OT (but don't let it get to you!) - Transformers in PSUs

[freebird1127]

Hey all,

I'm trying to figure out how to use a transformer that has dual primaries AND dual secondaries in a power supply to provide +12V and -12V.  I'll be using a 7812 and 7912 regulator, I've got everything else figured out... just the wiring of the transformer.  

Am I right in thinking that if I want the PSU to operate from 120V mains, I should wire the primaries in parallel?
(primaries in series for operation from 230V mains?)

I want to use a bridge rectifier module for rectification... so how do I wire the secondaries so that I have a centertap for grounding purposes, and a + and - VsecRMS for rectification and filtering?

[niftydog]

If the transformer is properly rated for 120V/240V operation, then I think you could just leave one primary unconnected.  Just make sure it doesn't short anything out!

But it really depends, because the transformer may be of some hoopy construction that I don't understand.  Do you know what it came out of or any detail about it at all?

Wire the secondaries in series and use two bridge rectifiers.  ie; the two secondary wires that you join together will become the common (ground) for the dual polarity supply.  Arbitrarily decide which of the remaining wires will be +ve and which will be -ve.  Then, mirror image the filter caps, insert your 78XX and 79XX and Bob's your uncle!

[R.G.]

Transformer windings have a voltage and a phase.

Most, but not all dual primary transformers want you to parallel the primaries for 120V, series for 240. You must connect the **RIGHT** two terminals together to series or parallel the windings.

For series, you do it this way: Number the primary wires 1, 2 for the first primary, 3, 4 for the second.

Connect 2 and 3.
Apply 120Vac to 1 and 2.
Measure the voltage between 1 and 4.
If this is 240V, you're fine, this is the correct series connection. Make the 2,3 connection safely insulated and you can connect 240Vac to 1 and 4.

If you measure almot 0V, the phase is incorrect, and you must reverse the connection of one of the windings. For instance, you can break the connection between 2 and 3, and hook 4 to 2. Now with 120Vac on 1, 2, you should read 240Vac between 1 and 4, and this is the connection to use.

For parallel connections:

Again, number leads 1, 2 for one primary, 3, 4 for the other.

Hook 2 and 3 together. Apply 120Vac to 1, 2. Measure the voltage from 1 to 4. If it's nearly 0V, you have the correct phasing for parallel. Hook 4 to 1 and you can apply 120Vac to both in parallel. Otherwise, flip 3 and 4.

Notice that if the voltage here between 1 and 4 is not almost 0, say more than 1V, you cannot parallel the primaries. They have too much voltage difference, and high circulating currents will flow if they're paralleled, accompanied by the release of much smoke. In this case, you must use one or the other primary, but not both.

Secondaries can be stacked for CT in the same way. Make one connection, measure the free ends. If it's twice the voltage of one winding, you're correctly series connected. If it's almost 0, invert one winding.

[Eric H]

If the transformer is properly rated for 120V/240V operation, then I think you could just leave one primary unconnected.  Just make sure it doesn't short anything out!!

Wrong! wire the primaries in parallel.  Is there no wiring diagram on this transformer? You're working with killing-power here.

Wire the secondaries in series and use two bridge rectifiers.  ie; the two secondary wires that you join together will become the common (ground) for the dual polarity supply.  Arbitrarily decide which of the remaining wires will be +ve and which will be -ve.  Then, mirror image the filter caps, insert your 78XX and 79XX and Bob's your uncle!

Why 2 bridge-rectifiers? You only need one.
look here:
http://sound.westhost.com/project44.htm

[niftydog]

Wrong! wire the primaries in parallel.

That depends on the construction of the transformer.

Consider a piece of equipment that can operate on many different mains voltages.  Usually they have a rotary switch on the back that you set to the mains voltage that you're intending to use.  This switch connects the appropriate windings in the appropriate fashion and leaves the others unconnected.

If the two windings are identical, then you could connect them in parallel.

If one winding is designed for 120V, and the other for 30V, then you're gonna get some strange results!  That's why I have asked if the OP has any further detail on the transformer.

Why 2 bridge-rectifiers?

Yes, sorry, mondayitis.

[Eric H]

Consider a piece of equipment that can operate on many different mains voltages.  Usually they have a rotary switch on the back that you set to the mains voltage that you're intending to use.  This switch connects the appropriate windings in the appropriate fashion and leaves the others unconnected.

Yes, though the dual primary/secondary is ordinarily designed specifically for 240/120 series-parallel , like a typical universal flat-pack .

If the two windings are identical, then you could connect them in parallel.

I assumed that to be the case:

"dual primaries AND dual secondaries"

--never assume, though

Why 2 bridge-rectifiers?

Yes, sorry, mondayitis.[/quote]

Hahaha, too much caffeine for me today --sorry .

[Eric H]

You must connect the **RIGHT** two terminals together to series or parallel the windings.

Believe it, or not, I was trying to find the text file I saved when you explained this process to me , quite some time ago. I'll save it again. I hate it when they don't mark this stuff on the transformer.

-Eric