The pre-regulator is a switching regulator (noisy) that holds the source voltage for the regulator at 3-4 volts above the regulated output.
For the record, I hacked a
DC-DC module from China as this was the cheapest solution to me.
Note that the LM2596 comes in both fixed and variable versions, the fixed voltage ones is unusable as a tracking regulator.
I use a transformer that delivers 1A@24VAC on this design, no other transformer has been tested.
And I'll use a current shunt resistor in TO220 package instead of the ten 10ohm resistors suggested, this will save quite some space on the PCB and provide more accuracy. This is the most expensive part on the regulator board so far.
The output stage that Richard (Amspire) designed seems to be usable as-is.
I spent most of the time trying out variations on the design, using parts that I had in-house.
My attempt to use a single 2N2955 pass transistor resulted in oscillation when there was put a load on it.
Current status:
I've made a input board with transformer, rectifier, pre-regulator, op-amp supply and reference voltage. Still some tweaking to do with this to get everything perfect.
The regulator itself is still on breadboard (using BC337/BC327 transistors). I've ordered some 2N2222/2N2907 to compare to the original design.
As far as I can see it seems to be function as intended.
On the regulator board I need to do some filtering of the pre-regulated source voltage as the noise is quite bad from the switcher, and just putting a large cap there is out of the question as the pre-regulator needs to "track" the output voltage. To high difference between input and output voltage will fry R10, and probably a few more parts.
Also, I need to address the issue with the current limit led. And I need to make a adjustable maximum current limit since I need to set this according to the available current.
I intend to post the schematic when I've got everything in place.