PT current rating Q

[Ptron]

I need someone to point me to an "Idiot's Guide to Power Transformer Ratings." I don't get why secondary current rating is given in DC. Unless I'm totally mis-imagining how a power supply works, it seems to me that the current in the transformer will be anything but DC, i.e. in a half-wave supply, each half of the winding is only supplying current during the portion of the cycle that it's voltage is higher than the first filter cap, no?

Is it just that they've done the math for you, assumed a capacitor input load, and are saying if your load draws this much, the transformer will handle it?

[Alex/Tubewonder]

Originally Posted by Ptron

Is it just that they've done the math for you, assumed a capacitor input load, and are saying if your load draws this much, the transformer will handle it?

In two words: that's right. AC ratings are for those who can do the math themselves as going back and forth between avarage, rms and dc can be confusing at times.

[Steve Conner]

Straight answer: Yes, they did the math for you.

Slightly more geeky answer: Average current is what forms the DC output to the load, but RMS current is what determines the heating in the transformer.

So, the difference between the transformer's AC and DC ratings is determined by the RMS-to-average ratio of the current waveform. That depends on the type of rectifier topology (bridge or FWCT), the type of filter circuit, the size of the filter capacitor(s), and a bunch of other stuff like resistance and leakage inductance of the PT.

So it's easier for the manufacturer just to do the math for you (but bear in mind that if you use a greatly different value of filter cap, the answer won't be valid any more: the bigger the cap, the lower the DC rating will be)

BTW, if you ever wondered why a transformer for a tube rectifier is so much bigger and heavier than one for a solid-state bridge of the same DC rating, now you know.

[Ptron]

Thanks guys. That's the info I was looking for.

I assume that's either common knowledge or a natural assumption that's eluded me ( wouldn't be the first time ) as you would kind of need to know that when selecting a PT.