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Getting voltages from transformer

blurimage

Jun 11, 2022
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Hi everyone.
I got a 240 V transformer that gives an output of 2 X 12 V AC.
I need to get 2 X 9 V dc and +-5 V dc. My first thought was to get the +9 V DC and ground from each of 2 X 12 V AC so i could get the power needed from the two coils and not from one . But it did not work as I expected because using the middle pin of the 3 pin 12 V AC as ground and placed the 12 v to the two bridges, one for each 9 V , the output was weird. And I haven't even added my circuit to get me +-5 V yet.
So the question is. Can we get a symmetric voltage +-5 V and from the same transformer a 9 V using the same ground for all of them?
 

Bluejets

Oct 5, 2014
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Once you get it rectified and cleaned up, you could use dc dc converters to give you whatever you like.

Depends largely on the transformer capacity (VA) and the load current you require, both of which you do not mention.
 

blurimage

Jun 11, 2022
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The transformer is at least 5 A . Now the 9 V is 2 A each and the +-5 V is 100 ma. So I need 4 A for the 9 Volts and 200 ma for the 5 V.
But my problem is when you ground the middle pin of the three pin of the transformer to create the +-5 V with regulators, how to make the 9 volt so we can have the equal amount of Amp from the left and right coil of the transformer(this is the secondary coil of the transformer)?
 

blurimage

Jun 11, 2022
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Wouldn't this be a problem if a get 4 A from the first secondary and 0 from he second secondary? Will the transformer work properly?
 

Harald Kapp

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Nov 17, 2011
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This is how one typically connects a transformer with two secondaries for one output only:
upload_2022-6-12_10-4-45.png
Now the two secondaries are evenly loaded. No worries with a single secondary being overloaded.
 

Bluejets

Oct 5, 2014
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Yup, ok but secondaries weren't overloaded , rating was 5A and load was 2 x 2A plus 100ma for the 5v.
Does no harm to have one secondary open circuit.
 

blurimage

Jun 11, 2022
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The 5 V is +-5 V symmetric so ground is C in the schematic, and if I get the two Ac voltages before the diodes with resistors I could get the +-5 in parallel with D1 and D2 too. Right?
 
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