Is it possible? To Isolate a transformerless topology?

[ali6x944]

I was working resently on a transformerless supply i metioned in earlyer posts and some said it was not safe to use becaues it is not isolated from the earth refrenced ac mains, so i want to know if i could isolate it somehow?
Help Please

[Brumby]

The way to isolate two sides of any circuit is to have the energy transferred by converting it into a form that allows complete separation.

Transformers do this by converting electrical energy into magnetic energy with a separate circuit that shares the magnetic field which then converts back to electricity.

Opto couplers do something similar using light, but the energy transferred is miniscule and useless for a power supply.

The only other "practical" option that comes to mind is a motor/generator setup where the intermediate form is mechanical energy - but that makes transformers a more attractive choice.

[bitshift]

From what I understand (I could very well be wrong), you can basically isolate circuits in three ways using either:

• Magnetics
• Capacitance
• Light

Light and capacitance are not too good at transferring large amounts of power so they are typically used to isolate things like fast changing digital signals. In these cases, both sides of the isolation barrier have their own power source.

[Brumby]

I would not consider capacitance to be an effective isolation mechanism.  It's inability to handle instantaneous voltage changes without consequences rules that out in my book.

Such a problem does not exist with magnetic, optical or mechanical mechanisms.

[bitshift]

I'm not too certain on the uses of the capacitive isolators but you can buy them http://www.ti.com/lsds/ti/isolation/digital-isolators-overview.page

[rs20]

I would not consider capacitance to be an effective isolation mechanism.  It's inability to handle instantaneous voltage changes without consequences rules that out in my book.

I wouldn't be so black-and-white about it -- any isolation solution will have some capacitance between primary and secondary, and indeed even magnetic isolators have Y-class safety caps bridging the isolation between primary and secondary. Why not actually transfer power over these caps?

Put another way, if the "consequence" of an instantaneous voltage change is that the resulting current gets immediately sunk into the mains earth connection; well then that hardly seems like a serious concern. Like I said, most magnetic isolators have these Y-class caps already.

With a capacitance chosen to not trip the RCD in your house, and arbitrarily high frequencies, you can theoretically transfer an unlimited amount of power. Of course in practice, you'd only be able to get away with very small power levels, but there are plenty of circuits these days that'll happily run on milliwatts, so it might even be an worthwhile solution in those cases.

[Brumby]

The key word there is "digital".

On the basis of high speed digital signals, a low value capacitor can effectively transfer signal information which would have a significant high frequency component while providing negligible transfer of low frequency (eg mains) and DC (leakage) - as is stray capacitance around a transformer.

The sole premise of this being able to claim any sort of isolation is the extremely low amount of energy that is actually able to cross from one side to the other - which is in direct contradiction to the requirements of a power supply of any significant VA capability.

[Jeroen3]

Is it possible to Isolate a transformerless topology?

Yes. With batteries.

[Kalvin]

There are few solutions using piezo-technology, for example:

http://www.izm.fraunhofer.de/en/abteilungen/rf---smart-sensor-systems/key_research_areas/piezoelectric-power-supplies.html

https://translate.google.com/translate?sl=auto&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.izm.fraunhofer.de%2Fde%2Fabteilungen%2Frf---smart-sensor-systems%2FForschungsschwerpunkte%2FPTSMPS%2FESPLIKA.html&edit-text=

I am not sure about the price or availability. Google finds also few other application notes using piezo-technology for galvanic isolation.

[orolo]

You can go all Tesla on your circuit and try EM coupling, via two coupled resonantors. Which is the basic idea behind a crystal radio. This would be a (somewhat) more efficient form of light coupling. I wonder if thermoelectric coupling could be viable in some exotic application.

[Zero999]

Capacitance isolation is fine. I've done it before to make a solid state relay. I built an oscillator with some logic gates and coupled the power to the gates of the MOSFETs via some capacitors and rectifier diodes.

I used the circuit on the right with a lower value of R1 and a 1MHz oscillator for the control signal. Safety wasn't an issue so I didn't need to worry about Y-rated capacitors. Using the old 2N7000 (this was a low power/signal switching application) I could switch the load at over 20kHz.

[Brumby]

I think there is a particular aspect of the design that is being overlooked by some...

