Xmas lights

[David Lesher]

I was at the Mart of K last weekend with a friend; they had LED Xmas
light strings. They had ~35 LED's and were priced at $10.

Got up front and found out they were half-off. He went back and got the
other 9 boxes of multicolor ones. Got back in line, and adjacent cashier
passed us an instant coupon just left behind; 20% off.....

Between that and the power saving...

They appear to be in series; with a lump in the line at each end.
The instruction sheet talks about there being no shunt, but the LEDs
are removable and they give you extras.

So if we have a rectifier in one lump, and series R in the other, some
things come to mind. One failed-open LED brings down the string, and it
has full line voltage across it. Hmm, is there a sparkgap in each LED
that seals closed when so abused? How'd they get UL/FM approval with
such?

We plan to do some measuring/testing after they are not needed, but
until then, has anyone beat us to it?

 

[Spehro Pefhany]

I was at the Mart of K last weekend with a friend; they had LED Xmas
light strings. They had ~35 LED's and were priced at $10.

Got up front and found out they were half-off. He went back and got the
other 9 boxes of multicolor ones. Got back in line, and adjacent cashier
passed us an instant coupon just left behind; 20% off.....

Between that and the power saving...

They appear to be in series; with a lump in the line at each end.
The instruction sheet talks about there being no shunt, but the LEDs
are removable and they give you extras.

Usually semis fail short unless they are physically damaged. One cause
of failing short is excessive power dissipation. There exists an
obvious risk of a cascade failure of a series string.

So if we have a rectifier in one lump, and series R in the other, some
things come to mind.

I doubt there's a rectifier, given that you have a series string of
diodes with reasonably high typical reverse breakdown voltages. There
might be a fuse to protect the wiring. There must be a resistor
(probably a fusible type to deal with the problem I mention above).

One failed-open LED brings down the string, and it
has full line voltage across it. Hmm, is there a sparkgap in each LED
that seals closed when so abused? How'd they get UL/FM approval with
such?

There's no additional safety issue with having line voltage across an
open LED. The primary risk is that someone gets themselves between the
"hot" and earth, for which the cure is insulation.

We plan to do some measuring/testing after they are not needed, but
until then, has anyone beat us to it?

No actual testing. I suspect they have evolved a bit over time.


Best regards,
Spehro Pefhany

 

[Adrian C]

There's no additional safety issue with having line voltage across an
open LED. The primary risk is that someone gets themselves between the
"hot" and earth, for which the cure is insulation.

Yup, The coffin.

 

[PeterD]

I was at the Mart of K last weekend with a friend; they had LED Xmas
light strings. They had ~35 LED's and were priced at $10.

Got up front and found out they were half-off. He went back and got the
other 9 boxes of multicolor ones. Got back in line, and adjacent cashier
passed us an instant coupon just left behind; 20% off.....

Between that and the power saving...

They appear to be in series; with a lump in the line at each end.
The instruction sheet talks about there being no shunt, but the LEDs
are removable and they give you extras.

So if we have a rectifier in one lump, and series R in the other, some
things come to mind. One failed-open LED brings down the string, and it
has full line voltage across it. Hmm, is there a sparkgap in each LED
that seals closed when so abused? How'd they get UL/FM approval with
such?

We plan to do some measuring/testing after they are not needed, but
until then, has anyone beat us to it?

When you buy cheap Chinese crap, you get to take your chances...

 

[Bob Eld]

I was at the Mart of K last weekend with a friend; they had LED Xmas
light strings. They had ~35 LED's and were priced at $10.

Got up front and found out they were half-off. He went back and got the
other 9 boxes of multicolor ones. Got back in line, and adjacent cashier
passed us an instant coupon just left behind; 20% off.....

Between that and the power saving...

They appear to be in series; with a lump in the line at each end.
The instruction sheet talks about there being no shunt, but the LEDs
are removable and they give you extras.

So if we have a rectifier in one lump, and series R in the other, some
things come to mind. One failed-open LED brings down the string, and it
has full line voltage across it. Hmm, is there a sparkgap in each LED
that seals closed when so abused? How'd they get UL/FM approval with
such?

We plan to do some measuring/testing after they are not needed, but
until then, has anyone beat us to it?

--
A host is a host from coast to [email protected]
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

You questions don't make a lot of sense to me. If you pull an led out of
it's socket or if one burns open, yes there will be full line voltage at
that place. But so what? That is the same as a incandescent strings that
have been available for 90 years.

When an led is plugged back into to open socket the voltage instantly falls
to the two or three volts to operate the led. That voltage is established by
the led's impedance and the available current, 10 mA or so. That current is
established by all of the other leds in the string and the ballast
resistor(s).

What has a spark gap got do with it? I don't understand your concern.

 

[Spehro Pefhany]

When you buy cheap Chinese crap, you get to take your chances...

Generally stuff like that sold in the first world is required to meet
reasonable safety standards, and generally due diligence takes place
to ensure that they do in fact meet the standards.

For example, fires in home electronics used to kill a lot of people.
They don't anymore. In the US TV fires only account for a handful of
deaths out of the thousands of other fire deaths, and people have lots
and lots of TVs these days, all of them made in the cheapest places on
earth to manufacture things.

 

[David Lesher]

Usually semis fail short unless they are physically damaged. One cause
of failing short is excessive power dissipation. There exists an
obvious risk of a cascade failure of a series string.

True.... But if one fails open...

I doubt there's a rectifier, given that you have a series string of
diodes with reasonably high typical reverse breakdown voltages. There
might be a fuse to protect the wiring. There must be a resistor
(probably a fusible type to deal with the problem I mention above).

The instructions warn about inserting the LED's backwards. [They are keyed.]

It'd been years since I looked, but I don't recall LED's having high
breakdown voltages. Five volts sticks in my mind....

There's no additional safety issue with having line voltage across an
open LED. The primary risk is that someone gets themselves between the
"hot" and earth, for which the cure is insulation.

As opposed to Edison base lamps; the LEDs have wires that strike me as
easy oh, insert only one into a sock, etc. The widespread use of GFI's
may save many a customer....

 

[Spehro Pefhany]

Usually semis fail short unless they are physically damaged. One cause
of failing short is excessive power dissipation. There exists an
obvious risk of a cascade failure of a series string.

True.... But if one fails open...

I doubt there's a rectifier, given that you have a series string of
diodes with reasonably high typical reverse breakdown voltages. There
might be a fuse to protect the wiring. There must be a resistor
(probably a fusible type to deal with the problem I mention above).

The instructions warn about inserting the LED's backwards. [They are keyed.]

Yeah, it will most likely kill that one LED. Unless you stick them all
in backwards, then it will be fine, IMO.

It'd been years since I looked, but I don't recall LED's having high
breakdown voltages. Five volts sticks in my mind....

Sure that's typically the *rating* (and all that is needed in normal
use), but the actual breakdown voltage is typically much higher, like
30V or more. That's actually bad news for the one that you plug in
backwards because the dissipation will be high.

As opposed to Edison base lamps; the LEDs have wires that strike me as
easy oh, insert only one into a sock, etc. The widespread use of GFI's
may save many a customer....

The ones I have are not abusable like that, but maybe the standards
are lower wherever you bought these.


Best regards,
Spehro Pefhany