±5V from up to +12v

It won't work like that. The 7805 needs the ground connected to the minus with respect to the positive input. You may be able to rig up something with a 7805 and a 7905.

The VoutA would be -7 with respect to the +12v, so the VinB would be at a at the right place respective the +12v.

Hook it up and get back to us.

You need to create a "splitter" which will force a ground node and absorb the current imbalance between the +5V and -5V outputs.

Do you have knowledge of the operating currents for each portion? ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

WW1 +-----+ |~ +5|------>+5 | | |~ 0V|--+ +-----+ | +--->GND +-----+ | |~ +5|--+ | | |~ 0V|------>-5 +-----+ WW2

Huh? Didn't Daniel imply he had a single +12VDC adapter as input? Currents have to balance. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

I could probably calculate it, but I don't yet have that, no.

If I don't have the appropriate "splitter", what will happen? Will I be drawing more current across the regulators, or will they just not regulate reliably?

Is there a better approach than this? One or more DC to DC converters perhaps? I'm open to alternative suggestions as well.

I was planning on using only one +12VDC adapter.

Here's the diagram of what I was thinking:

7805(A) +------+ | +5|------>+5 +--|Vin | WW | | 0v|--+ +-----+ | +------+ | |~ +12|--+ 7805(B) +--->GND | | | +------+ | |~ 0v| | | +5|--+ +---|-+ +--|Vin | | | 0v|-+---->-5 | +------+ | | | +----------------+

So, looking at this, I think I saw your earlier point about needing the splitter...

Well, I think I do, but every time I try to write it out, I can't figure out how to phrase it.

Example of problem... load on upper +5V is greater than load on lower

+5V, so current tries to flow _into_ lower +5V, but can't :-( ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

What's going to upset the "bottom" 7805, if it happens, is the "top" 7805 dumping more current onto its output than is being consumed. If that happens then it'll fall out of regulation (it only supplies current, it does not sink it).

The 7805/7905 should sorta-kinda work, maybe -- I'd give it a higher chance of success than the stacked 7805 chips.

You could load the "bottom" 7805 with a resistor, to make sure that it's always delivering current.

Or you could womp up a regulator from op-amps, particularly if the current draw is light.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Turns out, +12V input is insufficient, headroom-wise, to power a back-to-back combination of 7805/7905.

Rolling your own would still have headroom issues unless you use MOSFET's as the pass devices. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Yes, I thought of that but the OP said he had higher voltages available.

b +12 in -+--------[7805]-- +5 | | | a | `---[7805]-+--- 0V | | gnd in -------+-------- -5 that will work, but only if the -5 half has a larger load than the +5 half, you might need to add something there (a resistor?) to make sure.

if you have at-least one of the DC-DC convertsers with an isolated output you can stack their outputs

--
For a good time: install ntp 

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

A big enough load resistor from GND to -5V would fix that, at the expense of always burning up some power.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com

What advantages would that give over the stacked 7805 topology, It seems to me you'd have twice the hassle keeping the 0V where you want it than you have with the stacked 7805 topology,

With two 7805s where you can do something like this.

---------+-----+----------[7805]---------- +5 ^ | | | | |e [100] | | _\| | | Q1 |---+-[7805]---+-+---+----+---- 0V + /| | | | | 12V |c | [1K] | | - | | | |/ | | | +---| | | | | | |\| |/ | | | |/ e+--| | `--[1K]--------| | |\| | | |\| [1K] e| v | e| | | -------------------+------+-----+----+----- -5 Q2 Q3 Q4 Q1 eg: TIP42 Q2,Q3 eg: BC547, PN2222, or 2N2903 etc Q4 eg: TIP41

--
For a good time: install ntp 

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

Possibly. I'd probably opt for a TL431 to provide the load balance... your approach smacks of instability. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

d

As others have said you've got issues with the current flow. I did something similar, splitting 48V to +30, GND, -5. But I still needed to know the direction of current flow into the 'Gnd' ter minal... in my case it was always a current sink.

