If you want a more modern regulator, have a look at the LT1084.

Looks like a drop-in replacement. Which may be useful if the LM338s don't function...

 

Another way can be to connect the resistors all in series and short out the part not needed. Then, when the switch is open, as in the last position or between each position, the current limit goes to the minimum setting. And the reg always has all 3 terminals connected, never one floating.
Or, better, just wire the minimum current resistor permanently across the switch, and have the last position open.

 

I have corrected the switch error, and moved the pot/12k closer to the output pin - is that what you meant?

Yes.
Physically they should be connected directly to the output terminal.

 

If you use the switch to short out part of the chain of resistors and use a make before break switch then when switching the current limit the output curren will just change between the two values - it won;t do anything funny during switching.

For example:
https://www.ebay.co.uk/sch/i.html?_..._nkw=make+before+break+rotary+switch&_sacat=0

 

Physically they should be connected directly to the output terminal.

If the circuit seems OK I am now onto the physical placement. It looks as if the enclosure might not even contain a PCB. Just the SMPS, two bulky heatsinks with LM338s, a pot and a rotary switch, which will both have a few components mounted directly onto them. Currently checking out my bits box for 40V+ caps...

 

You can probably buy cheap fake LM338 ICs on ebay or buy real ones at a crazy high price at digikey:

Not sure what 'fake' means in this context.

Lots of items are now made in countries where IP is not strongly enforced. So we might get extra items run off the same production line which is making quality goods which are identical in every respect.

Then we might get output from that line which perhaps failed quality control in some respect. or which used inferior raw materials.

Then we find shadow factories producing to the same design and tooling, but perhaps with lower quality materials and workmanship, and finally factories producing similar external designs with completely different internal electronics.

These are all 'fake' in some respect, but in many cases are adequate for the use to which they will be put. No one would use 'grey' components in a professional avionics installation, but I think that there is a good chance that these LM338s will perform adequately.

I will keep a lookout for 'Chinese Amps' ratings of course!

 

People make videos of cutting open and exposing cheap junk from over there and sold on ebay and other sites from over there.
A few videos show what is inside a cheap Lithium battery, flour for its weight plus a tiny little real battery.
Here is one photo of maybe the LM338 IC:

 

Hello,

Here is a page with the picture of a genuine one:
https://zeptobars.com/en/read/LM338K-5A-LDO-TO-3-TO-220


And this is a picture of a fake one:

Bertus

 

I have an SMPS which puts out 35V at 5A, and it would be nice to have this adjustable. I have a couple of LM338s in my bit box, so I thought that I could just attach these to the SMPS to provide a box which has an adjustable voltage output from a couple of volts up to 30-odd, and also has a current-limiting function such that it can put out lower voltages at fractions of an amp.

There are numerous sample circuits available on the web - though nothing seemed to be quite what I wanted. This usually means that it's either not possible, or that it CAN be done, but this is not really the way to do it! I understand that running at lowered volts and amps means that I have to lose a lot of heat - luckily I have some quite big heatsinks in my box as well.... so I put together a schematic.

There may well be other problems that I am completely ignorant about. I would be grateful if someone could cast an eye over the proposal an tell me whether they think it will work or not before I put soldering iron to pcb...

thanks...

View attachment 229705

You could put a switcher just ahead of the LM338 device and that would reduce power dissipation by a large factor. That is the way a lot of bench power supplies do it.

With 35v input and 5v output at 1 amp, the power wasted is at least 30 watts.
With 35v input and 5v output at 2 amps the power wasted is at least 60 watts.
With 35v input and 5v output at 4 amps the power wasted is at least 120 watts.

So you can see that the power wasted can be enormous and it may be hard to find a heat sink that can do that without water cooling. There are some heat sinks that use heat pipes to help deal with the high power maybe you could look into that. You may have to modify them to fit your package types.
It would be much better to use a switcher just ahead of the linear LM type regulators though. That would greatly reduce the heatsink sizes.

I built a power supply some years ago using a device similar to your choice. It was made for decent current levels but the idea was to use a wall wart that was not too high in voltage at the input and so i could change the wall wart for different applications. For example, i could use a 9vdc wall wart or a 12vdc or a 15vdc or a 24vdc wall wart depending on what i was going to use it for. It was mostly for testing though not for a permanent application.

 

People make videos of cutting open and exposing cheap junk from over there and sold on ebay and other sites from over there.
A few videos show what is inside a cheap Lithium battery, flour for its weight plus a tiny little real battery.
Here is one photo of maybe the LM338 IC:

I once had an ebay USB hub which didn't work. Got the money back. Opened it up - there were no electronics inside at all...

 

You could put a switcher just ahead of the LM338 device and that would reduce power dissipation by a large factor. That is the way a lot of bench power supplies do it.

Hi - what do you mean by a 'switcher'? Another SMPS?

 

Hi - what do you mean by a 'switcher'? Another SMPS?

Hello again,

Yes, that's it. However, the new switcher could also be either manually adjustable (easier to build) or auto adjustable (easier to use). If it is auto adjust, it adjusts itself to just a little higher than the linear regulator. If you dont need super clean outputs you can just use a switcher alone.