Electronic – Understanding transformer ratings

power supplytransformer

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?

Best Answer

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.

Related Solutions

Electronic – Dual primary / dual secondary transformer heat up

The Farnell page has a little better description of the wiring needed:

Primary Pin Connections        |    Secondary Pin Connections           
Style   0V  115V    0V   115V  |    0V  Vsec    0V  Vsec
UI39    1   4       9    6     |    17  19      14  12

It looks like connecting per this table parallels the primary winding to set the turns ratio for 115V input.

Based on that idea, it looks to me like pins 1 and 6 should see the AC and pins 4 and 9 should be connected together, to maintain the correct phasing of the primary.

Flyback Transformer Saturation in Secondary

Saturation is a function of the core (there is only one core, and it links primary to secondary winding(s)). In the case of a flyback converter, energy is stored in the core and then released to the secondary. Saturation is thus only a function of the primary current, since there is no secondary current (the rectifier is reverse biased) when the magnetic field reaches the peak value.

In the general case, saturation would be related to the signed sum of the ampere-turns of each winding (each affecting the magnetic flux in the core), but that's not a factor in a flyback.

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Related Solutions

Electronic – Dual primary / dual secondary transformer heat up

Flyback Transformer Saturation in Secondary

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