Beginner trying to understand transformer power rating

[oldguyjohn]

Hi all, I'm trying to understand transformer power ratings. So example.

Transformer with center tapped secondary advertised as 230V - 20VA  20V + 20V

If I use both windings separately, say for positive and negative supply do I get 1 amp out of each winding or only 500ma?
If I connect the windings together in series I assume I get 40V but only at 500mA,  is that correct? or if both windings can supply 1amp can I draw 1amp at 40V as the current is then common to both?

[Marinated]

Hi all, I'm trying to understand transformer power ratings. So example.

Transformer with center tapped secondary advertised as 230V - 20VA  20V + 20V

If I use both windings separately, say for positive and negative supply do I get 1 amp out of each winding or only 500ma?
If I connect the windings together in series I assume I get 40V but only at 500mA,  is that correct? or if both windings can supply 1amp can I draw 1amp at 40V as the current is then common to both?

Hi. If the transformer has a single center-tapped secondary, then it has only one secondary winding. Your talk of using the windings separately, and connecting in series, makes more sense for a dual-output transformer. This has two separate secondary windings, which you can connect if you wish.

You can't draw 1A at 40V = 40VA from a secondary rated at 20VA.

[Aeternam]

You can't draw 1A at 40V = 40VA from a secondary rated at 20VA.

What happens if you do? Is it physically impossible for the load to draw more or is something bad going to happen?

[Audioguru]

In addition to the 10VA from each 20VAC output, if you rectify the 20VAC the peak voltage is 28.28V so the DCA must be reduced to 10VA/28.28V= 354mA for the positive and the negative because heating power is wasted in the rectifiers and voltage regulators.

EDIT: If you overload the transformer then it will get too hot and might catch on fire.

[Cliff Matthews]

Small transformers at this power level are often protected by built-in thermal fusing (or TCO, thermal cut-out).

A popular TV repair guy shows it here:

[retrolefty]

You can't draw 1A at 40V = 40VA from a secondary rated at 20VA.

What happens if you do? Is it physically impossible for the load to draw more or is something bad going to happen?

 core material will saturate, output volts drops, power dissipation increases, too high and winding insulation could fail and shorting of windings causes larger further problems upstream.

[T3sl4co1l]

You can't draw 1A at 40V = 40VA from a secondary rated at 20VA.

What happens if you do? Is it physically impossible for the load to draw more or is something bad going to happen?

 core material will saturate, output volts drops, power dissipation increases, too high and winding insulation could fail and shorting of windings causes larger further problems upstream.

Please don't spread misinformation:
In fact, core flux decreases, away from saturation, when loaded.  Flux is a voltage thing, not a current thing.

The limitation is entirely thermal: the winding gets too hot and smokes.

Even that isn't a big deal, for some transformers, like the type pictured above.  Small transformers, like these, are typically designed to operate into a short circuit for an extended period of time: they are "impedance protected".  (And some contain a fuse too, which helps even more with safety.)  The downside of this is, the output voltage varies widely with load current, and the efficiency is poor (perhaps 80% or less), so the transformer runs relatively hot, in spite its small size.

Roughly speaking, if you used only one winding, you could use about sqrt(2) more current through that one winding.  The reasoning is as follows: the transformer is designed to dissipate P = I^2 * R watts in the secondary winding area.  P is a design parameter, R is the DCR of the windings, and I is the rated current.  (R and I are for both windings loaded, however you choose to connect them.  If in series, then R is the total series resistance, and I is the current rating of a single winding.  If in parallel, R is half the DCR, and I is twice the single-winding rating.  The series and parallel ratings are usually stated explicitly in the datasheet, too.)  If you use only one winding, then you can dissipate the same total power (which results in the same temperature rise), but instead of that power being split between two equal windings (dissipating P/2 watts), you can dissipate it in just one.  This requires sqrt(2) times more current.

The actual figure will be less, because the windings aren't perfectly (thermally) conductive.  In any case, you're throwing away 30% of your total transformer capacity -- you never have any reason not to use both windings.

Tim

[oldguyjohn]

Thanks to all who replied. Particularly Audioguru for the basic explanation at a level I can easily understand. Some of the replies a bit too advanced for me at my present stage of learning but thanks anyway, I'm sure they are of use to more advanced readers and maybe will mean more to me in the future.