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Date: Wed, 20 May 2009 15:07:57 -0700
From: hilpert at cs.ubc.ca
To: General at invalid.domain
Subject: Re: S-100 power supply transformer
A slightly different take on this transformer requirements issue..
Hammond used to (perhaps they still do) provide a piece of paper with every
transformer, listing part numbers and specs. Included on the paper was a table
for voltage and current factors for basic rectifier/filter circuits: (extract)
Rectifier Filter Voltage Current VA
------------------- ------ ------------------ ---------------- -----------------
Half-wave cap in DCV = ACV * 1.16 DCA = ACA * 0.38 DCVA = ACVA*0.44
Full-wave center tap cap in DCV = ACV * 1.25/2 DCA = ACA * 1 DCVA = ACVA*0.63
Full-wave bridge cap in DCV = ACV * 1.25 DCA = ACA * 0.56 DCVA = ACVA*0.7
AC values are RMS; VA figures I added in.
They don't say precisely how these figures were arrived at, but reality is a
little different than theory (e.g. 1.25 vs 1.414 for the voltage output from
the filter, don't know whether that's reduced due to R losses or an average
with ripple under load).
I take it these values were provided as guidelines, as complete design takes
into account C size, tolerable ripple, etc.
--
TI published "The Voltage Regulator Handbook" in the 70's, which covers a
lot
of design issues for linear power supplies. Even it references a design
procedure from 1943 (Schade graphical techniques) for the rectifier/filter design.
What voltages were they talking about for these types
of numbers?
The 1.414 number is the best case. allowing some for the diode
and as you save, expected ripple.
Dwight
They came in the box with Hammond's low-voltage transformers (3 to 100VAC):
165,166,167 series.
It may be that there is some specificity to Hammond's design characteristics. I
mention them as an example of real-world relationships, and that there is some
derating of the VA capability of the transformer when used in these circuits.