Can you use LM317's and LM337's? Then I used the power supply negative lea d as -5V, used the 337 to make a 'ground' 5 volts above that, and the 317 f or the +30. You may be able to do something similar.. or if bi-directional current in the 'ground' then some sort of load resistor to the appropriate rail... (as others suggested.)

George H.

Or make the 12 V into 10 Volts, and then a power opamp as rail splitter. (I've got these TCA0372's in my parts box waiting for a project.) I guess it depends on what sorts of currents are involved.

George H.

On Mon, 04 Nov 2013 13:56:39 -0700, Jim Thompson wrote:

--- He said he had a bunch of wall-warts lying around, so my suggestion was to use two wall-warts to get plus 5 and minus 5V.

Current balance doesn't seem to be important:

Version 4 SHEET 1 880 936 WIRE -2000 -16 -2144 -16 WIRE -1632 -16 -1808 -16 WIRE -1440 -16 -1632 -16 WIRE -1632 16 -1632 -16 WIRE -1440 16 -1440 -16 WIRE -2000 112 -2000 -16 WIRE -1808 112 -1808 -16 WIRE -1632 176 -1632 80 WIRE -1632 176 -1760 176 WIRE -1440 176 -1440 80 WIRE -1344 176 -1440 176 WIRE -1232 176 -1344 176 WIRE -1088 176 -1232 176 WIRE -1024 176 -1088 176 WIRE -1632 224 -1632 176 WIRE -1568 224 -1632 224 WIRE -1440 224 -1440 176 WIRE -1440 224 -1504 224 WIRE -1344 256 -1344 176 WIRE -1232 256 -1232 176 WIRE -1088 256 -1088 176 WIRE -1632 272 -1632 224 WIRE -1440 272 -1440 224 WIRE -2000 368 -2000 192 WIRE -1936 368 -2000 368 WIRE -1808 368 -1808 192 WIRE -1808 368 -1856 368 WIRE -1632 368 -1632 336 WIRE -1632 368 -1808 368 WIRE -1440 368 -1440 336 WIRE -1440 368 -1632 368 WIRE -2144 432 -2144 -16 WIRE -2112 432 -2144 432 WIRE -2000 432 -2000 368 WIRE -2000 432 -2032 432 WIRE -1760 432 -1760 176 WIRE -1408 432 -1760 432 WIRE -1344 432 -1344 336 WIRE -1344 432 -1408 432 WIRE -1232 432 -1232 336 WIRE -1232 432 -1344 432 WIRE -1216 432 -1232 432 WIRE -1184 432 -1216 432 WIRE -1408 464 -1408 432 WIRE -1632 496 -1808 496 WIRE -1440 496 -1632 496 WIRE -1632 528 -1632 496 WIRE -1440 528 -1440 496 WIRE -1216 528 -1216 432 WIRE -1088 528 -1088 336 WIRE -2000 624 -2000 432 WIRE -1808 624 -1808 496 WIRE -1760 688 -1760 432 WIRE -1632 688 -1632 592 WIRE -1632 688 -1760 688 WIRE -1440 688 -1440 592 WIRE -1216 688 -1216 608 WIRE -1216 688 -1440 688 WIRE -1088 688 -1088 608 WIRE -1088 688 -1216 688 WIRE -1024 688 -1088 688 WIRE -1632 736 -1632 688 WIRE -1568 736 -1632 736 WIRE -1440 736 -1440 688 WIRE -1440 736 -1504 736 WIRE -1632 784 -1632 736 WIRE -1440 784 -1440 736 WIRE -2144 880 -2144 432 WIRE -2000 880 -2000 704 WIRE -2000 880 -2144 880 WIRE -1952 880 -2000 880 WIRE -1808 880 -1808 704 WIRE -1808 880 -1872 880 WIRE -1632 880 -1632 848 WIRE -1632 880 -1808 880 WIRE -1440 880 -1440 848 WIRE -1440 880 -1632 880 FLAG -1024 176 OUT+ FLAG -1024 688 OUT- FLAG -1184 432 0V FLAG -1408 464 0 SYMBOL ind2 -2016 96 R0 WINDOW 0 45 47 Left 2 WINDOW 3 45 79 Left 2 SYMATTR InstName L1 SYMATTR Value 50 SYMATTR Type ind SYMATTR SpiceLine Rser=,1 SYMBOL voltage -2016 432 R90 WINDOW 3 24 104 Invisible 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 WINDOW 0 -59 42 VRight 2 SYMATTR Value SINE(0 170 60) SYMATTR InstName V2 SYMBOL ind2 -1792 208 R180 WINDOW 0 42 65 Left 2 WINDOW 3 48 38 Left 2 SYMATTR InstName L2 SYMATTR Value .5 SYMATTR Type ind SYMBOL diode -1456 16 R0 WINDOW 0 -38 34 Left 2 WINDOW 3 -98 -1 Left 2 SYMATTR InstName D3 SYMATTR Value MURS320 SYMBOL diode -1648 272 R0 WINDOW 0 -38 34 Left 2 WINDOW 3 -96 72 Left 2 SYMATTR InstName D2 SYMATTR Value MURS320 SYMBOL diode -1616 80 R180 WINDOW 0 42 34 Left 2 WINDOW 3 24 69 Left 2 SYMATTR InstName D1 SYMATTR Value MURS320 SYMBOL diode -1424 336 R180 WINDOW 0 42 32 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D4 SYMATTR Value MURS320 SYMBOL ind2 -1792 720 R180 WINDOW 0 40 67 Left 2 WINDOW 3 45 40 Left 2 SYMATTR InstName L4 SYMATTR Value .5 SYMATTR Type ind SYMBOL diode -1616 528 M0 WINDOW 0 44 27 Left 2 WINDOW 3 22 -2 Left 2 SYMATTR InstName D7 SYMATTR Value MURS320 SYMBOL diode -1424 784 M0 WINDOW 0 45 32 Left 2 WINDOW 3 28 72 Left 2 SYMATTR InstName D6 SYMATTR Value MURS320 SYMBOL diode -1456 592 M180 WINDOW 0 -41 32 Left 2 WINDOW 3 -97 69 Left 2 SYMATTR InstName D5 SYMATTR Value MURS320 SYMBOL diode -1648 848 M180 WINDOW 0 -43 31 Left 2 WINDOW 3 -99 -6 Left 2 SYMATTR InstName D8 SYMATTR Value MURS320 SYMBOL polcap -1504 208 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 10000µ SYMBOL polcap -1568 720 M90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMATTR Value 10000µ SYMBOL ind2 -2016 608 R0 WINDOW 0 45 47 Left 2 WINDOW 3 45 79 Left 2 SYMATTR InstName L3 SYMATTR Value 50 SYMATTR Type ind SYMATTR SpiceLine Rser=,1 SYMBOL res -1360 240 R0 SYMATTR InstName R1 SYMATTR Value 1000 SYMBOL res -1248 240 R0 SYMATTR InstName R2 SYMATTR Value 10 SYMBOL res -1840 352 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 1g SYMBOL res -1856 864 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 1g SYMBOL res -1232 512 R0 SYMATTR InstName R5 SYMATTR Value 100 SYMBOL res -1104 240 R0 SYMATTR InstName R6 SYMATTR Value 10 SYMBOL res -1104 512 R0 SYMATTR InstName R7 SYMATTR Value 10 TEXT -1392 720 Left 2 !.tran 10 TEXT -1968 72 Left 2 !K1 L1 L2 1 TEXT -1416 752 Left 2 ;JOHN FIELDS TEXT -1416 784 Left 2 ;04 OCT 2013 TEXT -1960 592 Left 2 !K2 L3 L4 1 TEXT -2120 488 Left 2 ;120RMS