5 volt supply straight from 240v AC mains

There's no question that a wall wart is safer than a non-isolated supply, for some applications. But this vociferous reaction about how dangerous it is - is just not justified.

Many of us learned on vacuum tubes where the plate supply was an order of magnitude more lethal than any mains voltage one might encounter. Ever see the inside of a 100 KW transmitter power supply? Build a Tesla or Induction coil? Rail Gun? Coil Gun? Vacuum tube amp or transmitter? Line regulator? Repair a TV set? Power Factor correction circuit? etc..

When all you need is a small indicator or circuit it makes sense to use a cap to drop voltage - more efficient than a wall wart, takes up less space, less cost, lighter, no waste heat to speak of. The enclosure provides the shock protection.

For tinkering with circuits on a breadboard - or just learning electronics, I'd agree it is too dangerous. But the op mentions it doesn't need to be isolated, so he probably already thought of using a wall wart.

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IMHO, 99% of the time this topic comes up, it falls into the category of "if you have to ask, don't use it". The 1% are mostly sensible reliability and regulatory compliance concerns from people who do know fairly well what they are doing.

Best regards, Spehro Pefhany

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At least in Europe, all tube televisions and many tube radios had a universal (AC/DC) power supply, with a half wave rectifier generating the B+ line about 200-250 V, thus there was only a rectifier between the other mains plug terminal to the B+, while the other mains plug terminal was directly connected to the metallic chassis. Depending on the way the mains plug is inserted into the wall socket, you either have the Neutral in the chassis or the full 220 Vac Live in the chassis. Also the tube heaters were in series and across the Live and Neutral, possibly with a VDR in series to limit the inrush current.

When working with such equipment I have used two main principles, before starting to work, I _always_ checked the mains plug orientation by measuring the metallic chassis voltage. When working with active equipment, I put my left hand in the pocket and only work with my right hand inside the equipment. This avoids the risk of having the current flow through your heart. If you get a muscular cramp in your right hand due to an electric shock, you still have the left hand operational to cut the power.

Regarding low power devices powered by a series capacitor, I would suggest using capacitors intended for mains filters.

Instead of a single capacitor on the live side, put two in series, each connecting one side to the respective mains plug terminal, while the other terminal of each capacitor goes to the load (rectifier etc.). In this configuration, the small signal circuit is floating around 110 Vac. If you accidentally touch the small signal circuit, there is still the other capacitor in series between the mains voltage and you, limiting the current through your body. If you want to limit the worst case current to 30 mA, the normal circuit current consumption must be below 10-15 mA, since in normal operation, there are two capacitors in series.

Paul

This is a good idea because it also provides double protection against a capacitor short failure. Also due to the possibility of such a failure a series, low current fuse is advisable.

Dorian

I agree. I stuck a paper clip into a light switch when I was very young. I am luck to be alive, or at least not brain damaged or anything like that.

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Well, one out of two ain't bad... ;)

Yes it is. Without isolation every part of that circuit should be considered to be connected directly to the main, with the potential of upwards of 20A of current at 240V potential. It can be lethal.

The OP came across an nonchanlant about the safety issue. That's the reason for being vociferous.

But I don't believe the OP was one of those folks. Anyone trained on high voltage/high current equipment have a healthy respect for the safety issues involved. The OP's apparent disregard for such safety issues is what raised the alarm bells in my mind.

No. Each are potentially lethal and have specific safety procedures for building, using, and servicing such equipment. Right?

Can we at least agree that the attitude inside such equipment should not be "no big deal"?

Several issues here:

1) The OP wanted to drive a relay. It isn't clear that the cap can provide enough current to drive it.

2) The enclosure is the only isolation. Everything inside that case needs to be considered to be at line potential.

3) The OP said he wasn't concerned about safety issues.

The only point I got from the discussion was that he was concerned about the size of he circuit.

BAJ

I'm with Paul Keinanen on this one; thousands of devices in past and current use derive power through reactive coupling directly to the mains and in the U.S. have been and are U.L. approved.

