Is it possible? To Isolate a transformerless topology?

[rs20]

That's what I thought.

I still find a capacitive solution that provides the power required with a degree of real isolation somewhat elusive.

That's because it simply doesn't exist.  A capacitive dropper does not isolate you from the mains because the output of the capacitor is still referenced to ground (i.e. the ground you are standing on). 

Strawman detected! You've just leapt from "any capacitative solution" to "the standard capacitative dropper". Anyone reasonable agrees that a standard capacitative dropper does not provide isolation.

I think the OP's question is; what about the idea of a (hypothetical) topology that did provide galvanic isolation from primary to secondary, using only capacitors. Two interesting/reasonable (albeit very low power/inefficient/expensive) proposals have already been offered:

• Push-pull through pair of Y caps to be rectified on the secondary side (which is safety earthed on the secondary side to discharge the AC current that would come through those Y caps, just like the noise suppression caps in a traditional magnetic converter.)
• Relays/solid state switches that charge up a large cap on the primary side, and then "hand it over" to the secondary side. Fascinating concept; achieving safety ratings on those Relays/solid state switches would be "challenging" though.

I agree that this is probably more academic than practical.

[Brumby]

I think the OP's question is; what about the idea of a (hypothetical) topology that did provide galvanic isolation from primary to secondary....

I believe the Op was actually building a transformerless supply and, in response to concerns posted, wanted to find out if it could be isolated.  A practical solution was the objective.

[ali6x944]

https://drive.google.com/file/d/0B5vW-k7HbsL4d1JZUVpFUVhpVkk/view?usp=sharing
that what i want to isolate

[Zero999]

Either, providing it's properly designed. If the capacitive dropper supply uses low value Y1 capacitors for isolation, with proper clearances on the PCB, then it's no more dangerous than a transformer.

Wrong, capacitors do not provide isolation as already stated.  This should be entirely obvious since the voltage at the output of a capacitive supply is still referenced to ground.

As for the touching the output of such a supply, you are clearly either very trusting or can't see the inherent danger.  Just consider what happens if live and neutral are reversed with the typical resiator/capacitor/rectifier/zener design...suddenly the output is at full mains potential w.r.t ground.

Wrong, Y1 capacitors are connected across the primary and secondary in most switched mode power supplies. They provide more than adequate isolation at 50 to 60Hz.

It's possible to use 1nF Y1 capacitors and a 1MHz oscillator to couple the energy from the primary to secondary side. The 1nF capacitors will have a high impedance at 50Hz (3.18M) and an impedance of just 160Ohm at 1MHz. They're rated to withstand 8kV spikes, designed to fail open circuit and give the same degree of isolation as a bulky transformer.

[ali6x944]

this is the circuit i want to isolate
https://drive.google.com/file/d/0B5vW-k7HbsL4d1JZUVpFUVhpVkk/view?usp=sharing

[ali6x944]

photos of the real deal:
https://drive.google.com/file/d/0B5vW-k7HbsL4QnYwUHdJWWM5YVU/view?usp=sharing
https://drive.google.com/file/d/0B5vW-k7HbsL4eldnV0trMnlQdVk/view?usp=sharing

[Zero999]

Why not simply attach the schematic?

No, that's not isolated at all. What's the load in real life?

[JacobPilsen]

First of all, there must be also some "C3" in neutral wire.
Capacity of C1 or "C3" could not be over 820µF, due to 30mA in ordinary RCD.

[rs20]

First of all, there must be also some "C3" in neutral wire.
Capacity of C1 or "C3" could not be over 820µF, due to 30mA in ordinary RCD.

Is the aim here to not trip an RCD, or to not kill a human? What happens when lightning strikes? Why would an RCD be tripped anyway, when there is no Earth connection in sight?

[Zero999]

First of all, there must be also some "C3" in neutral wire.
Capacity of C1 or "C3" could not be over 820µF, due to 30mA in ordinary RCD.

