🚀 go-pugleaf

On Jun 10, 11:21 pm, Bill Bowden <wrongaddr...@att.net> wrote:
> On Jun 9, 5:42 am, F Murtz <hagg...@hotmail.com> 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
>
> Look up the LM338 three terminal regulator. It's rated at 5 amps and
> 35 volts from output to input.

But certainly not at the same time:

http://www.national.com/mpf/LM/LM338.html


> All you need is a couple resistors or a
> pot to set the output voltage. Use a 10 amp bridge rectifier to
> convert the transformer secondary AC to DC and a 20,000uF, or
> thereabouts filter cap. Connect the capacitor to the (+) input of the
> regulator and ground and adjust the resistors for the desired output
> votage. The regulator also has current limiting, but not sure what it
> is. You can get 1.2 volts to maybe 24 volts DC out, but you need a
> large heat sink   when the output is set low and drawing 5 amps.

Go read the datasheet before you believe this last bit.

Earlier, I posted the simple math that suggests that you want about 10
TO-220 packaged transistors to get that much power.


Here's a bit of ascii art for a way to share the power around:

                      NPN     R4
     -----------------   ----/\/\----
    !              Q2 \ /e           !
    !                 ---            !
    !  R2    PNP       !      R3     !
    +-/\/\---   -------+-----/\/\----+
    !       e\ / Q1                  !
    !        ---     U1              !
    !   R1    !      -------         !
  --+--\/\/---+-----! LM317 !----+---+---
                     -------     !
                        !        !
                        +-/\/\---
                        !
                        \
                        /
                        \
                        !
                       GND

The capacitors have been left out because I am too lazy to do them.

When U1 draws current R1 develops a voltage due to Ohms law.  Once
Q1's e-b draw is overcome, R2 sets how much voltage is needed on R1
per mA flowing in Q1.

R3 and R4 repeat the trick.  If you set the resistor values correctly,
you can spread the full load power over the three devices.

To get 10 or so devices, the Q2 R4 circuit becomes several transistors
each with its own emitter resistor.



>
> -Bill