Can I use one AND both legs of a transformer's secondaries?

[dlucy67]

I'm building a dual channel power supply as I want to power one device that needs 12V and a second device that needs 6V. I'd like to use just a single transformer to save on cost and enclosure space. I have selected a single transformer that has two secondaries:

  Signal Transformer's 14A-56-16
  http://www.mouser.com/ds/2/643/14A-892426.pdf
  120/240V dual primary and 16/8V dual secondary

I was planning to feed the series-connected secondaries and the resulting 16V @ 3.5 A into a series configuration of two LM338 voltage regulators to produce (from the first LM338) a 12V output (from the transformer + bridge 16V) and the on to a second LM338 to produce 6V (from the fed 12V). However, I was thinking the heat produced by the second LM338 dropping the 12V to 6V would be considerable.

So, would it be better to still tap the transformer's series-connected secondaries for 16V to supply one LM338 and then tap just one of the two 8V secondaries to supply the second LM338 (so it only needed to dump 2V instead of 6V)?

[Simon]

Yes you are correct, tap off the 8V to get your regulated 6V, much less heat and running cost.

[dlucy67]

Like so?

I know I'll need a bridge rectifier and reservoir cap prior each of the LM338's instead of just one set with the series-configuration of LM338's.

[SeanB]

How about using a cheap eBay DC Dc converter board to drop the second side 18VDC or so to around 8V, which will give you enough headroom for the linear regulator. Saves power, will give a higher current output on the 6V side and is cheap ( $2 each) so you can buy a spare one or two, or just order 2 of the buck converter modules with a built in controller and led display, and have a power supply with similar capability but with 3A output capacity over the full range. Slightly more noise on the output, but you can add extra LC filtering on the output to reduce that a little.

Probably cheaper to buy the switchers on fleabay than the LM338 anyway, plus you need a smaller heatsink as well.

[retrolefty]

So, would it be better to still tap the transformer's series-connected secondaries for 16V to supply one LM338 and then tap just one of the two 8V secondaries to supply the second LM338 (so it only needed to dump 2V instead of 6V)?

Yes, that would work well. Just be careful to wire the two secondary winding in series AND with proper phasing because if wrong phasing the two secondary voltages will subtract rather then add.

[retrolefty]

Like so?

I know I'll need a bridge rectifier and reservoir cap prior each of the LM338's instead of just one set with the series-configuration of LM338's.

No the top regulator should wire between the center and lower tap as you want a common circuit ground for your two regulated voltage outputs.

[Simon]

How about using a cheap eBay DC Dc converter board to drop the second side 18VDC or so to around 8V, which will give you enough headroom for the linear regulator. Saves power, will give a higher current output on the 6V side and is cheap ( $2 each) so you can buy a spare one or two, or just order 2 of the buck converter modules with a built in controller and led display, and have a power supply with similar capability but with 3A output capacity over the full range. Slightly more noise on the output, but you can add extra LC filtering on the output to reduce that a little.

Probably cheaper to buy the switchers on fleabay than the LM338 anyway, plus you need a smaller heatsink as well.

If the extra power is not needed why bother ? more heat to deal with. You could do the other way around for that matter. Parallel the secondaries for 6V and use a voltage doubling rectifier for the 12V.

[dlucy67]

No the top regulator should wire between the center and lower tap as you want a common circuit ground for your two regulated voltage outputs.

Ok, so are you saying they should be connected like so? Help me, the newbie, understand the difference. I very much so want a common ground (mostly so as to have a common ground bus on the board).

[retrolefty]

No the top regulator should wire between the center and lower tap as you want a common circuit ground for your two regulated voltage outputs.

Ok, so are you saying they should be connected like so? Help me, the newbie, understand the difference. I very much so want a common ground (mostly so as to have a common ground bus on the board).

 Not any better. Not to try and put you off but seem to be at the stage where you should be using
a proven schematic for you to work from. I don't have any schematic drawing software so I can't help directly but there are tons of circuits on the web and maybe someone here will post you something you can use.
Good luck and keep at it. 

[Benta]

I was planning to feed the series-connected secondaries and the resulting 16V @ 3.5 A into a series configuration of two LM338 voltage regulators to produce (from the first LM338) a 12V output (from the transformer + bridge 16V) and the on to a second LM338 to produce 6V (from the fed 12V).

This is the best idea and the only one that will work.
The LM338 needs at least 3 V between input and output, and you'll not reach that at 8 VAC (think storage cap voltage valleys).
On the other hand, it's no problem at 16 VAC.

There are more elegant solutions, but not with the LM338.

Personally, I'd use two 3 A LDOs (each with bridge and capacitor) and make two independent/floating 6 V supplies. You can then configure the outputs as you like.

[Simon]

yes low dropout regulators are a v ery good idea.