LED bulbs don't like MSW inverters?

[james_s]

I doubt that. But next time I am in the UK I might try to lay my hands on a 240 V bulb to bring back. They are hard to find these days since they have been banned for a long time. It's either a case of NOS, or the special "rough duty" kind that can still be found.

I have a whole box full of British lamps of various types, and I have an AC power source so I can test that when I get around to dragging it out. I have played with it in the past and don't recall there being any visible difference in output from kHz down to around 40Hz when flicker became visible. At 25Hz the flicker is very pronounced.

[IanB]

If anyone is interested, here is the optimal solution I found after my misguided foray into inverters and mains power. There appear to be readily available bulbs that run off a low voltage 12 - 24 V supply, and which have an E26 base to fit into a standard lamp fitting. These bulbs will therefore happily work with the 18 - 21 V supplied by a lithium tool battery, with no inverters or voltage converters required. This also leads to good efficiency and correspondingly economical use of the battery. The light quality is far more pleasant than the light you get from camping lanterns or work lights. In reality of course, you can put the battery on the floor out of sight, and the lamp appears just as if it were plugged into a wall outlet.

(It is important, of course, to remember that a low voltage bulb is installed. Some kind of tag on the plug would be indicated as a warning, or even a zip tie to prevent the plug being removed from the connector.)

[Zero999]

If anyone is interested, here is the optimal solution I found after my misguided foray into inverters and mains power. There appear to be readily available bulbs that run off a low voltage 12 - 24 V supply, and which have an E26 base to fit into a standard lamp fitting. These bulbs will therefore happily work with the 18 - 21 V supplied by a lithium tool battery, with no inverters or voltage converters required. This also leads to good efficiency and correspondingly economical use of the battery. The light quality is far more pleasant than the light you get from camping lanterns or work lights. In reality of course, you can put the battery on the floor out of sight, and the lamp appears just as if it were plugged into a wall outlet.

(It is important, of course, to remember that a low voltage bulb is installed. Some kind of tag on the plug would be indicated as a warning, or even a zip tie to prevent the plug being removed from the connector.)

Of just replace the connector, so it can't be plugged into the mains.

[Zero999]

No, a 12 to 24VDC lamp will not survive 120V to 240VAC. It depends on whether there's space to store the extra lamps. They don't take up much room, so keep them in the loft/attic/basement/garage etc. It's certainly the safer option.

[IanB]

Or just replace the connector, so it can't be plugged into the mains.

In North America, free standing lamps are the most common room lighting solution. In fact, many rooms may have no light fixtures at all (maybe five rooms in my house are like this depending on how you count open plan spaces). So the table lamp or floor standing lamp would be the normal lamp in the room, and during a power outage you would just temporarily replace the bulb and then plug it into a battery.

[james_s]

I've never seen somebody try it with an LED bulb but I have seen someone accidentally put a 12V RV incandescent bulb into a 120V socket. It was very bright, for a very short period of time, it made a loud pop and the whole inside of the bulb turned black and silver.

[james_s]

Highly doubtful. Resettable fuses are typically quite slow.

[IanB]

Fun experiments aside. I'm wondering just if a standard LED bulb would survive if given a very fast acting resettable fuse.

If you really wanted to do so, I think you could put something like a bidirectional TVS diode suppressor as a crowbar across the input and then have an inline fuse that blows if the TVS activates. You would have to replace the fuse in this instance, but fuses are cheaper than bulbs.

[themadhippy]

Hate to think how many triacs ive replaced in dimmer packs were they've blown to save the fuse

[james_s]

Hate to think how many triacs ive replaced in dimmer packs were they've blown to save the fuse

Unfortunately it's very difficult for a fuse to protect a semiconductor, they're just not fast enough. The fuse is there to prevent a fire, not to protect the equipment.

[2N2222A]

I think you could put something like a bidirectional TVS diode suppressor as a crowbar across the input and then have an inline fuse that blows if the TVS activates..

It wouldn't need to be bidirectional since it the 12V light is made to work on DC. But I don't think a TVS diode would work in all cases since it doesn't trip like a varistor. If someone plugged it in to 18 Volts the TVS diode would burn out without drawing enough current to blow the fuse. An SCR that is set to trigger at 18 Volts would work. The SCR datasheet should say what type of fuse (maximum value) is required to protect the SCR.

[james_s]

I think you could put something like a bidirectional TVS diode suppressor as a crowbar across the input and then have an inline fuse that blows if the TVS activates..

It wouldn't need to be bidirectional since it the 12V light is made to work on DC. But I don't think a TVS diode would work in all cases since it doesn't trip like a varistor. If someone plugged it in to 18 Volts the TVS diode would burn out without drawing enough current to blow the fuse. An SCR that is set to trigger at 18 Volts would work. The SCR datasheet should say what type of use (maximum value) is required to protect the SCR.

You don't necessarily know the polarity it will receive. Most DC sockets have positive on the center button but there is not really a standard as far as I know.

[IanB]

Bi-directional because the goal is to defend against plugging into the mains. If a TVS cannot sink enough current to blow a fuse before itself being destroyed, then back-to-back SCRs, or a TRIAC?

The light itself works on 12 to 24 V AC or DC, so the trip voltage would need to be around 35 V.

[2N2222A]

I used 18v TVS diodes in my 12v 555 solar charge controller build. Double that and its 36v.

Great big ones that can handle 1.5 kW. (Relative to SMD components they are big but not really relative to most THT.)

But for how long? A few miliseconds maximum? If it gets plugged in to a higher Voltage socket that does not supply enough current to blow the fuse, then all the energy has to go through the TVS diode. So it will need a large heat sink. With a thyristor it will trip and short out completely to blow the fuse.

You don't necessarily know the polarity it will receive.

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+.

[IanB]

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+.

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.

[Zero999]

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+.

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.

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]

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

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]

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.

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

[IanB]

Yes, the bulb uses 300 mA at 18 V, so a 1 A or 500 mA fuse should be fine, maybe even less.

[2N2222A]

I didn't realize the LED bulb works on AC. But the SCR crowbar can be put on the output of the rectifier then!

[IanB]

No, no, no, there is no external rectifier 

There is a low voltage AC/DC light bulb temporarily installed in a standard table lamp with a mains cord and a mains plug. The risk is that if somebody inadvertently plugged it into the mains, the bulb would blow up, possibly with a bang. Therefore we are considering some kind of black box that could be inserted into the mains cord to protect the bulb in this event.

But this idea is flawed, because the installation of the low voltage bulb is meant to be temporary, just used in the event of a power outage. If we went to the trouble of installing a black box in the mains cord, we might just as well cut off the mains plug and install a low voltage connector.

So this whole discussion in a sense is a dead end 

[themadhippy]

the bulb uses 300 mA at 18 V, so a 1 A or 500 mA fuse

id be more tempted with a 250mA fuse,