# Rules of thumb for power transformer current rating - derivation?

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I have this document:

which gives rules of thumb for sizing power transformers for certain DC output requirements. It says there are plenty of references for the derivations of these rules available - but I can't find any! At least on the web. Does anyone have any material they could link me to that would give the derivations for why say a bridge rectifier circuit with a capacitor filter needs a transformer with an AC rating of nearly twice the DC current, while say a choke input full-wave can draw 1.5 times the AC rating of the transformer? I hate using rules of thumb without knowing where they are derived from; I imagine it has something to do with the Fourier components of the current waveform in different configurations but I'm not smart enough to figure this out on my own. :(

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"Bitrex" schrieb im Newsbeitrag news:gfmdnWo1Wcbe6MjWnZ2dnUVZ snipped-for-privacy@earthlink.com...

Well, since the filter capacitor augments the DC-voltage up to sqrt2, it cannot be expected to deliver more power than the rms rating. Plus there are conducting losses in the windings and diodes which are higher with the intermittent current flow caused by the cap. With the same average current the rms-value goes up because the duty cycle is much shorter. A choke will lower the output voltage and increase the internal resistance. It will not reduce the power rating of the transformer, but can lower the max ripple current of the cap and diodes. ciao Ban Apricale, Italy

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"Bitrex"

** The math derivation would bore your backside off.

But a simple test would convince and be very instructive.

Shame if that idea causes you apoplexy.

** Choke input filters are relics of the dim, distant past.

And the link does not say what you claim anyway.

** Mainly from testing real, transformer based PSUs.
** Would apply to a great many other things too I suspect.

... Phil

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I do have this...

ftp://jjlarkin.lmi.net/XfmrScatter.JPG

John

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d

A quick way to go is to take the output power add any expected diode loss power etc and then multiply by 1.5 to get the VA rating of the transformer. It works for all cases except the half wave rectifier.

• posted

As a qualitative explanation, try to visualise it this way: A filter capacitor tries to charge up to the peak value of the rectified output voltage. This peak voltage is higher than the rms voltage of the transformer output. When the load draws current from the rectified and filtered output, the (load current)*(dc voltage) is higher than (rms voltage)*(load current). That extra power has to come from somewhere. It's made up for by drawing current higher than the dc load current from the transformer. The rms value of the transformer current is what causes the coils to heat up and therefore determines its ratings.

It's actually a bit more complex than that and, as Phil indicated, the mathemtical analysis is fairly involved. I use rule of thumb and guesstimate based on past experience. When I need a closer estimate, I turn to good ol' Radiotron Designer's Handbook (the 1952 edition!). It provides an extensive set of graphs for capacitor-filtered rectifiers.

It seems RDH is available for download as a pdf file, though I have no idea how legal it is.

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A mathematical derivation is what I asked for, but I'm glad someone is looking out for me, knowing what I'll find boring and what I won't.

Tube audio power supplies.

It's a good thing there are people like you, bright enough to derive all of electrical engineering from first principles. Thanks for the useless post, cocknose.

• posted

Thanks for the reference, I'll have to look at the RDH to see if it has the material I'm looking for. I've always meant to get a hardcopy of the book but 4th editions seem to go for stupid money on Ebay. I imagine the PDF version is legal if the last publication date of the RDH was before 1963 and the copyright on it was never renewed - it would then be in the public domain.

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I saw a copy each of RDH 4th Ed and RCA Receiving Tube Manual in good condition with a pavement bookseller when I visited Mumbai quite some time ago (I'm in India and it was still called Bombay then). They caught my eye and the seller asked only the equivalent of USD1 and 50 cents respectively for them. I was tempted but I already had a heavy luggage and copies of both at home. Besides, that was before we had internet here and I had no idea how valuable they had become. \$107 at eBay. Wow.

Not getting those two books is among my regrets in life. I would probably give them away to a deserving person rather than sell them at exhorbitant prices. When I visited Mumbai more recently, I went to the same location, solely to look for those two and similar books, but no luck.

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```--
Greed is the root of all eBay.```
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I found that site some time last year while looking for something else. It even has an older edition of my first book on electronics - Audels Radiomans Guide. I found that one too on a pavement "bookshop". That one was in New Delhi.

My connection is fairly OK. It's 2 Mbps and gives me 200 KB/s or better with a fast site. My best continuous d/l was a 4.14GB Knoppix LiveDVD in a single 5.5 hour session - an average of

220KB/s. (Yes, I *know* that some people enjoy several times that speed, but then there are others still struggling with dial-up).

Any idea where I can d/l a late 60s or 70s edition of RCA Transistor Manual? The 1968 edition was among my earlier books but I lost it. I'd like to keep at least a pdf version, more for sentimental reasons than for any practical purpose.

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Both Agilent and Onsemi have good refrence material on PSU capacitive input filtering in their handbooks.

Agilents

Heres Onsemis you want the linear and switching one

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Those links are great, thank you!

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Ed

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The 1964 (7th edition) seems to be the best of the lot, and strangely=20 one of the hardest to get. I have my original (rather beat up) physical=20 copy. Maybe it is time to recheck with pmillett.

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