decoding transformer connections

[dave]

I have a beefy mains transformer from a US made psu. The mains i/p voltages were
slider-switch selectable 110/240. As I now only have the xformer not the
switches I'd like if possible to know how to wire it up for mains 240 V use.

There are 2 primary winding and I can only measure their DC resistance. There
are 5 prim connections altogether:-

Winding 1, A-B measure 3 ohms

Winding 2. C-D-E measure 2.8 and 0.9 respectively.

The slider switches used to hook these two winding in some way for 240V as I
know it used to work ok. Should I just go for max resistance, feed in on A and E
and just link B-C? It may be the wrong way around of course (ie feed in on A-C
and link B-E) :-O Any way to tell beforehand - a pity to destroy it
experimenting.

Help!
Thanks

 

[MikeM]

dave wrote:

I have a beefy mains transformer from a US made psu. The mains i/p voltages were
slider-switch selectable 110/240. As I now only have the xformer not the
switches I'd like if possible to know how to wire it up for mains 240 V use.

There are 2 primary winding and I can only measure their DC resistance. There
are 5 prim connections altogether:-

Winding 1, A-B measure 3 ohms

Winding 2. C-D-E measure 2.8 and 0.9 respectively.

The slider switches used to hook these two winding in some way for 240V as I
know it used to work ok. Should I just go for max resistance, feed in on A and E
and just link B-C? It may be the wrong way around of course (ie feed in on A-C
and link B-E) :-O Any way to tell beforehand - a pity to destroy it
experimenting.

Help!
Thanks

I would hook Line1 to A

Connect B to C
connect Line2 to E

where Line1 and Line2 are the 240V mains

Turn things on, and measure the secondary voltage ( AC volts) with the
transformer unloaded.

By moving Line2 from E to D, you should be able to raise the output voltage by
about 10%...

(work with one hand in your pocket...)

MikeM

 

[MikeM]

One other suggestion:

Hook a 100W 240V lamp in series with either line for initial testing. If you
accidently connect a low impedance (dead short?) across the line downstream of
the lamp, the worst that will happen is that the lamp will light full brilliance.

With the transformer unloaded, the primary current in the transformer should be
low enough that the lamp will not light, the drop across the lamp will only a
few volts, and voltage across the primary should be most of 240V.

MikeM