Transformer center tap confusion and requirements

[cersepn]

If i need 6-9 VAC in for this PSU, is it possible to use a single center tap (series output) transformer of higher VAC and somehow wire it to give me 6-9 VAC? I'm looking at these transformers but the minimum output they have is 10VCT
Split Bobbin w/High Isolation?Lead Wires | Signal Transformer

Also, if i need +/- 18 VAC, is it possible to wire a center tap to give me dual secondaries?

 

[Frank Berry]

You can use a 12-18 volt center-tapped transformer and use one-half of the secondary.
+/- 18 VAC?
+ and - only applies to d.c.
What are you powering that requires +/- 18 VAC?
Can you post a schematic or other information?

 

[cersepn]

Thank you for your input, Frank. It is much appreciated! 🙂

I plan on using the other transformer to power the salas shunt, for use with the legato 3 i/v. I was confused, it should be 2 x 18 VAC instead
http://www.diyaudio.com/forums/power-supplies/192625-sslv1-1-builds-fairytales-65.html#post2754851

 

[discrete]

If i need 6-9 VAC in for this PSU, is it possible to use a single center tap (series output) transformer of higher VAC and somehow wire it to give me 6-9 VAC? I'm looking at these transformers but the minimum output they have is 10VCT
Split Bobbin w/High Isolation?Lead Wires | Signal Transformer

Two options:

1. Use a 10VAC CT model and ignore (isolate) the CT wire. 1 VAC extra should not matter. Vdc (peak) = ~14V, Idc max = 0.6Iac max

2. Use a 16VAC CT model and re-wire the circuit as a full wave rectifier:
http://sound.westhost.com/psu-f5.gif
I.E. tie the CT to the negative of the filter capacitors and do not populate bridge diodes 2 and 4 (from left to right on the linked pcb layout). Vdc (peak) = ~11V, Idc max = Iac max

 

[cersepn]

Thank you, discrete. Option 1 sounds good! What about the other 2x18 VAC requirement that i have for the other PSU?

 

[discrete]

Thank you, discrete. Option 1 sounds good! What about the other 2x18 VAC requirement that i have for the other PSU?

You will have to strip back the center tap wire and separate the two windings.
These are EI core type transformers, so not easy or recommended.

Use two 16 or 20VAC CT transformers and again isolate the CT wires?


I assume you are set against toroids, otherwise Antek has an excellent range and value for money:

Antek - Transformers - Grid View

 

[JoelS]

Some transformers have independent dual secondaries which can be used in series or parallel, or isolated from eachother (4 wires secondary). this is different from a single secondary with a center tap (3 wires). if the design call for 2x18VAC secondary then a single center tap will probably not cut it (unless they are just wired in series which is the same as a 36VAC with a center tap). a schematic would make it clear.
check these out:
http://www.signaltransformer.com/content/split-bobbin–low-power
("Secondaries - series or parallel" means 2 isolated secondary windings)

-Joel