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DIY power supply

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MarkF:
Okay...   Here is a cross between your original two LM338 design and Dave's current limiting case.  There are still a few odds and ends to work out.

   

Pirateguy:
i wasn't actually thinking of using 2 lm338 or an adjustable current limit,
but rather that using a regulator would cap the max current at whatever the
regulator's max capability is.

is this wrong?

i like the idea of having an adjustable current, but that might be a bit over my head at the moment.
i am decent at basic math, but i never learned any kind of shorthand in school or anything, so i still need
to learn to interpret these formulas n stuff.

like when it says '(stuff)morestuff', does that mean i calculate '(stuff)' and then multiply the result by 'morestuff'?
there seems to be a missing symbol there... :P
is there a guide or something out there that happens to focus on specifically those kinda questions?
i bet i can do it if i can just learn to translate it into english...

anyhoo, for now maybe i should build a variable voltage one for now and separate the variable current limit to
a later upgrade project?
in which case, single 338 +bypass transistor +polyswitch?

in that last pic, what is REF02?

MarkF:
You can build the adjustable voltage part of the last circuit (It's just like in the datasheets) and then add the variable current limit later.  The variable current section just monitors the current through a 1 ohm resistors and pulls the LM338 adj pin to gnd, which sets it's output to minimum, when the current reaches the set point.  Identical to Dave's video.  Just watch his explanation and think LM338 where he has the LT3080.

The REF05 is a 5V reference.  You would need the variant that has a 40V input capability.
   www.analog.com/media/en/technical-documentation/data-sheets/REF01_02_03.pdf


--- Quote from: Pirateguy on August 19, 2018, 05:30:51 pm ---i wasn't actually thinking of using 2 lm338 or an adjustable current limit,
but rather that using a regulator would cap the max current at whatever the
regulator's max capability is.

is this wrong?

--- End quote ---

Do not depend on any chip for current limiting.  That's the same as not using any fuses and just waiting for the meltdown.
I would still use a polyfuse on the adjustable supply just like you're doing for the fixed outputs from the ATX.

MarkF:
Here's an initial cut at what the LM338 Adjustable Voltage and Current Limit Power Supply PCB might look like:

   


Updated for AC or DC inputs (not all parts would be populated).

JS:

--- Quote from: MarkF on August 19, 2018, 08:16:23 am ---I forgot all about this:
  You might consider this power supply by Peter Oakes:

   

   

--- End quote ---
  That looks quite unstable, I run a simulation and it behaves awfully, specially with a little bit of capacitance at the load. Adding some resistance at the output makes it better but that screws regulation even further than the already badly placed current sense. Some inductance in parallel with the resistors helps but now it's getting tricky, not as simple as it looks. output wire parasitics might be enough in some cases but relying on that isn't good practice. I think Peter showed this as a first approach, to show a working prototype with a minimalistic approach but it doesn't mean it's a good design. It does work on steady conditions but will overshoot every time it can.

  Something similar would happen with the LM338 example, it might work but once in the real world it will catch a condition when it becomes  :scared: as current limiting doesn't look particularly stable. It's quite easy to make one or the other condition stable but the transition between both gets tricky to get right. I've explained this in a topic, I designed a CC/CV dummy load which simulates stable under a wide range of conditions, I haven't found one where it behaves erratically, overshoots badly or starts oscillating, I did build a prototype for the first attempt and it was ok, the second attempt after some math and more time on the sim looks even more promising but I haven't got around to build it yet. In the meanwhile it showed the possibility of adding constant resistance mode which looks nice so I might add that before finally building and for simple tests I have the first one which does an ok job for general testing.
  Going from there to a power supply adds a few challenges, they are similar but the load isn't designed to reject ripple for instance or accomodate for input voltage variations as it's only looking at the voltage across the pass elements, turning that around swaps all the control configuration bringing new stability issues so is not a straight forward conversion.
  There's a design around here of a dynamic load which seems great, much faster than mine, only current but with a pulsed current on top of the DC, so you can look at the dynamic response of the DUT.

  In all this time looking for a simple design of a lab PS which meets my desires I haven't found a project up to the task. I'm not being too picky about voltage and current ranges as I am with dynamic response under different loads. I expect my lab PSU to behave as it should with capacitive or inductive loads, avoiding overshooting and oscillations which might bring problems to the table later on when I'm testing something and trusting the PSU.
  One thing I'm considering is using two power devices to regulate current and voltage independently, which makes control much easier but that doesn't scale up nicely as two small devices are cheap and easy but once you get into higher powers needing multiple bigger devices it gets expensive and having twice as much as needed doesn't play nice.

JS

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