Re: [CAD_CAM_EDM_DRO] current rating of transformers

Andy Wander wrote:

>Yeah, but....
>
>I still say that the thing that limits the secondary current is the wire
>gauge used for the secondary winding.
>
>I don't think many transformers are built using wire that's a lot heavier
>than needing fro rated output current. Therefore, dropping the primary
>voltage(thereby dropping the secondary voltage) doesn't mean you can now
>pump more current through it to "get back" up to the VA rating.
>
>

Yes, that is absolutely true. You may have misinterpreted what I was
saying in an
earlier message. On transformers with 2 seperate secondary windings,
you can
parallel the winding for double current, or put it in series for double
voltage.

But, on a transformer with a center-tapped winding, you have a similar
option.
Take an example of a transformer with a 17 VAC winding with a center tap.
If you connect a full-wave bridge rectifier to the end terminals, and
don't connect
the center tap, you will get 24 VDC into a capacitor-input filter. (17
* 1.414)
Let's say this transformer is rated for 10 A in this configuration.
If you connect the center tap to the minus side of the cap, and connect
two diodes with the cathode to the + term of the cap and the anodes
to the transformer winding ends, you will get 12 V DC at 18-20 A DC.
That is because each half of the transformer winding now is only required
to conduct for half of the power line cycle. With the full-wave bridge,
the full
winding carries its full current EVERY half cycle.

Wire size is a major factor in secondary current rating. But, the depth
of the
winding affects the heat removal from the most interior parts, where the
"hot
spot" invariably is. Insulation class and the fill of transformer
varnish that
helps conduct heat away are all part of the equation. Also, the design
of the
iron to reduce eddy lossed in both the iron and the copper are significant.

Jon