Rail-splitting a wall wart

So we keep being asked for simple TIAs as incidental parts of our 
consulting work.  They're useful to have around anyway.

It's a pain getting them to work right if we put SMPSes inside, so we're 
kicking around the idea of using a rail splitter on our usual 24V wall 
warts to make +15/-9 or something like that.  We'd probably use a 
TCA0372 and a couple of big caps for the job.

Two-wire warts are isolated, of course, so as long as no bright spark 
decides to try running our box on the same wart as somebody else's, we 
should be fine, I think, but we haven't done it before.

Suggestions?

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Fri, 3 Feb 2023 14:55:32 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>So we keep being asked for simple TIAs as incidental parts of our 
>consulting work.  They're useful to have around anyway.
>
>It's a pain getting them to work right if we put SMPSes inside, so we're 
>kicking around the idea of using a rail splitter on our usual 24V wall 
>warts to make +15/-9 or something like that.  We'd probably use a 
>TCA0372 and a couple of big caps for the job.
>
>Two-wire warts are isolated, of course, so as long as no bright spark 
>decides to try running our box on the same wart as somebody else's, we 
>should be fine, I think, but we haven't done it before.
>
>Suggestions?
>
>Cheers
>
>Phil Hobbs

I like the little CUI dual-output dc/dc converters. Or one of the
LM266x inverting charge pumps. But they might be noisy.

The little 3-terminal switchers will invert too. Some are around one
dollar.

I'd expect that a TIA wouldn't need much current, so a rail splitter
is sensible.

Wouldn't a negative regulator, an LM337 or a 7905, work as a rail
splitter?

fredag den 3. februar 2023 kl. 21.50.16 UTC+1 skrev John Larkin:
> On Fri, 3 Feb 2023 14:55:32 -0500, Phil Hobbs 
> <pcdhSpamM...@electrooptical.net> wrote: 
> 
> >So we keep being asked for simple TIAs as incidental parts of our 
> >consulting work. They're useful to have around anyway. 
> > 
> >It's a pain getting them to work right if we put SMPSes inside, so we're 
> >kicking around the idea of using a rail splitter on our usual 24V wall 
> >warts to make +15/-9 or something like that. We'd probably use a 
> >TCA0372 and a couple of big caps for the job. 
> > 
> >Two-wire warts are isolated, of course, so as long as no bright spark 
> >decides to try running our box on the same wart as somebody else's, we 
> >should be fine, I think, but we haven't done it before. 
> > 
> >Suggestions? 
> > 
> >Cheers 
> > 
> >Phil Hobbs
> I like the little CUI dual-output dc/dc converters. Or one of the 
> LM266x inverting charge pumps. But they might be noisy. 
> 
> The little 3-terminal switchers will invert too. Some are around one 
> dollar. 
> 
> I'd expect that a TIA wouldn't need much current, so a rail splitter 
> is sensible. 
> 
> Wouldn't a negative regulator, an LM337 or a 7905, work as a rail 
> splitter?

Lm317 + LM337

https://www.eevblog.com/forum/beginners/lm317-dual-shunt-regulator/msg4632967/#msg4632967

On 03/02/2023 7:55 pm, Phil Hobbs wrote:
> So we keep being asked for simple TIAs as incidental parts of our 
> consulting work.  They're useful to have around anyway.
> 
> It's a pain getting them to work right if we put SMPSes inside, so we're 
> kicking around the idea of using a rail splitter on our usual 24V wall 
> warts to make +15/-9 or something like that.  We'd probably use a 
> TCA0372 and a couple of big caps for the job.
> 
> Two-wire warts are isolated, of course, so as long as no bright spark 
> decides to try running our box on the same wart as somebody else's, we 
> should be fine, I think, but we haven't done it before.
> 
> Suggestions?
> 
> Cheers
> 
> Phil Hobbs
> 
Wall warts are only isolated at DC! Some have horrible HF noise.

Only you know the kind of users but a lot of folk have trouble getting 
their mind around the idea of the non-positive supply rail not being 
ground. The near standard barrel DC jack with center pin positive 
encourages that notion so you may want to consider using a non-standard 
power connector?

piglet

 erichp...@hotmail.com wrote:
----------------------------------------------
> > 
> > Two-wire warts are isolated, of course, 
>
> Wall warts are only isolated at DC!

**  Being made to meet Class 2 means having galvanic isolation from the AC supply and broad immunity against insulation failure without reliance on safety grounding.


