🚀 go-pugleaf

In article <484d2532@dnews.tpgi.com.au>, F Murtz wrote:

>I would like to make an adjustable power supply using a transformer with 
>a 24v 5a single wound secondary I would like the voltage to be 
>adjustable from lowest to highest possible given transformer I would 
>like to have regulated current
>
>There is another possibility I also have a larger transformer with two 
>32v 4.7a secondaries If anyone has a schematic for an adjustable power 
>supply with adjustable current regulation making the most of the 
>transformer specs I would be eternally grateful

  One thing to watch for:  RMS current being drawn from the transformer 
exceeding the DC output current.  The bigger the filter capacitor, the 
worse this gets.

  Assuming that you will be using an adjustable voltage regulator, you 
should be OK with a few volts of filter capacitor ripple, so don't make 
the filter capacitors too big.

  I would not try for more than 3-4 amps of DC from a 5 amp transformer 
unless the rating is for achievable DC with a "capacitor input" 
rectifier/filter circuit (capacitor immediately following the rectifier, 
as opposed to having a choke between the rectifier and the capacitor).

  24 volts times sqr(2) is 33.9 volts.  Allow loss of a couple volts in 
a bridge rectifier, a couple volts minimum drop in a voltage regulator 
that is doing its job well, a couple more for a current regulator, and 
that is about 28 volts.  Allow 4 volts ripple on a moderate filter 
capacitor, and you get 24 volts.

  As for a filter capacitor value:  4 volts ripple at 4 amps over the 
approx. 6 milliseconds or so that the capacitor has to supply current
works out to 6,000 microfarads.  I would not use more.  Greater filter 
capacitance makes the current waveform drawn from the transformer spikier, 
which makes the RMS current drawn from the transformer greater.  I might 
go for 4700 microfarads and only count on drawing 3-3.5 amps DC without 
overheating the transformer.

  BEWARE - with mild-moderate overloads, transformers this size can take a 
couple hours to reach excessive temperature.

 - Don Klipstein (don@misty.com)