Refer to this:
N-mosfet as an external switch for mc34063

hello everybody,
I'm working in a switching transformer less power supply based on the great mc34063 ic.
I want a fare current  capability and a duel output voltage supply that could reach 0v, so i will put it like this:
220/110v ac,60hz-------> -15v -0- +15v @ 3A, because I'm using a switching supply we would have some voltage ripple,i could deal with 100mv pk-pk .
 
after some work i got these measurements -raw rectified dc-:
@ 220v AC, i got 306.6v dc and a max current of around 325mA,
@ 110v AC, i got 152.2v dc or so, and a max current of around 175mA,
so the first problem is the big voltage drop that should be present to run the mc34063 and the second problem is the the output current capability, the third is efficiency related problems. 
oh! by the way, I'm using a capacitor based transformer less design, I already dealt  with inrush current problems, so u don't need to worry about that.
please help!
thanks .

The OP was after a +/-15V split supply at 3A.  He is after power, not signalling.

Unless I've missed something.............

[ali6x944]

it is power, I'm talking about power here

[Brumby]

That's what I thought.

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

[macboy]

That's what I thought.

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

If you only consider that the capacitor are connected from primary to secondary side (for lack of better terms) then you are stuck. The capacitors will transfer energy, but they also AC-couple one side to the other. This means that this solution is only viable if the potential difference between sides is a fixed DC voltage. Any AC potential difference is transferred directly through the capacitor. Big power = big caps = low Z AC coupling.

However, consider a solution where a capacitor is first connected to a DC source on the primary side, then disconnected and connected to a reservoir capacitor on the secondary side. Repeat frequently, and you can transfer a significant amount of energy without having a capacitor directly connected between sides. This is called a charge pump, and while it is commonly used to boost and/or invert voltages (e.g. MAX232 RS-232 line driver uses them to derive +/-10 V rails from a 5 V supply), it could in theory also be used to isolate.

[rs20]

If you only consider that the capacitor are connected from primary to secondary side (for lack of better terms) then you are stuck. The capacitors will transfer energy, but they also AC-couple one side to the other. This means that this solution is only viable if the potential difference between sides is a fixed DC voltage. Any AC potential difference is transferred directly through the capacitor. Big power = big caps = low Z AC coupling.

...which you then just shunt away by shorting the ground of the secondary to mains Earth. Problem solved (provided your primary-to-secondary C's aren't too large.)

[Brumby]

However, consider a solution where a capacitor is first connected to a DC source on the primary side, then disconnected and connected to a reservoir capacitor on the secondary side. Repeat frequently, and you can transfer a significant amount of energy without having a capacitor directly connected between sides. This is called a charge pump, and while it is commonly used to boost and/or invert voltages (e.g. MAX232 RS-232 line driver uses them to derive +/-10 V rails from a 5 V supply), it could in theory also be used to isolate.

I see what you're saying and it sounds quite reasonable.  You could do this mechanically with relays - but that's not a particularly practical solution.  I'd be interested in the effectiveness of the isolation for a solution that is.

[mikerj]

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).  For any significant power transfer you need a transformer to provide isolation (ignoring solutions like motor/generator sets which tend to be impractical).

[Zero999]

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).  For any significant power transfer you need a transformer to provide isolation (ignoring solutions like motor/generator sets which tend to be impractical).

A transformer also connects the secondary to ground, via the inter-winding capacitance.

[Brumby]

Yes, but that's like talking about ants in a conversation on elephants.

[mikerj]

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).  For any significant power transfer you need a transformer to provide isolation (ignoring solutions like motor/generator sets which tend to be impractical).

A transformer also connects the secondary to ground, via the inter-winding capacitance.

Pretty much irrelevant in the case of low frequency power transformers from a safety perspective.  If you were grounded, would you be happy to touch the output of a capacitive dropper type supply?  How about the output of a low voltage transformer?

[Zero999]

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).  For any significant power transfer you need a transformer to provide isolation (ignoring solutions like motor/generator sets which tend to be impractical).

A transformer also connects the secondary to ground, via the inter-winding capacitance.

Pretty much irrelevant in the case of low frequency power transformers from a safety perspective.  If you were grounded, would you be happy to touch the output of a capacitive dropper type supply?  How about the output of a low voltage transformer?

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.

[Psi]

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.

[bson]

I'm not too certain on the uses of the capacitive isolators but you can buy them http://www.ti.com/lsds/ti/isolation/digital-isolators-overview.page

Useful for high-speed isolation where the impedance of an inductance is too high and optics can't be switched fast enough.

[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.