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I think I read somewhere it is not a good idea to use 2 regulators in one circuit i.e. say an lm338 to go from 40v to 29v and then an lm350 to go from 29v to 18v...

You can't make blanket statements like that. I have a power supply modification for a homebrew EPROM programmer and I'm generating 3 different voltages using 3 LM317's.

Technically, the LM338 in the circuit I drew for you didn't need to be on the 29V rail. But operating it from the unregulated 40V supply would have doubled its power dissipation from about 30W to 60W. Doing that would have required an even larger heatsink.

A disadvantage, if you could call it that, is that the power zener needs to dissipate power from the current through the LM338. You have to dissipate the power somewhere and I think splitting it across 2 devices makes heat dissipation easier.

 

Well thats a little yucky. Aside from buying the right transformer or using bolton $20 zeners is there a solution with a little less voltage swing? I think I read somewhere it is not a good idea to use 2 regulators in one circuit i.e. say an lm338 to go from 40v to 29v and then an lm350 to go from 29v to 18v... Side note- If needed I have a few giant heatsinks that came from the same device the transformer came from (old Bogen solid state).

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View attachment 306020

Yes, but the bottom of D2 goes to 0V, not to the source.
If you want 29V, then a 33V will do the job.

 

Yes, but the bottom of D2 goes to 0V, not to the source.
If you want 29V, then a 33V will do the job.

What would the zener rating need to be?

 

What would the zener rating need to be?

400mW will do.

 

400mW will do.

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