series / parallel transformer switch wiring

[GCA]

hey all- dumb question, but after about 40 drawings on paper I can't for the life of me figure out how to properly wire a dual primary transformer with a DPDT switch for series vs. parallel operation. My brain is about dead. I have friends in Europe and I'd like it to work for them on 220 without my having to make a new power supply.

Now here's the fun part - there are a ton of places on the web that talk about wiring pickups to do the same thing, but all of the pictures and drawings I can find lay out the switch dead-bug style. I can't learn crap from that. Anyone have a schem?

I've also come up with a way to use a thermistor and a SPST switch to do the same thing, except what it's really doing is completely turning off one winding for 220 and will soft-start only in 110. In my power supply I have everything figured out (according to psud2, figured out to within 1% ripple in plate and filament current both with a voltage doubler... I think I can live with that. > ), but this stupid switch wiring is killing me. Since I don't want my next post to be posthumous because I did it wrong, anyone know where I can find an existing schem to do the same thing?

[Enzo]

Look at the schematic for any unit that has a 120/240 switch. For example:

[Enzo]

Make real sure you get the windings in the right phase with each other.

[GCA]

Thx. That's what I needed. It's confusing, though. On first glance it looks like it's reversed and the windings are in parallel when in the 115V position, and series in the 230.

i actually had trouble finding a schem with one!

[octal]

Uh, the windings are SUPPOSED to be in parallel at 115V and in series for 230.
I've converted a lot of Euro equipment to US and vice versa, so I've studied a lot of multi-tap primary wiring diagrams.

N.

[GCA]

Then I must not be understanding right.

Suppose each winding has 10 turns. There's a single secondary with 20 turns.

So... the 115V receptacle would be at a 10:20 ratio, and the 230 at a 20:20?

Wait. nm. That makes sense. Step up @ 115V receptacle. Get 220 on the other side no matter the setting. Twice the ratio at 115.

See? Told ya it was a little confusing. Is why i had to ask

[vaibhav1203]

wow... that schematic is very helpful... as i have a EX250 from samson...whose fuse is blowing off everytime i poer it up... using that schematic i figured out that the power board is fine... it is the amplifier board that has a short between the +45 and -45. by any chance would you have a schematic for the power amplifier board too??...thanks for ur help

[J M Fahey]

GCA, *forget* the secondary.
You will have two equal 115V primaries, with, say, 400 turns each (a more realistic value).
In series, the turns add, you have a 230V 800 turns primary.
In parallel, you have a 115V 400 turn primary.
In parallel, turns do not add; in fact, they need the *exact* same number of turns or your transformer overheats and eventually burns; that's why you need a good digital counter when winding *or* wind bifilar (two wires at the same time); that's what I had to do in the beginning, when I used a turns counter pulled from a cassette recorder.
Why then wire both in parallel if the turns do not add?
Because what adds is the copper section/area/surface, meaning you add current carrying capacity.

[Jazz P Bass]

Make sure you send a fuse for the 220V wiring that is half the rating of the 110.