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Underdriven 12V LED chips in a 120VAC system

Audioguru

Sep 24, 2016
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Leds are driven with current, not voltage because each LED has a different voltage and the voltage required changes with temperature changes.
Cheap no-name-brand (factory rejects?) electronic items do not last long anyway.
 

primuspaul

Feb 7, 2018
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It depends on what you mean by "properly 12V rated", "work properly", and "shortening the life".

I would tend to say no, but, based on your experience reading the LED resource, what do you think?
I think if the diodes prevent reverse voltage, the quantity of LEDs prevent overvoltage, and a heatsink prevents overheating, it should work.
 

primuspaul

Feb 7, 2018
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Leds are driven with current, not voltage because each LED has a different voltage and the voltage required changes with temperature changes.
Cheap no-name-brand (factory rejects?) electronic items do not last long anyway.
Yes, but by reducing the voltage, I'll reduce the current, won't I?
 

(*steve*)

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Yes, but by reducing the voltage, I'll reduce the current, won't I?

If the unknown LED is nominally rated at 12V, what voltage is safe to restrict the current to a safe value if you use an unknown heatsink at an unknown ambient temperature?

I don't know the answer, do you?
 

primuspaul

Feb 7, 2018
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If the unknown LED is nominally rated at 12V, what voltage is safe to restrict the current to a safe value if you use an unknown heatsink at an unknown ambient temperature?

I don't know the answer, do you?
I can just hook it up and and feel the LEDs with my hand. Should be safe as long as I'm not standing in a puddle of water or using both hands.
 

Audioguru

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The chip producing the heat is deep inside an LED and it could be over-temperature when the outside of its case is just very warm. It takes time for the case to get hot, meanwhile the chip is cooking.
 

vassock

Nov 30, 2015
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The chip producing the heat is deep inside an LED and it could be over-temperature when the outside of its case is just very warm. It takes time for the case to get hot, meanwhile the chip is cooking.
What do you mean by "chip?" Do you mean a "chip" inside the "chip?"
 

(*steve*)

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At low currents you can see that the module you have has a number of bright points on the surface, each of these is an individual LED. It's not uncommon for each of these to be rated near 1W each, but it can vary substantially.

The specs you linked to indicate that the base of the module is "pure copper". I suggest that this claim is probably "pure bull....", and the base is aluminium. That in itself is not a huge deal, but it's indicative that you can't trust the specs.

I would insert stuff here about how thermal runaway works, but I looked you to a resource describing it and I will just assume you're familiar with it.

Now, back to those individual LEDs that make up each module. At low current do all of them light up simultaneously and at the same brightness? If so, that's a good thing. However a "feature" of many cheap eBay modules is that they are rejects where the LEDs do not light up evenly. If yours do not light up evenly, request a refund from the seller on the basis that they are faulty. Then throw them away.

When you say that each of these LED modules is 12V, and that you are running the string from 120VAC, are you setting up the string to operate at 170V? How will you LEDs react if the voltage goes 10% high or low? What happens it there's a voltage spike of (say) 300V? What happens if one module fails short circuit? What is the design current, and how will it vary with temperature, component variation, and applied voltage?

None of the above are silly questions, and they're questions that I have no way of answering for your design.
 

Audioguru

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There is a likely possibility of electrocution since this project has high voltage and no mains transformer then other electronics chat forums would close this thread. But I can't find the whistle for me to make an alert to the moderators.
EDIT: I found and used the "report" button.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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But I can't find the whistle for me to make an alert to the moderators.

And you only just noticed this?

But yeah, We should have warned the OP that he could kill himself, and that the insulation resistance of these COB LEDs are unknown.
 

primuspaul

Feb 7, 2018
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At low currents you can see that the module you have has a number of bright points on the surface, each of these is an individual LED. It's not uncommon for each of these to be rated near 1W each, but it can vary substantially.
Yes, I've seen those running a 12V chip at 5V. I just wasn't sure what he meant by "chip." I thought the consolidation of the individual LED bulbs inside a chip is what made the chip.
The specs you linked to indicate that the base of the module is "pure copper". I suggest that this claim is probably "pure bull....", and the base is aluminium. That in itself is not a huge deal, but it's indicative that you can't trust the specs.

I would insert stuff here about how thermal runaway works, but I looked you to a resource describing it and I will just assume you're familiar with it.

