ą5V from up to +12v

[Daniel Pitts]

I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

Thanks,
Daniel.

 

[Tom Biasi]

On 11/4/2013 12:54 PM, Daniel Pitts wrote:

I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

Thanks,
Daniel.

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.

 

[Daniel Pitts]

On 11/4/13 10:20 AM, Tom Biasi wrote:

On 11/4/2013 12:54 PM, Daniel Pitts wrote:
I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

Thanks,
Daniel.


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.

 

[Tom Biasi]

On 11/4/2013 2:35 PM, Daniel Pitts wrote:

On 11/4/13 10:20 AM, Tom Biasi wrote:
On 11/4/2013 12:54 PM, Daniel Pitts wrote:
I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

Thanks,
Daniel.


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.

 

[Jim Thompson]

On Mon, 04 Nov 2013 09:54:20 -0800, Daniel Pitts
<newsgroup.nospam@virtualinfinity.net> wrote:

I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

Thanks,
Daniel.

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 | |
| Voice480)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.

 

[John Fields]

On Mon, 04 Nov 2013 09:54:20 -0800, Daniel Pitts
<newsgroup.nospam@virtualinfinity.net> wrote:

I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

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

 

[Jim Thompson]

On Mon, 04 Nov 2013 14:48:39 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Mon, 04 Nov 2013 09:54:20 -0800, Daniel Pitts
newsgroup.nospam@virtualinfinity.net> wrote:

I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

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 | |
| Voice480)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.

 

[Daniel Pitts]

On 11/4/13 12:16 PM, Jim Thompson wrote:

On Mon, 04 Nov 2013 09:54:20 -0800, Daniel Pitts
newsgroup.nospam@virtualinfinity.net> wrote:

I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

Thanks,
Daniel.


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?
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.

 

[Daniel Pitts]

On 11/4/13 12:56 PM, Jim Thompson wrote:

On Mon, 04 Nov 2013 14:48:39 -0600, John Fields
jfields@austininstruments.com> wrote:

On Mon, 04 Nov 2013 09:54:20 -0800, Daniel Pitts
newsgroup.nospam@virtualinfinity.net> wrote:

I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

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.

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.

 

[Jim Thompson]

On Mon, 04 Nov 2013 13:56:23 -0800, Daniel Pitts
<newsgroup.nospam@virtualinfinity.net> wrote:

On 11/4/13 12:56 PM, Jim Thompson wrote:
On Mon, 04 Nov 2013 14:48:39 -0600, John Fields
jfields@austininstruments.com> wrote:

On Mon, 04 Nov 2013 09:54:20 -0800, Daniel Pitts
newsgroup.nospam@virtualinfinity.net> wrote:

I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

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.

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 | |
| Voice480)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.

 

[Tim Wescott]

On Mon, 04 Nov 2013 11:35:32 -0800, Daniel Pitts wrote:

On 11/4/13 10:20 AM, Tom Biasi wrote:
On 11/4/2013 12:54 PM, Daniel Pitts wrote:
I'm trying to design a remote controlled volume controller, and the
chip I've chosen to control the volume needs Âą5v for the analog side
of the circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference
point, making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an
easier way to do what I'm trying to do? It seems like most Âą5v chips
I've found require a bit more complicated external components, and are
far more expensive.

Thanks,
Daniel.


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.

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

 

[Jim Thompson]

On Mon, 04 Nov 2013 09:54:20 -0800, Daniel Pitts
<newsgroup.nospam@virtualinfinity.net> wrote:

I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

Thanks,
Daniel.

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 | |
| Voice480)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.

 

[Tom Biasi]

On 11/4/2013 6:40 PM, Jim Thompson wrote:

On Mon, 04 Nov 2013 09:54:20 -0800, Daniel Pitts
newsgroup.nospam@virtualinfinity.net> wrote:

I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

Thanks,
Daniel.


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

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

 

[Tim Wescott]

On Mon, 04 Nov 2013 15:18:55 -0700, Jim Thompson wrote:

On Mon, 04 Nov 2013 13:56:23 -0800, Daniel Pitts
newsgroup.nospam@virtualinfinity.net> wrote:

On 11/4/13 12:56 PM, Jim Thompson wrote:
On Mon, 04 Nov 2013 14:48:39 -0600, John Fields
jfields@austininstruments.com> wrote:

On Mon, 04 Nov 2013 09:54:20 -0800, Daniel Pitts
newsgroup.nospam@virtualinfinity.net> wrote:

I'm trying to design a remote controlled volume controller, and the
chip I've chosen to control the volume needs Âą5v for the analog side
of the circuit, and +5v for the digital.

