5 volt supply straight from 240v AC mains

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

Reply to
Byron A Jeff
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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

Reply to
msg

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

Reply to
Byron A Jeff

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.

Reply to
Mark Fortune

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.

--
Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.
Reply to
Hans-Bernhard Broeker

I think the reason is heavily weighted for liability costs control; and it is perverse when a 'pigtail' mains cord could have been standard issue on wall wart inputs to alleviate the crowding.

I would hope 'she' (Alison) will reply; I had inferred from some of her earlier posts that she has been in that world.

Regards,

Michael

Reply to
msg

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