> -----Original Message-----
> From: piclist-bounces@mit.edu On Behalf Of William Chops Westfield
> Sent: Thursday, March 24, 2005 12:58 AM
>
> OK.  So exactly how do transformer ratings work, anyway?  Like many
> people, my first 5V power supply had a 6.3V filament transformer and
> an LM309k, since the 6.3V is RMS voltage and yields 8.8V when
> rectified and filtered, which is plenty for the regulator dropout.
>
> And yet, there's not necessarily a lot of power in the peaks of those
> waveforms.  If a transformer is rated 6.3V at 100mA, just how much
> current can I draw from the filtered output without the voltage drooping
> significantly?

The voltage will only be 6.3 if the current is 100mA. At lower current the
xformer will give a higher voltage. This is specified as xformer load
regulation, it is expressed as a percentage rise in voltage at no load. Load
regulation of excellent xformers can be 10% or less and with very cheap wall
warts it can be 30% or worse. A cheap 6.3V @ 100mA xformer may deliver 6.3V
+ 30%  = 8.2V with no load. The load line is usually very linear so @ 50mA
the voltage will likely be 6.3V + 15%.

Above when you calculated the output as 8.8V you forgot to subtract the
diode losses. On average you can estimate 0.7V drop per diode for a total of
1.4V. So that 8.8V would really be closer to 7.4V. If you check the data
sheets for the rectifiers you use you will find that the Vf will vary
depending on the current. A typical 1N400X 1A diode has a Vf of 0.5V @ 1mA
and 1.1V @ 1A. These values are for a steady state DC current, in an AC/DC
power supply the average Vf will be lower because of the alternating
current, the 1N400X has an average Vf of 0.8V @ 1A.

Next you need to consider line regulation, some high end xformers will
actually reduce the effect of line voltage variations. However most xformers
currently available have zero line regulation so the entire line fluctuation
is reflected at the output. For US 110/120VAC power it's usually safest to
design for line voltage varying 90VAC to 140VAC. The xformer should specify
the line voltage at which the xV @ xA spec is measured.
An example:
Xformer spec - primary 120VAC secondary 12VAC @ 100mA.
This shows a 10:1 reduction @ the rated current. So, with a line voltage of
90VAC you will get 9VAC @100mA and a line of 140VAC will give 14VAC @ 100mA.

I hope you can see that it is easy to have a supply drop out of regulation
or burn up do to the variations in the line voltage and/or load current, if
they are not considered during the design.

> (I wouldn't think that the size of the filter cap would
> matter that much for a continuous load, right?)

The magnitude of the 120Hz ripple will increase with current so you need to
size the capacitor such that the ripple is acceptable at maximum current. If
the capacitor is too small the magnitude of the ripple may cause the
regulator to drop out in the ripple valley.

If you can find a copy of the 1980? National Semiconductor "Voltage
Regulator Handbook" it has excellent information on xformers, rectifiers and
filter caps for designing linear supplies. I searched national.com but, it
is no longer available. IIRC the 1980 "Audio/Radio Handbook" had some of the
same good power supply design information and you can buy a reprint of it at
http://www.audioxpress.com/bksprods/books/bkaa59.htm.

Paul

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