Transformer-less power supply question

Could some electronics guru please clarify the following ? All traditional transformer-less power supply designs suffer from the AC mains isolation issue. I was wondering how viable the following solution would be to tackle this issue. In traditional transformer-less power supply designs, the connection to the 'live' is made via a parallel RC combination, where R is very high and C limits the current, but the voltage is unaffected. Then there is a voltage limiting Zener diode and then a rectifier diode and so on. In traditional transformer-less designs, the 'neutral' connection does not have any protection, and the problems arise here. So, what if the neutral connection was passed through a similar parallel RC pair (R is very high) and then grounded, instead of connection to the 'live' sub-circuit ? Would this solve the safety issue ? May I have the opinion of the electronics gurus on this ? Thanks in advance.

Reply to
Daku
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So you would like to split the limiter network into two halves, one in each connection to the mains?

You need to halve the impedance on each branch, to keep the target device current feed as before. This doubles the risk of undesired current into the person touching the device (a double shock).

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Tauno Voipio
Reply to
Tauno Voipio

As you didnt say what circuit you're dealing with, I'm guessing its a basic R,C,D,Z circuit. The usual series RC limits mean i and peak i from live, but as you point out there's often no neutral limiting, which is easily implemented. But:

- peak i limit is usually dangerously high for human contact

- limiting in N and L leaves the output at 1/2 mains v, or thereabouts

- such circuits can be made touchable by keeping i limits low enough, but generally theyre still not permitted in the 1st world, and one needs to take care tha a single failure cant cause a nasty shock.

Ultrasimple RC PSUs could be safely and cheaply implemented for very low current devices, such as wallclocks and unlit LCD display devices such as temperature/RH monitors. So I'm in favour of them being permitted, subject to teh usual safety requirements. I'd suggest permitting a peak possible i output of 0.1mA.

NT

Reply to
NT

Yes it is a basic R,C, D and Z circuit. However, what I had in mind was a parallel RC at the mains outlet, with a very high R (e.g., 50.0 M). That should bring the current down to safe limits and the Zener diode will then reduce the voltage also. What exactly do you mean by 'RC' PSU ? Just R and C ?

Reply to
Daku

"Daku"

** Wrong.

The R and C are usually in series.

The R limits the peak current at switch on and the C sets the steady current.

The C values used are many times greater than permitted where body contact is possible.

... Phil

Reply to
Phil Allison

"Daku"

Yes it is a basic R,C, D and Z circuit. However, what I had in mind was a parallel RC at the mains outlet, with a very high R (e.g., 50.0 M).

That should bring the current down to safe limits

** Madness.

The C will pass a far too much current to be legal.

Does this trolling lunatic think any C is safe in series with the AC supply ?

... Phil

Reply to
Phil Allison

upply

Here in Europe its standard and widespread practice to have capacitors from mains poles to an unearthed appliance case. What current's safe is a question of magnitude.

As Phil pointed out, you need R and C in series. This way

- either one can fail without catastrophic consequences

- C controls mean i, which is energy efficient, R controls peak i, which is necessary for the diodes.

NT

Reply to
NT

Not a good idea - it doubles the possibility of a shock.

Original: AC "hot" ---R--C---D---+--o + | Z There is a path from + through a person | to ground. AC neutral ------------+--o -

Proposed: AC "hot" ---R--C---D---+--o + | There is a path from + through a person Z to ground, and a path from - through a | person to ground. AC neutral -R--C-------+--o -

The proposed circuit is essentially voltage divider, which will raise the voltage at negative terminal above ground.

Ed

Reply to
ehsjr

All except some laser powered fiber systems sitting 100-1000 kV above ground ;-)

At least in Europe, the plug can be inserted either way, so live and neutral are interchangeable.

At least in Europe, the 230 V plug can be inserted either way, into the 230 V supply.

Putting two X-capcitors across the 230 Vac mains will produce a mid point potential of 115 Vac.

Using a bridge rectifier and a 10 V zener diode across the rectifier will produce 110 Vac to neutral on one side and 120 Vac on the other side relative to ground.

The IEC standard defines as "safe" anything below 50 Vac (PELV/SELV), on the other hand GFI's will trip at anything above 30 mA, so this limits the maximum capacitor values and hence maximum power dissipation for any capacitively connected device.

If anyone tries to circumvent these limits, I have no problems reporting these people to the relevant authorities.

Reply to
upsidedown

Ummm.. he said the R is very high- that should be a hint.

The 'R' he's talking about is the bleeder resistor, which is required across the dropping cap to discharge it quickly when the plug is removed. There should also be a much smaller R in series.

