Galvanic isolation without transformers ??

On a sunny day (Thu, 27 Nov 2014 12:15:53 -0500) it happened Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote in
<Jrydnfl1m-vUwerJnZ2dnUU7-cudnZ2d@supernews.com>:

>On 11/27/2014 01:52 AM, Phil Allison wrote:
>> daku...@gmail.com wrote:
>>>
>>> Could some electronics guru please help ? Maybe this is a very
>>> silly question, so please pardon me. A voltage transformer is used
>>> to provide galvanic isolation, in a power supply, amongst other
>>> features. Is there another way to achieve galvanic isolation ?
>>
>>
>> ** I guess you could say 93 million miles of space provides us with
>> galvanic isolation from the sun - though allowing plenty of EM
>> radiation to arrive and do good things.
>
>Actually both the Sun and the Earth are immersed in a conducting medium 
>(the solar wind) so their relative potential is very nearly zero. 
>(Feynman has a lecture on that.)
>
>The atmosphere is usually a pretty decent insulator, of course, but it's 
>the last few miles that do the isolating, not the first 93 million.
>
>Cheers
>
>Phil Hobbs

Has anybody mentioned piezo electric transformers yet?
 http://en.wikipedia.org/wiki/Piezoelectricity
can be made very small.

On Wed, 26 Nov 2014 21:21:58 -0800 (PST), dakupoto@gmail.com wrote:

>Could some electronics guru please help ?
>Maybe this is a very silly question, so 
>please pardon me.
>A voltage transformer is used to provide 
>galvanic isolation, in a power supply, 
>amongst other features. Is there another
>way to achieve galvanic isolation ? Thanks
>in advance.

---
It's entirely possible to isolate a load from the mains using the
dielectric of a couple of capacitors as the isolating medium, but
whether it's practical is another matter.

Do you have an application in mind or is your interest merely
academic?

On Wednesday, November 26, 2014 9:22:06 PM UTC-8, daku...@gmail.com wrote:

> A voltage transformer is used to provide 
> galvanic isolation, in a power supply, 
> amongst other features. Is there another
> way to achieve galvanic isolation ?

Powered lamp, air gap, photovoltaic "solar panel"
Motor, belt/hydraulic pump/nonconductive shaft, generator
Buzzer, ceramic rod, contact-microphone-and-rectifiers
Hydrolysis apparatus, low-pressure gas tubing, fuel cell
Heat pump, circulating air or fluid, thermopile
Battery factory...

In article <Jrydnfl1m-vUwerJnZ2dnUU7-cudnZ2d@supernews.com>, 
pcdhSpamMeSenseless@electrooptical.net says...
> 
> On 11/27/2014 01:52 AM, Phil Allison wrote:
> > daku...@gmail.com wrote:
> >>
> >> Could some electronics guru please help ? Maybe this is a very
> >> silly question, so please pardon me. A voltage transformer is used
> >> to provide galvanic isolation, in a power supply, amongst other
> >> features. Is there another way to achieve galvanic isolation ?
> >
> >
> > ** I guess you could say 93 million miles of space provides us with
> > galvanic isolation from the sun - though allowing plenty of EM
> > radiation to arrive and do good things.
> 
> Actually both the Sun and the Earth are immersed in a conducting medium 
> (the solar wind) so their relative potential is very nearly zero. 
> (Feynman has a lecture on that.)
> 
> The atmosphere is usually a pretty decent insulator, of course, but it's 
> the last few miles that do the isolating, not the first 93 million.
> 
> Cheers
> 
> Phil Hobbs

What, No Aether ?

Jamie

Maynard A. Philbrook Jr. wrote:
> In article<71bb8bf7-c263-4add-8e10-c668a7603645@googlegroups.com>,
> dakupoto@gmail.com says...
>>
>> Could some electronics guru please help ?
>> Maybe this is a very silly question, so
>> please pardon me.
>> A voltage transformer is used to provide
>> galvanic isolation, in a power supply,
>> amongst other features. Is there another
>> way to achieve galvanic isolation ? Thanks
>> in advance.
>
> Photo coupling?
>
> Jamie
>
   Yep; one of many ways.