I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with
higher voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference
point, making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an
easier way to do what I'm trying to do? It seems like most Âą5v chips
I've found require a bit more complicated external components, and
are far more expensive.

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.

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 :-(

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

 

[Jasen Betts]

On 2013-11-04, Daniel Pitts <newsgroup.nospam@virtualinfinity.net> wrote:

On 11/4/13 12:16 PM, Jim Thompson wrote:

One idea I had was to chain 7805s together:


+12v connected to VinA & VinB

GndIn connected to GndA

VoutA connected to GndB

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.

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

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 ---

 

[Jasen Betts]

On 2013-11-04, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most Âą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

Thanks,
Daniel.


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

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 ---

 

[George Herold]

On Monday, November 4, 2013 12:54:20 PM UTC-5, Daniel Pitts wrote:

I'm trying to design a remote controlled volume controller, and the chip
I've chosen to control the volume needs ą5v for the analog side of the
circuit, and +5v for the digital.
I have several options for input voltage, mostly based on old AC
adapters I have laying around. In particular I have a "selectable"
adapter which can select up to 12v, but I may have others with higher
voltages (I haven't looked through them recently).

One idea I had was to chain 7805s together:
+12v connected to VinA & VinB

GndIn connected to GndA
VoutA connected to GndB

If I understand correctly, I could then use VoutA as my reference point,
making GndA -5v and VoutB +5v.

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.



Thanks,

Daniel.

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' terminal... in my case it was always a current sink.

Can you use LM317's and LM337's? Then I used the power supply negative lead as -5V, used the 337 to make a 'ground' 5 volts above that, and the 317 for 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.

 

[George Herold]

On Tuesday, November 5, 2013 9:49:28 AM UTC-5, Jim Thompson wrote:

On 5 Nov 2013 07:52:36 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:



On 2013-11-04, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:
big snip... includeing Jasen's circuit

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

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.

...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 | |

| Voice480)460-2350 Fax: Available upon request | Brass Rat |

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I love to cook with wine. Sometimes I even put it in the food.

 

[George Herold]

On Tuesday, November 5, 2013 10:39:31 AM UTC-5, Phil Hobbs wrote:

On 11/05/2013 09:57 AM, George Herold wrote:
On Tuesday, November 5, 2013 9:49:28 AM UTC-5, Jim Thompson wrote:
On 5 Nov 2013 07:52:36 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

On 2013-11-04, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:
big snip... includeing Jasen's circuit

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

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.

Careful with those--they work great, except that the heat sinking is
quite a bit less great. The best package runs 22 K/W junction to
"case", whatever that means in a plastic-encapsulated package with no
power tab. That's quite a bit considering that the chip can dissipate
over 40W before thermal shutdown kicks in. (Steady-state thermal
transfer calculations would put the die at about 900C with an infinite
room-temperature heat sink--a nice bright red-orange glow.)

They run warm to the touch just with their quiescent current.

Grin, Hey, the TCA0372's are burning a hole in my parts bin :^)
(maybe the OP only needs ~100mA)
I must admit when you (or someone else?) pointed out these 1 amp IC's in a dip8 package I was a bit 'concerned'.
(I'd really like a nice to-220 style metal tab.. or a surface mount with 'power pads' on the bottom.)

George H.

As a rail splitter, you have to watch out for the chip hitting thermal
shutdown, which will probably make one rail collapse. A couple of beefy
Zeners on the output would help prevent Joergish noises.



Cheers



Phil Hobbs



--

Dr Philip C D Hobbs

Principal Consultant

ElectroOptical Innovations LLC

Optics, Electro-optics, Photonics, Analog Electronics



160 North State Road #203

Briarcliff Manor NY 10510



hobbs at electrooptical dot net

http://electrooptical.net

 

[Jim Thompson]

On 5 Nov 2013 07:52:36 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

On 2013-11-04, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

If I do this, am I going to let the magic smoke out? Is there an easier
way to do what I'm trying to do? It seems like most ą5v chips I've found
require a bit more complicated external components, and are far more
expensive.

Thanks,
Daniel.


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

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

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 | |
| Voice480)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.