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LED bulbs don't like MSW inverters?

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Zero999:

--- Quote from: IanB on December 19, 2022, 09:29:55 pm ---
--- Quote from: 2N2222A on December 19, 2022, 07:32:39 pm ---It's a diode so it will conduct forward biased anyway, which is what it needs to do in a fault condition. No need for back to back TVS diodes. Reverse polarity protection that will save the fuse can be done with a 1N4007 which won't have much loss at 18V+.

--- End quote ---

The goal is to save the low voltage bulb if someone plugs the lamp into the mains. This should happen by applying a crowbar as a short circuit across the input, so that the bulb does not experience an excessive voltage. Since the mains has alternating cycles, it is ideal if both polarities of the mains cycle are shorted out, preventing the bulb from seeing even one cycle of high voltage mains. The fuse is going to be something like a 3 amp mains rated fast blow fuse, which when presented with the short circuit from the crowbar should blow almost instantly.

--- End quote ---
A TRIAC is non-polarised.

I doubt a 3A fuse would blow quickly enough to protect the crowbar circuit, unless very high current devices are used.

IanB:

--- Quote from: Zero999 on December 20, 2022, 09:28:16 am ---I doubt a 3A fuse would blow quickly enough to protect the crowbar circuit, unless very high current devices are used.

--- End quote ---

Interesting problem. I see that Triacs may have a "Value for fuse" definition: "To protect the device, the I²t rating of the fuse used in series with it must be lower than this specified value."

I have not done a parametric search to see what this means in practice with regard to Triac and fuse combinations.

https://www.st.com/resource/en/application_note/an2703-parameter-list-for-scrs-triacs-ac-switches-and-diacs-stmicroelectronics.pdf

2N2222A:
I'm trying to just make the point that a diode can be used to protect the whole thing from reverse polarity, and an SCR can be a crowbar to blow the fuse if it is plugged in to too much Voltage. It's also possible to skip the diode and use a triac. A varistor or TVS diode will catch fire or destroy itself before the fuse blows in some cases. Varistors are mounted beside thermal fuses to prevent fires.

IanB:
Yes, but reverse polarity protection is not needed, since the bulb can run on AC.

A triac would be fine as a crowbar just as long as it does not blow up to protect the fuse. This is where the maximum I²t rating of the triac comes into play.

themadhippy:

--- Quote ---A triac would be fine as a crowbar just as long as it does not blow up to protect the fuse. This is where the maximum I²t rating of the triac comes into play.
--- End quote ---
and the I /T of the fuse,maybe use a lower rated fuse than seems sensible,as done by at least one old dimmer manufacturer

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