Mauricio;

You want a 280 watt power supply, right?  Well, you are experimenting with a linear power supply.  You are forced to accept the following parameters:

A bridge/resistor/capacitor filter is in reality a peak detector circuit (not an average or RMS circuit).  When there is no load, the capacitor charges to full peak voltage.  You have set that to the target.  When you draw a heavy current, you are not allowing the capacitor to keep the peak voltage on it.  If you look with a scope, you will see a big ripple with an average of 24V
and a peak to peak of probably 8VDC, at obviously 60 Hz (50? where are you?).  You will approach the RMS value.  0.707 times the starting value of 28 is 19.8.

The big filter cap on a linear supply will vary with load and line voltage variation.  You thus have to plan for the worst line and worst load.  Therefore, at no load and high AC line, you will have far too much voltage at the cap.  You must then follow this capacitor with a series pass regulator as you showed in your scanned picture.  Since you have to deliver at least 280W, you
must be prepared to burn off a lot too, a wild guess would be 100W.

An autotransformer is not ideal for this application.  They offer you no isolation whatever, which is dangerous, and are not exactly highly efficient.  You are better with a good old fashioned 'fillament' transformer, designed for the 380 or more watts that you need.

400W power supplies do not an easy project make.  In modern designs, you would definitely approach this design with a well designed off line switcher.  280 watts with 85% eff would waste 49 watts, a lot, but doable.  You would no doubt have to have a custom flyback wound for the project.  Again, not a good candidate for your first power supply.

I suggest you look into the off the shelf 'lab' power supplies which are available.  They are rugged, can withstand abuse, and have agency regulations assigned to them.  If you only need a few power supplies, this is by far the best way to go.

G'luck
Chris Eddy

Mauricio Jancic wrote:

> Hi guys,
>         I have a cuestion for those Analogic electronics fans.
>         I must make a power supply for a proyect I'm currently on, it must be adjusted at 28 v, and it must draw about 10 amps with a 5% stability. I look at some motorola rectifiers desing manual and I ended in a 50 A bridge rectifier, with a 68.000uF GIGA capacitor and a series 2.5 ohm NTC. At the input i put a autotransformer wich can draw 12 A and is rated from 0 to 250 v.
>
>         Ok so far, now, i select 24vac at the input (wich must be approximate 34vdc at the output of the rectifier). In order to mantein a good stability in high currents I know that the relation given by  w*Rl*C >= 40 where w = 2*pi*f  (50Hz), Rl is load resistance wich is 2.8ohm@10A and C is the capacitance.
>
>         So, i asembled the enclosed high current regulator circuit and "feed it" with the above mentioned power supply. Now, when I try to load the circuit the voltage falls to about 24 vdc when it was at 28 vdc with out load.
>
>         Any Ideas??
>
>         Important: The circuit below its been modify for availability reasons, as the LM195 (power transistor) i'm using the TIP142 and as the 2N... i'm using the TIP4X series (PNP OR NPN i don't remember, but is the same as the 2N..)
>
>         Any help will be apreciated, Mauricio Jancic
>
>
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