>Some have horrible HF noise. 
> 

**  Indeed, many SMPS warts have a 1000 to 2000pF class Y cap from the hot side to the output.


....    Phil

On Friday, February 3, 2023 at 3:50:16 PM UTC-5, John Larkin wrote:
> On Fri, 3 Feb 2023 14:55:32 -0500, Phil Hobbs 
> <pcdhSpamM...@electrooptical.net> wrote: 
> 
> >So we keep being asked for simple TIAs as incidental parts of our 
> >consulting work. They're useful to have around anyway. 
> > 
> >It's a pain getting them to work right if we put SMPSes inside, so we're 
> >kicking around the idea of using a rail splitter on our usual 24V wall 
> >warts to make +15/-9 or something like that. We'd probably use a 
> >TCA0372 and a couple of big caps for the job. 
> > 
> >Two-wire warts are isolated, of course, so as long as no bright spark 
> >decides to try running our box on the same wart as somebody else's, we 
> >should be fine, I think, but we haven't done it before. 
> > 
> >Suggestions? 
> > 
> >Cheers 
> > 
> >Phil Hobbs
> I like the little CUI dual-output dc/dc converters. Or one of the 
> LM266x inverting charge pumps. But they might be noisy. 
> 
> The little 3-terminal switchers will invert too. Some are around one 
> dollar. 
> 
> I'd expect that a TIA wouldn't need much current, so a rail splitter 
> is sensible. 
> 
> Wouldn't a negative regulator, an LM337 or a 7905, work as a rail 
> splitter?

A regulator typically can't sink current, so while it would hold the voltage down, it won't prevent the voltage from rising. 

-- 

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

Phil Hobbs wrote:
> So we keep being asked for simple TIAs as incidental parts of our
> consulting work.  They're useful to have around anyway.
>
> It's a pain getting them to work right if we put SMPSes inside, so we're
> kicking around the idea of using a rail splitter on our usual 24V wall
> warts to make +15/-9 or something like that.  We'd probably use a
> TCA0372 and a couple of big caps for the job.
>
> Two-wire warts are isolated, of course, so as long as no bright spark
> decides to try running our box on the same wart as somebody else's, we
> should be fine, I think, but we haven't done it before.
>
> Suggestions?

My audio mixer amp uses a BUF634P to split a 12VDC wall wart into ±6VDC: 

<https://crcomp.net/mixeramp/index.php>

Also, the audio mixer amp's notes contain a lot of useful links:

<https://crcomp.net/mixeramp/index.php#Notes>

Danke,

-- 
Don, KB7RPU, https://www.qsl.net/kb7rpu
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.

On Friday, February 3, 2023 at 11:55:48 AM UTC-8, Phil Hobbs wrote:
> So we keep being asked for simple TIAs as incidental parts of our 
> consulting work. They're useful to have around anyway. 
> 
> It's a pain getting them to work right if we put SMPSes inside, so we're 
> kicking around the idea of using a rail splitter on our usual 24V wall 
> warts to make +15/-9 or something like that. We'd probably use a 
> TCA0372 and a couple of big caps for the job. 

You aren't in need of high current, are you?   Two zeners and a bit of R for
bias, and C for zener noise.   Go ahead and waste a watt.   50 mA and 9.1V

On Fri, 3 Feb 2023 14:55:32 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>So we keep being asked for simple TIAs as incidental parts of our 
>consulting work.  They're useful to have around anyway.
>
>It's a pain getting them to work right if we put SMPSes inside, so we're 
>kicking around the idea of using a rail splitter on our usual 24V wall 
>warts to make +15/-9 or something like that.  We'd probably use a 
>TCA0372 and a couple of big caps for the job.
>
>Two-wire warts are isolated, of course, so as long as no bright spark 
>decides to try running our box on the same wart as somebody else's, we 
>should be fine, I think, but we haven't done it before.
>
>Suggestions?
>
>Cheers
>
>Phil Hobbs

This should work:

https://www.dropbox.com/s/1v66augsxfmlx3f/Rail_Splitter_1.jpg?raw=1

On Friday, February 3, 2023 at 2:55:48 PM UTC-5, Phil Hobbs wrote:
> So we keep being asked for simple TIAs as incidental parts of our 
> consulting work. They're useful to have around anyway. 
> 
> It's a pain getting them to work right if we put SMPSes inside, so we're 
> kicking around the idea of using a rail splitter on our usual 24V wall 
> warts to make +15/-9 or something like that. We'd probably use a 
> TCA0372 and a couple of big caps for the job. 

Is that "24V" a regulated 24VDC? Then you should be able to get away with TCA0372 buffering a 5:3 ratio input voltage divider to drive the circuit COM for the 15/9V dual. It will be a strictly ratiometric supply with all the regulation being performed by the wall wart, obviously. You'll want to add common/differential mode filtering on the input to kill the kinds of RF frequencies that will cause rectification DC shifts in your TIA circuit.

If it's 24VAC, then you need to add a bunch of additional stuff, and accounting for transformer regulation and mains tolerance, you won't have enough headroom for even an LDO. You'll have to regulate to +15V and maybe use a switched capacitor inverter for the -9V.

https://www.analog.com/en/parametricsearch/11511#/


> 
> Two-wire warts are isolated, of course, so as long as no bright spark 
> decides to try running our box on the same wart as somebody else's, we 
> should be fine, I think, but we haven't done it before. 

That isolation is a so-called double insulation for safety. It's quite well coupled to a lot of interference on the mains.



> 
> Suggestions? 
> 
> Cheers 
> 
> Phil Hobbs 
> 
> -- 
> Dr Philip C D Hobbs 
> Principal Consultant 
> ElectroOptical Innovations LLC / Hobbs ElectroOptics 
> Optics, Electro-optics, Photonics, Analog Electronics 
> Briarcliff Manor NY 10510 
> 
> http://electrooptical.net 
> http://hobbs-eo.com