Don't you mean 820nF?

Anyway, that's not good enough. If this were to be used for isolation, the total capacitance between the primary and secondary should be under 5nF and it needs to be a Y1 rated capacitor or two Y2 capacitors in series.

[JacobPilsen]

Is the aim here to not trip an RCD, or to not kill a human?

Both.
RCD is definined such way.

What happens when lightning strikes?

Basicaly the same as with transformer tested for same voltage (5000V for Y2).

Don't you mean 820nF?

Yes, sorry.
(But it should be 415nF.)

If this were to be used for isolation, the total capacitance between the primary and secondary should be under 5nF and it needs to be a Y1 rated capacitor or two Y2 capacitors in series.

In this case we have only 360µA to use.

[Zero999]

If this were to be used for isolation, the total capacitance between the primary and secondary should be under 5nF and it needs to be a Y1 rated capacitor or two Y2 capacitors in series.

In this case we have only 360µA to use.

Yes, that's the sort of leakage current allowable across double/reinforced insulation. Only tiny Y1 capacitors and high value, high voltage resistors are allowed.

It's possible to get more useful current but not at 50 to 60Hz. The solution to this is to power an oscillator from the capacitive dropper, couple the energy to the secondary side via tiny Y1 capacitors and rectify it with fast Schottky diodes.

[ali6x944]

no you misunderstood me, what i was trying to say is i built this circuit and i want to isolate it, how?   

[Zero999]

no you misunderstood me, what i was trying to say is i built this circuit and i want to isolate it, how?

With an isolating transformer.

[rs20]

Isolation isn't just something you tack on as an afterthought. If you decide you want isolation, you should probably throw out your existing design and start from scratch with your requirements.

Having said that, you can just add an isolated DC-DC converter, although by the time you've arranged all that you'd be better of just buying a off-the-shelf wall wart.

[mikerj]

Either, providing it's properly designed. If the capacitive dropper supply uses low value Y1 capacitors for isolation, with proper clearances on the PCB, then it's no more dangerous than a transformer.

Wrong, capacitors do not provide isolation as already stated.  This should be entirely obvious since the voltage at the output of a capacitive supply is still referenced to ground.

As for the touching the output of such a supply, you are clearly either very trusting or can't see the inherent danger.  Just consider what happens if live and neutral are reversed with the typical resiator/capacitor/rectifier/zener design...suddenly the output is at full mains potential w.r.t ground.

Wrong, Y1 capacitors are connected across the primary and secondary in most switched mode power supplies. They provide more than adequate isolation at 50 to 60Hz.

The presence of Y1 caps across the primary and secondary creates a direct path from the AC main inlet to the DC output, ergo it's no longer truly isolated, albeit the maximum leakage current should be safe.  However, since the leakage current is limited to a very small value, it's not actually going to be very useful for powering anything (micropower circuits maybe).  Up the capacitor value to get more current and we are back to where we started.

Strawman detected! You've just leapt from "any capacitative solution" to "the standard capacitative dropper". Anyone reasonable agrees that a standard capacitative dropper does not provide isolation.

I think the OP's question is; what about the idea of a (hypothetical) topology that did provide galvanic isolation from primary to secondary, using only capacitors. Two interesting/reasonable (albeit very low power/inefficient/expensive) proposals have already been offered:

• Push-pull through pair of Y caps to be rectified on the secondary side (which is safety earthed on the secondary side to discharge the AC current that would come through those Y caps, just like the noise suppression caps in a traditional magnetic converter.)

Still not isolated, but now you've added a ground connection to guarantee the output is referenced to ground.  That will get rid of those pesky leakage currents, but what would happen if you tried to e.g. float the output on top of another voltage which is ground referenced?  Bang.  If the output was isolated you could do this with impunity (obviously limited by the maximum isolation voltage in the supply).

• Relays/solid state switches that charge up a large cap on the primary side, and then "hand it over" to the secondary side. Fascinating concept; achieving safety ratings on those Relays/solid state switches would be "challenging" though.

