Well I did not design the circuit or the board.

Good for you! Though I've got some sympathy for the guy who did: it's all too easy to design something to work under particular circumstances, even if it's really meant to be more versatile, and then someone tries to run it at the end of its range, and it's "Gotcha" time.

Could anyone show me a schematic of a stupid-simple switching regulator circuit?

The LT1076 circuit that Ifixit posted is pretty typical of switching converters, and my guess is that little $25 module that you found has something similar inside it, with the two resistors replaced by a pot.

http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,C1033,P1007

You could build your own equivalent of that, and if you're making 500 of them, the cost shouldn't be too painful. As I said before, you could customize its mounting to fit your existing layout.

 

I would make a 3 pin converter that fit the pinout of the 338. The daughterboard type. Just like the dimensions engineering product with the proper pinout. You may also consider contacting them to see if they will do a 500pc run with the pinout you need. You may also get volume pricing....maybe.

 

Good for you! Though I've got some sympathy for the guy who did: it's all too easy to design something to work under particular circumstances, even if it's really meant to be more versatile, and then someone tries to run it at the end of its range, and it's "Gotcha" time.

Could anyone show me a schematic of a stupid-simple switching regulator circuit?

The LT1076 circuit that Ifixit posted is pretty typical of switching converters, and my guess is that little $25 module that you found has something similar inside it, with the two resistors replaced by a pot.

http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,C1033,P1007

You could build your own equivalent of that, and if you're making 500 of them, the cost shouldn't be too painful. As I said before, you could customize its mounting to fit your existing layout.

That looks as simple as it gets. They claim 80-90% convertion efficiency on the datasheet, if it were 80% (which might be a bit optimistic) at 7V/1.5A (10.5W) the 24V will pull around 0.55A and dissipate around 2W (a far cry from the LM317's 25.5W).

Retched, the LM338 has the same problem, 26W is a lot of power in a small space. I'm assuming 7V at 1.5A.

 

I took IFixit's schematic and plugged it back into LTSpice. Made a few "tweaks" to improve the efficiency a bit.

How's 87.4% efficient at a 1.5A load sound?

Note that if you need more than 1.5A, I suggest going to an LT1074 regulator instead. That will allow you to use a smaller inductor, too.

 

Retched, the LM338 has the same problem, 26W is a lot of power in a small space. I'm assuming 7V at 1.5A. Retched, the LM338 has the same problem, 26W is a lot of power in a small space. I'm assuming 7V at 1.5A.

I was suggesting using the switcher daughter board that fits in the existing 338 holes.