John S wrote:
> On 11/26/2014 11:21 PM, dakupoto@gmail.com wrote:
>> Could some electronics guru please help ?
>> Maybe this is a very silly question, so
>> please pardon me.
>> A voltage transformer is used to provide
>> galvanic isolation, in a power supply,
>> amongst other features. Is there another
>> way to achieve galvanic isolation ? Thanks
>> in advance.
>>
>
> Is this for power transfer or just for sensing?
   Level of power unstated.

On Thursday, November 27, 2014 12:45:53 PM UTC-5, John Fields wrote:
> On Wed, 26 Nov 2014 21:21:58 -0800 (PST), dakupoto@gmail.com wrote:
> 
> >Could some electronics guru please help ?
> >Maybe this is a very silly question, so 
> >please pardon me.
> >A voltage transformer is used to provide 
> >galvanic isolation, in a power supply, 
> >amongst other features. Is there another
> >way to achieve galvanic isolation ? Thanks
> >in advance.
> 
> ---
> It's entirely possible to isolate a load from the mains using the
> dielectric of a couple of capacitors as the isolating medium, but
> whether it's practical is another matter.
> 
> Do you have an application in mind or is your interest merely
> academic?

Thanks to each of you for your responses. 
My interest is mostly application/work 
related and a little academic. We build, 
amongst other things, power supplies of 
all varieties -- linear, switched mode(
full galvanic isolation)  and a few very 
low current output(20 - 30 mA) reactive 
ones(zero galvanic isolation). Recently,
we were discussing the general issue of
galvanic isolation, and one of the items
discussed was capacitive isolation. Indeed,
it is very much possible to provide galvanic
isolation with capacitors for AC, and in 
fact, capacitive reactance may be used 
to reduce high mains side current(5A) 
on manageable isolated load side, as well.
May be this scheme could be used in some
low power power supplies. Surely this would 
reduce the time needed to wind transformer
coils.

daku...@gmail.com wrote:

> Recently,
> we were discussing the general issue of
> galvanic isolation, and one of the items
> discussed was capacitive isolation. Indeed,
> it is very much possible to provide galvanic
> isolation with capacitors for AC, and in 
> fact, capacitive reactance may be used 
> to reduce high mains side current(5A) 
> on manageable isolated load side, as well.
> May be this scheme could be used in some
> low power power supplies. Surely this would 
> reduce the time needed to wind transformer
> coils.


** You are talking complete bollocks. 

Capacitive *galvanic* isolation from the AC supply only works for signals - not power. 

The largest value cap you can safely and legally connect to the AC supply and have a human handle the other end is about 2nF.  Even with an earthed metal enclosure, the largest allowed value is about 5nF.

There is a very simple reason PSUs all use magnetics to achieve isolation. 



...  Phil 

On Thu, 27 Nov 2014 20:57:10 -0800 (PST), dakupoto@gmail.com wrote:

> Recently,
>we were discussing the general issue of
>galvanic isolation, and one of the items
>discussed was capacitive isolation. Indeed,
>it is very much possible to provide galvanic
>isolation with capacitors for AC, 

Sure you can use capacitive coupling, especially with balanced feed
and using protocols, not containing a DC component (Such as Manchester
coding).

>and in 
>fact, capacitive reactance may be used 
>to reduce high mains side current(5A) 
>on manageable isolated load side, as well.

If you are going to measure live line currents, what is wrong with a
traditional current transformer ?

To do current measurements, you would need a shunt resistor  and then
couple the difference voltage  and drop down to some "safe" levels  by
capacitors will cause a lot more headache compared to a current
transformer. With a current transformer, you get all this for "free"
even without a significant voltage drop.
 

