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.
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.
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 ?
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.
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.
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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).
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.
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.
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