[sdiy] Multiple LM317s - serial or parallel?

[Justin Owen]

>> I have an LM317 in a TO-220 package outputting 12V DC from a 15V DC supply. 
>> I have a second LM317 in a TO-92 package outputting 5V DC and a third outputting 3V.

> Just out of curiosity, what are you running with the 3V regulator, and why can't you just run it on 5V?  Ditto with 12V,
> why can't you just run it on 15V?  It seems like an awful lot of trouble to have four separate voltage sources in a circuit!

Hey David,

The 15V is a wall wart. One of the design constraints for this project was that it needed to run off just about any 15V DC 300-500mA wart. That means my supply - straight from the wart - could be anything from approx. 14V upwards. Adding the 12V Reg makes sure I have a nice, stable supply to the board - at exactly 12V. Jay was right about the voltage drop - but this first VReg is LDO.

The 5V is for a 6N138 optocoupler for a MIDI interface - there's a few other things on the 'to do' list that also want 5V.

The 3V is for a TLV5618 DAC and I've added a fourth 1.25V VReg for the DAC VRef.

At 3V VCC and 1.25 VRef on the DAC I get an output of exactly 0-2.5V (which is a CV source). Running the DAC from 5V and using an op-amp buffer to supply the VRef worked-ish - but not as well. The current set up is rock solid (so far...).

At some point the micro that is driving the DAC and reading yhe MIDI input is also going to need a supply that isn't my USB port. You're right - I'm going to  lot of trouble, it's a pretty ambitious/important project.

Cheers,

Justin