Now, back to those individual LEDs that make up each module. At low current do all of them light up simultaneously and at the same brightness? If so, that's a good thing. However a "feature" of many cheap eBay modules is that they are rejects where the LEDs do not light up evenly. If yours do not light up evenly, request a refund from the seller on the basis that they are faulty. Then throw them away.

When you say that each of these LED modules is 12V, and that you are running the string from 120VAC, are you setting up the string to operate at 170V? How will you LEDs react if the voltage goes 10% high or low? What happens it there's a voltage spike of (say) 300V? What happens if one module fails short circuit? What is the design current, and how will it vary with temperature, component variation, and applied voltage?

None of the above are silly questions, and they're questions that I have no way of answering for your design.
I thought 170 was just the peak with the average heat produced being equivalent to 120. What happens if I get bad voltage from the mains? Hopefully the fact that the LEDs are underdriven will help protect them. What if one burns out? I'll have to unplug the system and replace it.
 

primuspaul

Feb 7, 2018
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There is a likely possibility of electrocution since this project has high voltage and no mains transformer then other electronics chat forums would close this thread. But I can't find the whistle for me to make an alert to the moderators.
EDIT: I found and used the "report" button.
You can electrocute yourself changing a light bulb if you're not careful.
 

(*steve*)

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I thought 170 was just the peak with the average heat produced being equivalent to 120. What happens if I get bad voltage from the mains? Hopefully the fact that the LEDs are underdriven will help protect them. What if one burns out? I'll have to unplug the system and replace it.

Again, if you had read the resource I pointed you to, you would realise that a small increase in voltage can cause a large change in current, AND that you can't predict the actual current for a specific voltage.

As @Audioguru has pointed out, LEDs should be driven from a current source, not a voltage source.

The difference between the average and the peak voltage for AC is 40%. LEDs are not operated by heating (as incandescent bulbs are) so the equivalent heating isn't particularly relevant. One of the very first things in the resource I pointed you to is the difference between incandescent bulbs and LEDs.

A 40% increase in voltage will cause an increase in current significantly greater than 40%. It might be 100% more, 400% more, or 1000% more. This might make the LED just go POP and die immediately, or it might heat it causing the next peak to draw more current, which heard it more, causing it to draw more current until it goes POOF. Or it might appear to do nothing, but you find that it's life deceases from the typical 20,000 to 40,000 hours to 5000 hours, or 1000 hours, or 100 hours.

And when it fails it might fail short circuit which will cause all the other modules to fail in an ever increasing cascade.

There fact that you see some segments light more brightly than others means that these will be more highly stressed than the others in that LED which may lead to a cascade failure in the LED module.

Driving a LED from a voltage source and asking how long it will last is a bit like removing your hands from the wheel while driving and asking how long before you crash.
 

BobK

Jan 5, 2010
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I have not read the whole thread, so this might be repetitious, but...

You have to understand the difference between the forward voltage of and LED and the operational voltage range of typical devices. I think you are interpreting 9-12V as being the range of safe operating voltages. That is not the case. The forward voltage of an LED is the voltage at which it will draw the operating current, which is the safe current to operate the LED at continuously with proper heat sinking. The forward voltage is one specific voltage for any given LED. The range is telling you that the voltage will be somewhere in that range.

So, if you have a and LED with a forward voltage of 9-12V that means that it will draw its rated current somewhere in that voltage range. If it happens to be 9V, then applying 12V will either kIll the LED immediately or shorten it life dramatically.

Theoretically, you could operate the LED at the lowest voltage in the range and be safe. But that would result in reduced output in practically every LED you run that way. This is why you need a regulated current source to optimally and safely drive high power LEDs.

Bob
 

dave9

Mar 5, 2017
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Where are you getting brand name 5W LEDs for under $1 each?
FastTech, for example the following, they aren't what you'd want for everyday household lighting (tint and CRI aren't the best) but to just get a lot of light cheap w/o dubious generic issues:

https://www.fasttech.com/products/0/10004538/2058900-cree-xt-e-450lm-4500-5000k-led-emitter-5-pack

They have other bins and other Crees that are also reasonably priced. There are other options even cheaper the more you buy, but I've had good experiences with FastTech and they seem to get things to me a week sooner than most Chinese merchants.

https://www.aliexpress.com/item/Hot...-Emitter-Bulb-mounted-on-20mm/1393623625.html
 
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