Congratulations, you just (re)invented a flying capacitor converter   There are COTS off-line switcher ICs that do just this, but you are then at the mercy of a semiconductor device between you and 110/230VAC.  Can you absolutely guarantee that device will fail in a safe state?

I'm honestly quite surprised at how many people don't understand what galvanic isolation is.  As a simple thought experiment, take the output of your proposed supply and connect one or the other output terminals to ground referenced voltage (e.g. 240v live terminal).  If it's galvanically isolated then nothing exciting will happen and no significant additional current will flow.

[Jeroen3]

Any topic regarding galvanic isolation leads to a debate here. Maybe it needs a Fundamentals Friday?

[Zero999]

Either, providing it's properly designed. If the capacitive dropper supply uses low value Y1 capacitors for isolation, with proper clearances on the PCB, then it's no more dangerous than a transformer.

Wrong, capacitors do not provide isolation as already stated.  This should be entirely obvious since the voltage at the output of a capacitive supply is still referenced to ground.

As for the touching the output of such a supply, you are clearly either very trusting or can't see the inherent danger.  Just consider what happens if live and neutral are reversed with the typical resiator/capacitor/rectifier/zener design...suddenly the output is at full mains potential w.r.t ground.

Wrong, Y1 capacitors are connected across the primary and secondary in most switched mode power supplies. They provide more than adequate isolation at 50 to 60Hz.

The presence of Y1 caps across the primary and secondary creates a direct path from the AC main inlet to the DC output, ergo it's no longer truly isolated, albeit the maximum leakage current should be safe.

The situation is no different to a big ass isolation transformer with a large inter-winding capacitance.

  However, since the leakage current is limited to a very small value, it's not actually going to be very useful for powering anything (micropower circuits maybe).  Up the capacitor value to get more current and we are back to where we started.

Use a high frequency oscillator to couple the energy across the capacitive isolation barrier.

A transformerless psu followed buy a small cheap isolated dcdc works well but is of cause not transformerless any more. But still safer than a cheap mains to DC transformer module. Also, you can often run most things off the non isolated side and only use the dcdc for external interfaces that need power. That way you can use a lower power dcdc.

Be careful, some of those isolated DC:DC converters only have functional insulation between the primary and secondary and are not to be relied upon for protection against shock.

[ali6x944]

could i not use the bleeder resister? and include a 3A fuse and an MOV?

[Zero999]

could i not use the bleeder resister? and include a 3A fuse and an MOV?

Yes you could but a bleeder resistor, fuse and MOV won't provide isolation or any protection against shock.

What is the power supply for? Why not just buy a proper isolated power supply?

[Brumby]

I'm tending to agree with that.

It's like - "I've built a car and want to know how to make it safe - but I don't want to use seatbelts."  After all the discussions, debates, alternatives, pros and cons have been thrashed out - it would seem the simplest - and most straightforward - solution is to put in the seatbelts.

So, for this case, use a transformer.  If size is an issue, go for a switchmode solution.


(But I do like the idea floated earlier about Dave doing a blog about this subject.)

[ali6x944]

yes i liked it too

[ali6x944]

but i can not simply buy and use a proper isolation transformer for this project because space constraint, but i was going to use a switching solution  a -mc34063- because it is the simplest to use and i have a lot of it, but due to over voltage problems i could not use it.
so i searched in my junk box and i found this UC1846-EP and i was wondering if i could use it instead ...
any way thanks a lot everybody 

[Jeroen3]

What are the size contraints?
You can get a 50 Watt power supply in a real small box today. Apple's macbook adapter is 45Watts and is really small. And it has several transformers!

Do you want to create a 50 Watt power supply on a SD card?

[ali6x944]

they are 5.5cm length , 5cm width, and 2.2cm in height.
if we were talking about line transformers i can't use them, but for switching no problem to have them.
for the power out 45 watts @ 15v & 3A.