Rules of thumb for power transformer current rating - derivation?
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From: Bitrex on 19 Jan 2010 04:02 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. :(
From: Ban on 19 Jan 2010 06:06 "Bitrex" <bitrex(a)earthlink.net> schrieb im Newsbeitrag news:gfmdnWo1Wcbe6MjWnZ2dnUVZ_vKdnZ2d(a)earthlink.com... >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. :( 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
From: John Larkin on 19 Jan 2010 08:00 On Tue, 19 Jan 2010 04:02:34 -0500, Bitrex <bitrex(a)earthlink.net> 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. :( I do have this... ftp://jjlarkin.lmi.net/XfmrScatter.JPG John
From: Phil Allison on 19 Jan 2010 07:59 "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. > 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. > 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
From: MooseFET on 19 Jan 2010 09:44
On Jan 19, 1:02 am, Bitrex <bit...(a)earthlink.net> wrote: > I have this document: > > http://www.audiofaidate.org/it/materiale/20_020_020_001_PowerTransfor... > > 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. :( 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. |