A silly question about LED - Page 1
Forum section: electronics / beginners

author: muffenme
author_rank: Contributor
timestamp: March 11, 2020, 07:00:45 pm
content: Say I have a LED that normally operate at 3v @ 100mA DC.Can I run this same LED at 300v @ no more then 1mA?I'll try it but I don't have the test equipment, regulated power supply, to do such a test. Remember it just a silly question.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 11, 2020, 07:07:53 pm
content: Quote from: muffenme on March 11, 2020, 07:00:45 pmNo.

author: muffenme
author_rank: Contributor
timestamp: March 11, 2020, 07:13:31 pm
content: Thanks,Another silly question, have you tried it? 300v DC.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 11, 2020, 07:22:48 pm
content: QuoteNo.

author: mikerj
author_rank: Super Contributor
timestamp: March 11, 2020, 07:42:16 pm
content: You could run ~100 LEDs in series on 300v 1mA, but a single LED would drop your 300v 1mA current limited supply to around 3v. If it wasn't current limited, the LED would go bang.

author: brucehoult
author_rank: Super Contributor
timestamp: March 11, 2020, 07:56:19 pm
content: Quote from: muffenme on March 11, 2020, 07:00:45 pmThe voltage of the supply to the LED doesn't really matter, an LED will keep a more or less constant voltage drop across itself of around 2 V or a bit less for red and 3V or a bit more for blue, with green somewhere between.What matters is the current. LEDs effectively have a quite low resistance of something like 10 to 50 Ohms.It's quite possible your LED might draw 100 mA when placed across a 3 V power supply. But place it across a 5 V supply and it will make a nice little *pop* sound.But if you put it in series with a suitable resistor then it can be ok.What resistor? One that will keep the current to the desired level all by itself if directly connected to the voltage. Say for example 10 k Ohms across your 300 Volts. That will give 30 mA. If you then put the LED and resister in series across the 300 V then all should be well. Just make sure the resister is rated for that Voltage -- and for the 9 Watts of heat it will dissipate.Your LED might well show brightly enough with 1 mA (but much less than at 100 mA!), in which case 270 or 330 kOhm would work and dissipate about 0.3 W which is a lot easier to handle.

author: floobydust
author_rank: Super Contributor
timestamp: March 11, 2020, 08:09:51 pm
content: It's not practical to lower 300V to 3V at a lot of current like 100mA. It (the voltage drop) ends up wasted as heat (30W!) in the dropping resistor and way too inefficient.I run LED's at 1mA from 300VDC in place of neon lamps, that is only 0.3W dissipated as heat in the dropping resistor.If you want your LED to run at full current 100mA from 300V, you would use a mains SMPS to lower the voltage.

author: digsys
author_rank: Unknown
timestamp: March 11, 2020, 10:01:59 pm
content: There is also an important forward breakdown voltage issue. At the moment of voltage application, when the Led hasn't started to conduct, it has up to 300VDC across it, depending on the rate the voltage "ramps up", or even full 300V if it is instantly applied. Many Leds etc have a breakdown limit, often under 100V or even less. It may survive a few / several "starts", but can eventually break down.IF you "have" to run it at 300VDC, use resistors or even zener diodes to wash off as much voltage as possible.IOW don't try it :-)Edit: My above struck-out comment was mostly incorrect, sorry. The Led arrays had other active silicon in them, which I failed to notice. So changing the comment to "may not be true with Leds which incorporate other active elements" :-)

author: floobydust
author_rank: Super Contributor
timestamp: March 11, 2020, 10:51:51 pm
content: OP might be thinking in terms of power- that a 3V 100mA LED can be run at 300V 1mA for the same resulting power. It is the same input power only if an LED could work at 300V, but that is not the case- they are 3V devices.Only in big LCD TV's do you see series strings of say 100 white LED's for the backlight, running at 300V.

author: JohnG
author_rank: Frequent Contributor
timestamp: March 11, 2020, 11:48:52 pm
content: With 300V, I'm pretty sure it will light up very briefly. After the LED melts and evaporates, I am pretty sure you might still get some light from the plasma arc, but it will probably be a different color.John

author: gcewing
author_rank: Regular Contributor
timestamp: March 12, 2020, 02:09:43 am
content: Lighting the LED at full brightness will require 100mA flowing through it, regardless of anything else. To do this efficiently, you'll need some kind of power supply circuit to drop the 300V down to around 3V and limit the output current to 100mA.You can probably find an off-the-shelf unit designed for powering LEDs from the mains that would do this. They're designed to run from AC, but some of them would probably work when fed from DC, since the first thing they do is usually to rectify the input.Whatever you do, DON'T connect 300V directly to the LED. That would destroy it very quickly and spectacularly. (Unless you're Big Clive and this is the effect you're going for...)

author: james_s
author_rank: Super Contributor
timestamp: March 12, 2020, 02:37:02 am
content: The LED backlight in my TV is driven by almost 300V. It has a chain of something like 72 LEDs in series, works fine. As long as the power supply is current limited it's irrelevant (unless an open circuit fault occurs) whether the open circuit output is 30V or 300,000V, it's going to sag down to ~3.5V or whatever the forward voltage of the LED is when it is connected. LEDs are current driven and need something to limit the current, and the voltage follows the current.

