Xmas lights

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.

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).

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.

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

Best regards, Spehro Pefhany

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Yup, The coffin.

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Adrian C

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

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.

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.

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

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....

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....

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

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.

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

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com