also ave to take the peak current into account when the transformer re-charges the filter cap.

 

For a regulator with 12V DC output it would be a lot better to use only one of the 12V windings.

You'll have about 16V n the Elco then, which is enough margin for the ripple voltage over the Elco.

 

More voltage means more heat in the regulator, higher load for the transformer and more money to your electricity distributor.

 

 

 

 

Paul van der Hoeven.
Bunch of old projects with AVR's:
http://www.hoevendesign.com

Last Edited: Tue. Mar 21, 2017 - 11:07 AM
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Joseph1 wrote:
Guys I have a transformer named as 25VA. It has two 12V tappings on secondary.

Actaully, if your drawing is correct, is has two 12V secondary windings.

 

See the attachment how I have configured.

It makes like much easier if you embed the image in the post where we can see it:

 

See the instructions here: http://www.avrfreaks.net/wiki/em...

 

I want to know how much current I can get from this transformer? Is it 25/24 = 1.04Amps?

Yes: VA = Volts x Amps - so 25VA / 24V = 1.04A

 

 

 

If I need 2V,5V,12V (by adding regulators) do I still can get 1.04 Amps?

The rating applies to the Transformer - it is irrelevant how you distribute it.

 

But, as noted, getting 2V, 5V, 12V from the above arrangement using linear regulators will be grossly inefficient!

 

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Paulvdh wrote:
You can not draw 1A of current out of the transformer in this way.

Eh???

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If the two 12V windings are independent, I would parallel the two 12V windings.

David (aka frog_jr)

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Taking care to connect them the correct way around!

 

surprise

 

http://www.electronics-tutorials...

 

Last Edited: Mon. Mar 20, 2017 - 03:30 PM