Yes. There is nothing inherently unsafe about the arrangement, provided such contact is prevented.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

"Spehro Pefhany" "Phil Allison"

** The OP is an idiot who says any damn thing.
** He was talking about no such thing.

** Which is irrelevant to the topic here.

Fuckwit.

... Phil

Reply to
Phil Allison

The arrangement that I proposed is: Original arrangement:

AC"hot" -- R------D--C--o+ | | | -C- Z | AC neutral-----------------o-

Modification: AC"hot"----R(approx 50M)------D----o+ | | | | - R(0.5M)-C------ Z C(big) | | GND AC(neutral) ---R(approx 50M)--- | | | - R(0.5M)-C------ GND

So the neutral gets grounded through high resistances.

path from + through a person

path from + through a person

and a path from - through a

round.

Reply to
Daku

The issue is when those components fail. It doesn't matter how you connect the resistors because if one fails then there is no isolation. The same thing happens with a transformer BUT the likelihood is lower and there is galvanic isolation(which is really the most important aspect).

There are many ways to "isolate" the mains but transformers offer the most cost effective vs quality.

Your method is more expensive than a single resistor version and as long as long as the neutral is true then does not offer any added benefit. You could add complexity to increase safety such as some type of relay or switch based disconnect.

Read up on

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Opto's would be ideal if they were efficient and could handle large currents. There failure mode would almost surely be an open circuit. Transformers have the added benefit of being able to isolate and transform while having a pretty low chance of shorting. One can design a transformer to almost have no chance of shorting on failure by wrapping the primary and secondary separately(hence the only electrical path would be by using the magnetic core.

Once you get away from transformers and opto's the short circuit failure mode drastically increases. Resistors, caps, and transistors all have a relatively high short circuit failure rate.

Mechanical switches, while having a near zero chance of short circuit failure(assuming ideal driver) are not used since they have no galvanic isolation when on and limited lifetime compared to the other devices.

Although you could, for example, use two relays to charge a cap slowly then discharge it into the load with great galvanic isolation(you would actually need 4 similar to what you have suggested for the neutral), they are not cost effective or useful for large power transfers.

One can replace the relays with solid state switches but had the cost of significantly reduced galvanic isolation(relatively), a significantly higher short circuit failure mode, and generally a higher parts cost. With modern day SMPS it simply isn't a feasible option. This method is essentially a bi-switching cap topology and can be modified slightly to provide a SMPS topology.

Ultimately you'll have to determine the effective cost that your scenario will impart. This includes the hazard cost. If it is unlikely(say 1 in 10^12 or greater) that someone will die or be seriously injured then by a malfunction or improper use then the isolation issue should not be a problem.

If you add a few more parts such as a fuse, crowbar, and overvoltage zener you can significantly decrease the short circuit likelihood for a few pennies more.

If this is a one off then your method should be fine. Even if your method has a 1 in 10^4 chance of killing you it is a rather low probability(about the same as you dyeing in a vehicle accident).

Reply to
Jeffery Tomas

Your schematic is not readable, but it looks like you have 500K in series with the cap. If that is the case, the current would be limited to safe levels by the 500K resistor all by itself. Worst case, figuring 240V mains and a shorted series cap, your body could draw only ~.000336 amps peak if you touched the positive or negative terminal (figuring those terminals at 1/2 mains voltage due to the voltage divider) while you were grounded, and half that at 120V mains. You'll get even less continuous current available at the DC output, with 500K series resistance in each leg.

Ed

Reply to
ehsjr

Several generations of vacuum tube guitar amps were based on only mildly electrocuting the player. Bunches of these schematics are readily available online.

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Many thanks,

Don Lancaster                          voice phone: (928)428-4073
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Reply to
Don Lancaster

Ferinstance -- <

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>

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Many thanks,

Don Lancaster                          voice phone: (928)428-4073
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Reply to
Don Lancaster

What was really cute is that they went to a lot more trouble to protect the first audio stage than they did the player. Some tubes got bent out of shape if their heater to cathode voltage was too high.

An example at <

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Many thanks,

Don Lancaster                          voice phone: (928)428-4073
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Reply to
Don Lancaster

Mmmm.. ~50nF caps from one side of the mains to ground- so a couple of mA, potentially. Probably not made/tested to modern class "Y" specs either.

I see this one had a switch to select the desired level of electrocution:

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Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

Gack! Do not play in bathtub..

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

And you had to love the "all American five" radio for that!

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Even a "fancy" Hallicrafters radio had the problem - damn thing tried to kill my aunt.

Ed

Reply to
ehsjr

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