On Thu, 27 Nov 2014 20:57:10 -0800 (PST), dakupoto@gmail.com wrote:

>On Thursday, November 27, 2014 12:45:53 PM UTC-5, John Fields wrote:
>> On Wed, 26 Nov 2014 21:21:58 -0800 (PST), dakupoto@gmail.com wrote:
>> 
>> >Could some electronics guru please help ?
>> >Maybe this is a very silly question, so 
>> >please pardon me.
>> >A voltage transformer is used to provide 
>> >galvanic isolation, in a power supply, 
>> >amongst other features. Is there another
>> >way to achieve galvanic isolation ? Thanks
>> >in advance.
>> 
>> ---
>> It's entirely possible to isolate a load from the mains using the
>> dielectric of a couple of capacitors as the isolating medium, but
>> whether it's practical is another matter.
>> 
>> Do you have an application in mind or is your interest merely
>> academic?
>
>Thanks to each of you for your responses. 
>My interest is mostly application/work 
>related and a little academic. We build, 
>amongst other things, power supplies of 
>all varieties -- linear, switched mode(
>full galvanic isolation)  and a few very 
>low current output(20 - 30 mA) reactive 
>ones(zero galvanic isolation). Recently,
>we were discussing the general issue of
>galvanic isolation, and one of the items
>discussed was capacitive isolation. Indeed,
>it is very much possible to provide galvanic
>isolation with capacitors for AC, and in 
>fact, capacitive reactance may be used 
>to reduce high mains side current(5A) 
>on manageable isolated load side, as well.
>May be this scheme could be used in some
>low power power supplies. Surely this would 
>reduce the time needed to wind transformer
>coils.

---
While capacitors are attractive for use as series voltage droppers
because of their contribution to the circuit's impedance being
lossless, the problem which quickly rears its ugly head is that for
substantial current into a load the capacitive reactance must fall
to a low value. 

That means a large capacitance must be used to feed a moderately
high-powered load, which quickly gets to be a more expensive
proposition than using a transformer.

For example, consider 120 volt 60Hz mains being used to drive a
12V/12mA load through two capacitors for "perfect" isolation from
both sides of the mains:

            C1
120L>-----[CAP]----+
                   |R1
                [1000R]
            C2     |
120N>-----[CAP]----+ 

        FIG 1.

In order to make life easier, though, we can combine C1 and C2 -for
now - and have our circuit look like this:

      E1             E2
     /      C1      /
120L>-----[CAP]----+
                   |R1
                [1000R]
       <--I1-->    |
120N>--------------+ 

        FIG 2.

Now, since the current in a series circuit is everywhere the same in
that circuit and we want the 12 mA through R1 to drop 12 volts out
of the 120 available from the mains, we can find the value of the
capacitor required to drop the remaining 108 volts, with 12 mA
through it, by first considering the impedance, Z, of the circuit,
where:

          E1      120V
     Z = ---- = -------- = 10000 ohms
          I1     0.012A

and, since E1 and E2 aren't in phase and are driven by sinusoidal
mains by considering that:

     Z&#4294967295; = R1&#4294967295; + Xc&#4294967295;.

Next, since we know Z and we know R we can rearrange to get Xc&#4294967295;:

     Xc&#4294967295; = Z&#4294967295; - R1&#4294967295;

         = 100e6 - 1e6

         = 99e6 ohms

Then, since Xc is the square root of that,

     Xc = sqrt 99e6R ~ 9950 ohms.


and, since:

             1
    Xc = ---------
          2pi f C

we can rearrange to solve for the capacitance:

             1
    C = ---------- 
         2pi f Xc

                  1
      = ---------------------     
         6.28 * 60Hz * 9950R

      ~267nF 

Finally, to return to the configuration in figure 1, the capacitors
are equal-valued and in series, so the value of each must be twice
the value of the single cap shown in figure 2, or 534nF each.

That's about half a microfarad each for a 12mA load, so if the load
goes to 120mA the caps go to about 5.5&#4294967295;F each, and to about 55&#4294967295;F
each for a 1200mA load.

Because of that and because of the safety issues surrounding using
caps for off-line voltage droppers, transformers start looking
better than caps somewhere between three and ten VA, I'd guess, with
anything notmuch higher than that being no contest.    

 
John Fields