5 volt supply straight from 240v AC mains

[Dorian McIntire]

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

 

[¦¦¦SHAD0W¦¦¦]

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.

 

[Byron A Jeff]

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.

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.

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.

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.

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..

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"?

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.

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.

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.

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

BAJ

 

[default]

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

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

Sure. Safety is a 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.\

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.

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

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

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.

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

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.

 

[Byron A Jeff]

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.

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

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

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.

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.

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

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

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.

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

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

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.

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

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.

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

 

[Hans-Bernhard Broeker]

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.

Yes it is.

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.