Rules of thumb for power transformer current rating - derivation?
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From: pimpom on 19 Jan 2010 11:59 Bitrex wrote: > I have this document: > > http://www.audiofaidate.org/it/materiale/20_020_020_001_PowerTransformer_FilterRatings.pdf > > 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. :( 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.
From: Bitrex on 19 Jan 2010 17:07 Phil Allison wrote: > "Bitrex" > >> I have this document: >> >> http://www.audiofaidate.org/it/materiale/20_020_020_001_PowerTransformer_FilterRatings.pdf >> >> 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. > > ** 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. > 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. > >> 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? > > ** Choke input filters are relics of the dim, distant past. > > And the link does not say what you claim anyway. > http://www.hammondmfg.com/pdf/5c007.pdf Tube audio power supplies. > >> I hate using rules of thumb without knowing where they are derived from; > > ** Mainly from testing real, transformer based PSUs. > > >> 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. :( > > ** Would apply to a great many other things too I suspect. > > > > > ... Phil > > > > 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.
From: Bitrex on 19 Jan 2010 17:19 pimpom wrote: > Bitrex wrote: >> I have this document: >> >> http://www.audiofaidate.org/it/materiale/20_020_020_001_PowerTransformer_FilterRatings.pdf >> >> 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. :( > > 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. > > 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.
From: pimpom on 20 Jan 2010 02:54 Bitrex wrote: > pimpom wrote: >> >> 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. >> >> > > 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. 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.
From: Michael A. Terrell on 20 Jan 2010 07:52
pimpom wrote: > > 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. You can download those and other old electronics books for free, if you have a good internet connection: <http://www.pmillett.com/technical_books_online.htm> -- Greed is the root of all eBay. |