Stupid question (dual voltage pt)

[yalesmith]

I purchased a dual voltage vox transformer and the instruction seem a bit odd. I also don't want to hook it up and burn my house down:). If someone can take a look and let me know what they think it would be greatly appreciated. I want to run at 120 volts primary.

http://www.classictone.net/40-18050.pdf

Thanks

[martin manning]

Just parallel the two primary windings together like it says on the data sheet.

[yalesmith]

just seems odd to me, the 240 volt hookup makes sense but connecting all four seems oddly wrong.

[Phil_S]

Not oddly wrong, it's versatile. Transformers are all about turns ratios. If you need to step up 240 to 480, it's 1:2. If you need to step up 120 to 480 it's 1:4. For the sake of the example, assume a fixed secondary of 480 turns, and a split primary with 120 turns on each side of the split. If you put the two primaries in series, you have a ratio of 1:2. If you only use one half of the primary, you have 1:4.

The problem is current. If you only use half the primary, you are only using half the capacity. If you run both primaries in phase, you don't sacrifice any capacity for current. IOW, if the PT secondary is rated for 100mA, you get the full 100mA when you put the two halves of the primary in series. You also get the full 100mA when you put the two halves of the primary in parallel. With only one half of the primary, you still get the full voltage on the secondary, but only 50mA capacity.

Does that help?

[rdjones]

Not oddly wrong, it's versatile. Transformers are all about turns ratios. If you need to step up 240 to 480, it's 1:2. If you need to step up 120 to 480 it's 1:4. For the sake of the example, assume a fixed secondary of 480 turns, and a split primary with 120 turns on each side of the split. If you put the two primaries in series, you have a ratio of 1:2. If you only use one half of the primary, you have 1:4.

The problem is current. If you only use half the primary, you are only using half the capacity. If you run both primaries in phase, you don't sacrifice any capacity for current. IOW, if the PT secondary is rated for 100mA, you get the full 100mA when you put the two halves of the primary in series. You also get the full 100mA when you put the two halves of the primary in parallel. With only one half of the primary, you still get the full voltage on the secondary, but only 50mA capacity.

Does that help?

This is the correct way to construct a dual voltage primary, two windings that are arranged either in series or parallel.
In cases where the primary is a single tapped winding you lose efficiency at the lower voltage because, quite simply, only half the primary winding is being used.
The result is more heat and lower current capacity.

You do have to observe proper polarity when connecting the primary windings and it does make a dual voltage switch more complicated.

rd