Question: Toroidal transformer rating

[siuguyguy]

Recently I built a new class-d amp, and planned to put a longtime-idle toroidal connect to it. On the spec label of the toro it stated that its 270VA, secondary: 0-20-35(x2 in pair) + 24-0-24(x1) . The d amp stated that its onboard psu need >15VAC with minimum 5A power supply, and i plan to feed the amp with set 0-20. But I wonder if I just use a set of 0-20, the current capibility of this single set 0-20 is simply 270/20 = 13.5A?

I had once see that some transformer (i see this from a online shop, its a r-core) had explicit speicified different rating on various secondary. But I had not see these on toroidal. Is that there's something similar to toroidal like r-core? The shop that the toroidal bought from already closed now and no way to ask them anymore.

Thanks for advice!

[richie00boy]

The current capacity of transformers is limited mainly by the wire gauge. So the max current of your transformer is likely to be determined by the highest voltage of each winding and the VA.

In your case it's difficult because of the 24-0-24 winding and no stated current capacity of that. If that winding was not there, you would have the full VA available so current would be:

270VA / 2 = 135VA per winding

135VA / 35V = 3.857A

[singa]

originally posted by siuguyguy
On the spec label of the toro it stated that its 270VA, secondary: 0-20-35(x2 in pair) + 24-0-24(x1) .

270/3=90VA.? 2X windings of 0-20-35V?

[rjb]

Lets approach it from a different perspective.

The rating of a transformer is normally set by the allowable heat rise due to the losses in it. This heat rise is usually limited by the insulation used, which if it gets too hot will fail and cause shortcircuits which will destroy the transformer.

The losses are in two forms, hysterisis loss in the transformer coil, and resistive loss in the windings.
Different designers allocate these in different ways, but lets say they are made equal.

This heat is dissipated by the transformer through radiation and convection, so for a winding, the centre of it will be hotter than either the transformer core, or the outside of the winding.

The insulation is in three forms, surface treatment of the wire, (" synthetic varnish"), sheet insulation between windings, and the winding former itself. Typically it is the first of these that is critical. So the temperature at the centre of a winding is usually the most important.

Damage to the insulation is cumilative, so a transformer can stand quite high very short term over-loads, but a much lesser over-runif applied long term will eventually cause failure.

If one winding is not used, but the same VA loading used by increasing the current in the used winding. the core loss is the same. It might then be expected the total rated resistive losses could be applied to the used winding. However because this will now get hotter, and the heat cannot get away quite so readily as from two windings, the heat loading and hence the spot temperature within the centre of the used winding will be higher than the normal temperature had all windings been used. This will eventually cause the insulation to fail.

So the answer is that an unused winding will allow some extra current to be drawn by the other winding, but not 100%.

How much extra current can be drawn from the used winding will depend on the transformer design.

[siuguyguy]

Thanks all for advice!

I just dig out the old manual of toro. In fact the toro is come with a 3886 kits and the manual stated that the 24-0-24 is speically made by seller that for kicking speaker protector circuit, with 0.2A(but 0-20-35 not mentioned rating). I think it should be safe to use the 0-20 for the new d amp.

And certainly, i should thanks all you for let me learn so much about toro