author: Zero999
author_rank: Super Contributor
timestamp: March 12, 2020, 09:12:03 am
content: Quote from: digsys on March 11, 2020, 10:01:59 pmI've not heard of that one before. It seems implausible. Please post some data sheets which list this?The LED will also have a capacitance, so as long as the current is limited, the voltage across the die will not ramp up very rapidly and if the current isn't limited, the LED will go pop anyway.

author: johankj
author_rank: Contributor
timestamp: March 12, 2020, 10:07:10 am
content: QuoteBut doesn't that spell trouble if one LED desides to conduct, and then the remaining 71 LEDS have to take the load, resulting in a cascading failure?

author: johankj
author_rank: Contributor
timestamp: March 12, 2020, 10:12:53 am
content: Quote from: johankj on March 12, 2020, 10:07:10 amI just realised that that would be bad if they were in parallel too.

author: Zero999
author_rank: Super Contributor
timestamp: March 12, 2020, 11:05:34 am
content: Series is generally better than parallel for LEDs.The problem with a parallel connection is the LEDs need to be well-matched to ensure reasonable current sharing. The forward voltage also has a negative temperature coefficient which means that as the LED heats up, it will draw more current, which can be an issue if some LEDs have better heat sinking than others, as will be the case if some of them are in middle of the heat sink and others are near the edge. An LED failing short circuit will also quite likely result in a short circuit and smoke, although the bad LED might simply burn up, leaving the rest of the LEDs working, but at a slightly higher current and more prone to failure.Series connection is better because the current through all of the LEDs is the same. If it's a proper current regulator and not just a resistor, then an LED failing short circuit will not change the current. The only downside with series connection is if an LED fails open, all of the other LEDs will stop working.

author: johankj
author_rank: Contributor
timestamp: March 12, 2020, 11:13:35 am
content: Quote from: Zero999 on March 12, 2020, 11:05:34 amYep, that makes total sense.

author: kosine
author_rank: Regular Contributor
timestamp: March 12, 2020, 11:54:15 am
content: Surprisingly, LEDs can actually be run with a 20kV supply and still work:Of course, most of the voltage is being dropped by the spark, so the LEDs will have only a couple of volts across them at a few mA. But it's a neat demo all the same.

author: digsys
author_rank: Unknown
timestamp: March 12, 2020, 12:18:50 pm
content: Quote from: Zero999Which can be mitigated by placing zener diodes across groups of Leds. How many at a time depends on how many you can afford to "lose" if one in that group goes O/C. Pick a zener that is marginally higher than the total of that group. Same principle as used in Solar panels. This works very well when using a constant current supply.

author: digsys
author_rank: Unknown
timestamp: March 12, 2020, 12:37:02 pm
content: Quote from: Zero999Unfortunately, it is destructive testing that I carried out many years ago when manufacturing huge dot matrix LED arrays for factory / warehouse displays / messaging. Part of extensive testing, as I couldn't afford failures. I'd have to find and dig up my design / manufacturing notes. Not impossible, but not in the next few days. I still keep a couple of the Led displays for sentimental reasons, so could even run the tests again.I did find that 20KV test video very interesting, and would have to compare / evaluate both test methodologies and Led types. I think I have a pdf somewhere as well that covers the breakdown phenomena. TBC

author: james_s
author_rank: Super Contributor
timestamp: March 12, 2020, 08:21:11 pm
content: Quote from: Zero999 on March 12, 2020, 11:05:34 amThis actually happened, and is why I ended up with the TV. One of the LEDs failed intermittent open, it arced internally and burned up and in the process this caused about 5 other LEDs to fail. I bought a salvaged strip on ebay and replaced all the damaged LEDs, then added a transzorb across each strip to try to prevent this sort of cascading failure in the future.I've had the TV now for longer than the original owner had it from new and it's been fine.

author: Zero999
author_rank: Super Contributor
timestamp: March 12, 2020, 09:08:38 pm
content: Quote from: digsys on March 12, 2020, 12:37:02 pmIt sounds like ESD testing to me.Are you sure it wasn't the current, rather than voltage, which killed the LEDs? I doubt it's possible to get hundreds of volts across the LED, without a proportionally high current.The open circuit voltage shouldn'tt make any difference. You could have a power supply with an open circuit voltage of 1MV, but if it has an output impedance of 100M, the LED don't be damaged, since the current will still be under 10mA.

author: digsys
author_rank: Unknown
timestamp: March 13, 2020, 12:41:51 pm
content: Quote from: Zero999[/quoteWell, seems I wasmostlywrong ! The Kingbright LED array I used must have had some other silicon in it, which is what failed. I just "assumed" they were pure LEDs. A quick check of a variety of Leds in stock confirms they are fine with forward biased HV. So I'll change my original comment to - "may not be true with Leds which incorporate other active elements" .. possibly flashing type etc

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2020, 01:48:56 pm
content: Quote from: digsys on March 13, 2020, 12:41:51 pmYes LEDs containing active circuitry such as colour changers and flashers are very sensitive to overvoltage. When a power supply is used with an open circuit voltage, greater than the LED's rating, a suitable zener diode should be connected in reverse parallel to limit the voltage.

author: ebastler
author_rank: Super Contributor
timestamp: March 15, 2020, 08:49:43 am
content: I think capt bullshot nailed it with his two initial replies. I believe the subsequent discussion is not adding much, especially in view of the OP's level of technical expertise. (Nothing wrong with "silly questions", this is the beginners' section after all.) I would merely add the following in response to the original post:Don't.