understanding transformers

I am working on an arduino controlled signal generator.

One of the parts I need to have is a power supply with 12+ and 12- (that’s 24 v total) I have been using a recycled computer power supply, but want to make it a little more compact.

I have the attached schematic as a starting point; and I have this transformer

The problem I am not getting is there is not a ‘common’ pin as in the schematic.

Could someone help me understand what I should do? (Wrong transformer?)

thanks

Join/connect pin 6 and 7 to ground (the center pin of X1). Pin 5 and 8 go to the bridge rectifier. Leo..

Have you checked the current requirements of your signal generator circuit?

The power supply circuit you linked is for 1 A max from each rail yet your transformer is only rated 0.1 A. If your current is less than 0.1 A you could reduce the values of the reservoir capacitors and perhaps use 78Lxx and 79Lxx regulators. If the current is more than 0.1 A you need a better transformer.

Russell.

and the negative regulator is a 7912

For your future information, you'll notice the transformer windings have dots showing in one end of each winding. These are there to indicate the "polarity" of the windings. They are AC and therefore don't have polarity but I'm using the term for convenience. The dots actually show the "start of the windings. So, if you connect the windings in series (to increase output voltage) the "dot" of one winding should be connected to the "no-dot" of the other. If you want to connect identical voltage windings in parallel (to increase current capability) the "dots" of the two windings are connected together and the "no-dots" are connected together.

You can connect dissimilar windings in series (say 12v and 6v to give 18v) but you must never connect dissimilar windings in parallel as this will effectively short circuit the transformer resulting in its destruction.

They absolutely do have polarity. The dots indicate that a positive going voltage on the input yields a positive going voltage on the output.

For someone who asks "understanding transformers" it was essential to keep it simple. The term you refer to should be "phasing" rather than "polarity"

Wow… Great replies! Thank you!!!

Wawa:
Join/connect pin 6 and 7 to ground (the center pin of X1).
Pin 5 and 8 go to the bridge rectifier.

Thanks Leo - but wouldn’t the phasing be off? (90 or 180degrees?) hmm… I should use this as a good oscilloscope lesson; don’t know why I didn’t think of that.

russellz:
Have you checked the current requirements of your signal generator circuit?

It’s about 150ma max; when I measured it off a bench supply it was about 80ma

russellz:
The power supply circuit you linked is for 1 A max from each rail yet your transformer is only rated 0.1 A. If your current is less than 0.1 A you could reduce the values of the reservoir capacitors and perhaps use 78Lxx and 79Lxx regulators. If the current is more than 0.1 A you need a better transformer.

Thanks for pointing that out. I misread the page on the transformer. i will double check requirements & swap out parts (caps/regulators/transformer) as needed.

jackrae:
The dots actually show the "start of the windings.
So, if you connect the windings in series (to increase output voltage) the “dot” of one winding should be connected to the “no-dot” of the other. If you want to connect identical voltage windings in parallel (to increase current capability) the “dots” of the two windings are connected together and the “no-dots” are connected together.

You can connect dissimilar windings in series (say 12v and 6v to give 18v) but you must never connect dissimilar windings in parallel as this will effectively short circuit the transformer resulting in its destruction.

Good information! Thank you. I get worried about a/c’s phasing getting in the way, so I didn’t want to ‘try and see’ w/o more information.

I assume you already know how to connect the inputs of the transformer (for your particular residential ac voltage ?)

raschemmel: I assume you already know how to connect the inputs of the transformer (for your particular residential ac voltage ?)

Yes, I already have it wired up & tested the combination of pins for the lower voltage. All seems good; I plan on putting the thing together today & testing it.

Thanks!

KeithRB:
They absolutely do have polarity. The dots indicate that a positive going voltage on the input yields a positive going voltage on the output.

If they had polarity it wouldn’t work with a negative going voltage the same way, which it does.

Windings have ‘sense’ rather than polarity, and its only the relative senses of windings that matters for
a transformer - current flowing into the dot ends of the windings all goes the same way round the core.