PT primary wiring question

[Tony Bones]

Whatever switch you use, wire it as shown below. The schematic looks more complicated than it is. The layout shows how simple it is.

edit: Pay close attention to the polarity of the two windings. It is assumed that BLU and BLK wires are the starts of the two windings whilst the yellow stripe indicates the finish. In simple terms, you want to connect the windings in parallel for 120VAC operation and series for 240VAC operation.

I agree that the switch should not be easily accessible. Maybe inside the chassis?

[Leo_Gnardo]

Wiring the 2 primaries in parallel does not increase the magnetic field, they just act as a single 120V primary with doubled copper cross section. The magnetic field depends on amperes x turns and each primary carries half the current so the result is the same as with one primary and full current.

The difference is in the copper resistance and as there are safety limits for max. current per square mil (current density), a transformer having a continuous 240V primary with a 120V tap typically needs a heavier wire gauge for the full winding (requiring more winding space).

OK, well so much for that (my) theory. Thanks for your explanation!

[mozz]

Find any HP or Tektronix piece of test equipment (50's on up) that has dual voltage tranny. Wire it up like that, they often use a slide switch.

[J M Fahey]

Slide switch version:



In principle:

Top winding has top end always to Mains HOT.

Bottom winding has bottom end always to Neutral.

Top winding , through switch, sends its bottom end either to Neutral (120V setting) or to a short wire bridge (240V setting)

Bottom winding , through other half of the switch, sends its top end either to HOT (120V setting) or to short wire bridge (240V setting)

So in a nutshell:

120V setting: both windings send top ends to HOT , bottom ends to Neutral, so they are in parallel and you use all the copper available (good).

240V setting: top winding sends top end to HOT, always; bottom winding sends bottom end to Neutral, always; free ends get together through short wire and so both windings become in series and you use all the copper available (good).

As suggested above, get the 120-240V labelled slide switch, which is harder to move by accident, you need a flat screwdriver tip and must be done on purpose. Any other switch is dangerous.

Some add a small piece of acrylic covering the switch, and you must remove 2 screws to allow switching.

[nsubulysses]

I really appreciate all the replies. As I would have expected, my knowledge on this subject and scenario has increased tremendously. Now I know just what to do. Thank you!

ALso, the switch is OUTSIDE the chassis but INSIDE the headcase. It is a big clunker and I agree that the 120/240V slide switch would be better.

[J M Fahey]

Brazilians have an absolutely schizophrenic Mains system; half of it 110V , half of it 220V , mixed at random even in the same city, even across a single street or , say, 0 to 1200 house number is 110; 1300 to 2400 os 220; and so on.

So every amplifier sold there has a 110-220V switch , not hidden somewhere but straight at the front panel, and just by or on top of the mains power switch, so you can not ignore it, period:

[Leo_Gnardo]

Brazilians have an absolutely schizophrenic Mains system; half of it 110V , half of it 220V , mixed at random even in the same city, even across a single street or , say, 0 to 1200 house number is 110; 1300 to 2400 os 220; and so on.

Touring Brasil 1982, found that to be true, also some areas were 50 Hz, some 60. And in some remote ares we never got to, 25 Hz !?! Somebody must have found some old subway generators & hauled 'em into the jungle & outback.

Japan still has that 50/60 Hz thing going from what I hear.

By the way, happy Argentine Independence Day, Juan! What goes on down there? Parades 'n fireworks?

[J M Fahey]

Thanks

Not much of a celebration because parades are being hosted by our current President, Mauricio Macri, who is HUGELY unpopular.
So much so that he only allowed about 4000 people to assist, all of them hardcore members of his Party (normal would have been 500k to 1M citizens on their own) to avoid being booed to death.

Specially considering that Elections campaign started 2 days ago and he´s considered a favorite ... to lose.

[loudthud]

In the late 80's I worked at a standalone musical instrument repair shop. I got stuck with all the boomerang (they keep coming back) repair jobs. One of the boomerangs was an active guitar that was owned by a big used guitar dealer that would send us his repairs. The power supply would shut off usually after the customer took it home. Turned out one of the dual primaries was open. Nobody had checked that previously because they were in parallel and everything seemed to work. It would operate for about an hour, then blow the fuse.