What linguistic features of the original post lead you to believe that?

Again, if you have followed Alison's posts I don't really believe you can draw that conclusion, it appears that she has a lot of experience and sagacity.

Right, and every Boy Scout troop in the 1960s built or experimented with such things.

Just common application of 'best practices' that _anyone_ working with electronics should possess.

Alison is best probably referenced as 'she'. Give her the benefit of the doubt when parsing that sentence; I suspect she implied that this is not something for external consumption or approval and that she has the intent to maintain safety within her internal environment.

Her original idea, to derive +5V without switch-mode circuitry is a clever one and as has been demonstrated by this thread eminently do-able, however I feel that the real-estate used by the discrete parts (including zener regulation) would not be significantly increased by the addition of a switching regulator, which would make the design more robust.

Regards,

Michael

Right.

Sure. Safety is a big deal -

Yeah, I know. In the first part he says "drive a single LED directly from the mains without a transformer" or words to that effect. Later in the post the relay comes in - a relay would complicate a reactive dropping circuit to the point where it might not be feasible unless the relay took little current to drive or was a solid state type - but I would agree there most relays would make a transformer desirable - and the physical size of the cap might make it impractical.

Yes.

Well that could be for a variety of reasons - like maybe the case is bullet proof and gasketed or he plans to pot the circuit in epoxy or it is all to go in a receptacle enclosure. Or perhaps he just isn't concerned with safety because he's not that competent.

If size was an issue I must have missed that. Sounded like he wanted a time delay between two circuits turning on - or a time delay relay? That could be implemented easily by just using one non isolated LED driver to switch a solid state relay on with an R-C network to delay power to the relay - most SSR's have built in hysteresis to make the switch work without chattering and with reasonable repeatability. They make AC and DC SSR's these days.

Well OP? Feel free to jump in and clarify a thing or two.

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I'm well aware of those type of commercial devices designed by professional engineers who are cognizant of safety issues:

I quote:

"This doesn't need to be insulated from the outside world, safety is not a concern,"

That's linguistic enough for me. It does need to be insulated from the outside world and safety is a concern.

I didn't see it from the quote in the original post. And that's what I based the assessment upon.

But this isn't the 60's anymore. That's the problem. Truly any knucklehead with Internet access thinks they can tackle any problem with only a minimal amount of instruction.

The lack of informedness on the issue leads to potential Darwin Award nominees.

There's a different set of practices between isolated low voltage work and non isolated high voltage, high current work.

Better not to give the benefit of the doubt in this situation don't you think?

Nothing in the original design spec pointed out how any aspect of this design path is more clever than simply using a $2 wall wart.

BAJ

Being a former boy scout myself, I would expect any well trained boy scout to consider any safety issues, no matter how remote. My troop at least, were certainly educated in the dangers of high voltages and electrocution in basic first aid... that may have even been in the cub scouts.

If that didnt teach us, the numerous horror stories recalled over a campfire and singsong certainly did.

Strike "some", write "the vast majority of" applicable cases. Basically The wall wart is either safer than a non-isolated supply, or impossible to use for some reason other than safety (e.g. too much power required, voltage output so high that the cable from the wall wart to the gadget would be more dangerous than the mains cable it's supposed to replace,...).

Or do you really think that all the manufacturers of low-power-usage electronic equipment out there use wall warts just for the perverse fun of crowding up your power bars? The wall wart became a design pattern for a reason: it hits an almost perfect equilibrium between cost, usability, safety and reliability.

Fully seconded. If people need to be shouted at from a close distance to avoid a realistic danger of accidentally electrocuting themselves or other people, then by all means, let's get them shouted at. Violent explanation never seriously hurt anyone --- violent electricity does. A much more brutal, hands-on approach (such as: showing the apprentice a realistic electrocution accident on video) would be even better, but we can't do that over the internet.

"Many" of us may indeed have learned that way. But odds that the OP is among them are plainly negligible. If that guy had been around since tubes were taught, he wouldn't have had to ask what he did.

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Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
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