# Understanding transformer ratings

I have a transformer that has the following specifications:

• Primary: 220V
• Secondary: 12+12V
• Rating: 5A

Does this mean the transformer has two 2.5A windings? Or is each winding capable of delivering 5A as long as the total rating is not exceeded?

If I use it as a 24V transformer, is the total current 2.5A?

What happens (besides heating) if the transformer's current rating is exceeded?

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Is that all the information on the type plate? Are those two separate windings or a winding with a center tap? – jippie Feb 22 '14 at 17:22
@jippie It's center-tapped (3 wires on the secondary). – hjf Feb 22 '14 at 17:32
It is common to see the transformer's current to exceed some 10~15 times the rated, but momentary and non repetitive. – GR Tech Feb 22 '14 at 21:52

## 2 Answers

Primary: 220V Secondary: 12+12V Rating: 5A

This notation generally means that there are two secondaries, each rated at 12V and 5A. It could also mean a center-tapped secondary that's rated at 24V @ 5A overall.

If I use it as a 24V transformer, is the total current 2.5A?

No, if you put the two secondaries in series, you get 24V @ 5A.

What happens (besides heating) if the transformer's current rating is exceeded?

Isn't heating bad enough? Transformers generally aren't all that good at getting rid of heat (high thermal resistance between the windings and ambient). Once the internal insulation breaks down (or catches fire), it will fail fairly catastrophically because of the shorted-turn effect.

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It has two wires on the primary, and three on the secondary so It's, at least "for all practical purposes", a center-tapped transformer. So that's putting me off a little about the "5A" rating. – hjf Feb 22 '14 at 17:30

Just to add a bit of extra info to Dave Tweed's answer: -

What happens (besides heating) if the transformer's current rating is exceeded?

It gets warmer under heavier load conditions and this is due to copper losses ($I^2 R$) in the windings. But, a side effect of this is that it might also increase the cores magnetic permeability and this means a higher flux density and slightly higher core losses.

Nothing so sinister really until the transformer is powered off and then back on again. Inrush current will have increased (especially if re-powered under no-load conditions) and this could blow a fuse.

When this happens, folk naturally think - why should it blow a fuse on repowering with no-load just because it's warmer. It's because the peak flux density seen in the the first AC cycle can be twice that at all other times: -

Why should the load being disconnected make this worse? If fully loaded the volt drops in primary due to the load current slightly decrease the magnetizing current and can partially avoid the full effects of saturation.

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how hot does a transformer need to be for this to happen? how quickly does it rise? – hjf Feb 22 '14 at 19:55
some materials can double there permeability in 100 degrees centigrade rise. – Andy aka Feb 22 '14 at 20:01
As a rule of thumb the transformer should not get above 105 degrees C.... – Spoon Feb 24 '14 at 20:01