audioguru

Hi Hero,
You are correct. The transformer will be overloaded if it has the same current rating as the project's output current.

The power passing (VA) rating of a transformer should be used to specify it. Then the power wasted by heating the recifiers and regulator will be included.

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Uncle $crooge

Hero999

What make the situation even worse is that the power isn't drawn thoughout the AC cycle but in surges at the high parts of the waveform. These surges overload the transformer in extreem cases causing core saturation which increases losses (and therefore heating) even further.

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audioguru

I don't think a transformer manufacturer would make a transformer so poor that its core saturates when it has a rectifier and filter capacitor feeding a load and it is operating within its ratings.

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Uncle $crooge

Someone Electro

There are still those ultra cheap tranformers that barly work under low power conditions

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Il give you shocking experience.

eblc1388

I've seen many transformers feeling so hot to the touch by just idling without any load connected. This is a sign of inferior core material or some core saturation occurring. Could be all cost related.

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L.Chung

Roff

Several years ago I bought a Triad isolation transformer. It gets too hot to touch, even with no load. I emailed Triad, saying that I thought it had a shorted turn or something. They wrote back saying that that's the way it's designed. Didn't give me a lot of confidence in Triad.

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Ron

audioguru

A cheap transformer gets hot without a load because its primary doesn't have enough turns to make enough inductance. The cheap primary is just a long piece of wire. Sure it gets hot.

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Uncle $crooge

Nigel Goodwin

Don't you mean gave you none at all?, and never used them again!.

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Roff

I guess I'm master of the understatement.

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Ron

Roff

Yeah, I thought about that. More inductance means more turns of heavier gauge wire, and that leads to a larger core, both of which raise the cost. Well, I seem to recall selecting it because it was cheap (in the true sense of the word, as it turns out). You get what you pay for.
When I get home, I'll check to see if it's rated for 220V also. If it is, that sucker should be glowing red on 220.

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Ron

Hero999

I don't either, but if you connect a 1A load to a capacitor and rectifier connected to a 1A transfomer it will draw huge 3A pulses with an RMS value of 1.414, which will overload it.

I've teasted my theory using a scope, a LM317 constant current source, a transformer, capacitor and a small 0.5ohm current sensing resistor (two 1ohms in paralell.

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Nigel Goodwin

The relevent word there is 'PULSES', it's NOT 3A continuous, and the transformer is (or any decent one will be?) designed for it.

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audioguru

I think a rectifier charges the main filter cap with current pulses that are about 10 times the load's current. All power supply parts are designed for it: the transformer, rectifier bridge and main filter cap.

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Uncle $crooge

Hero999

The magnitude of the pulses will depend on the filter capacitor. Somehow I find if difficult to believe that a 10A pulse won't cause the core to saturate, there again the pulses are quite short so I don't know.

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audioguru

I have never measured a cheap transformer that has a cheap soft iron core instead of carbon steel, maybe has a core that isn't even laminated with insulated laminations, and without enough turns on its windings to be efficient. Some guys have and insist that the peak voltage is the same as the RMS voltage due to the transformer compressing the voltage peaks.
I wouldn't say the cheap transformer saturates its core, I would say that its peak current operates it on the curve of its magnetic hysteresis graph, so it is approaching core saturation.

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Uncle $crooge