Author Topic: LED strips.  (Read 10386 times)

0 Members and 1 Guest are viewing this topic.

Offline ShadyK

  • Contributor
  • Posts: 39
LED strips.
« on: January 27, 2014, 05:13:55 am »
Hello everyone.

I have a quick question.
I want to make a circuit with 4 separate strips on. For shelving would i just rung wires from each strip to the 12v WW?
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #1 on: January 27, 2014, 05:34:20 am »
I have some LED strips installed under my desk and wired 3 of them directly to a 12v 2A Router transformer and after two weeks the strips got very dim so I've searched the internet and it seems LED's need some constant current drivers to run properly so I got a regulated driver made for LED strips and no problem since.

The driver looks like this, and I hooked all 3 of them at this unit:

If life was perfect would it be boring?
 

Offline Huluvu

  • Regular Contributor
  • *
  • Posts: 68
  • Country: de
    • ECM Home
Re: LED strips.
« Reply #2 on: January 27, 2014, 06:01:08 am »

The driver looks like this, and I hooked all 3 of them at this unit:


Hi Elektroshok .... please be careful not to "Elektroshock" yourself ....
I have recently checked one very similar LED Driver and was shocked about the missing Earth Ground connection.
Second thing I noticed was a far too late current limiting as specified on the label (3,6A instead of 1,5A)

Rgds
"Yeah, but no, but yeah, but no..."
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #3 on: January 27, 2014, 06:35:15 am »
Hi Huluvu, fortunately I didn't get electrocuted but then again never touched the wires while the stripes were on, I've soldered the wires, put heat shrinking tubes on and only then powered them so it's a thing to keep in mind.
If life was perfect would it be boring?
 

Offline Melkor

  • Contributor
  • Posts: 11
  • Country: ro
Re: LED strips.
« Reply #4 on: January 27, 2014, 06:54:26 pm »
While LEDs do benefit from constant current driving, most LED strips (monochrome ones) use individual resistors for current regulation so they can be used with constant voltage power sources.
Did you measured the current draw of your LED strips and the power source's voltage under load ?
 

Offline Towger

  • Super Contributor
  • ***
  • Posts: 1344
  • Country: ie
Re: LED strips.
« Reply #5 on: January 27, 2014, 08:56:44 pm »
As per Melkor's post.

Also from my own experimenting those 5050 300 LEDs for 5M ebay strips (from two different sources UK and China!) draw about 3A at 12V, and not the 5A (72W) per the adverts.  That does not mean that some types may have lower value resisters in them, to drive the LEDS harder. By upping the input voltage to around 14v they do draw 5A, but get much hotter than I would like to run them at for longer than a minute or two. From memory, this brought the voltage per led up to the max allow in the 5050 datasheet.  In any event even at 12v then get hot, so ideally they should be stuck to a metal base to act as a heatsink.
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #6 on: January 27, 2014, 09:04:07 pm »
Just measured one of my strips and the Amp draw is 0.5A and I had 3 strips wired in parallel on a 12v 2A transformer and as I said it did not manage to power the strips for to long and gave out. The 12v LED Driver is 1.5A and it's doing the job for 5 months now so I would no recommend a regular 12v transformer. The same problem I had with five automotive LED's



I've powered them with with a 12v wall charger and after a month or so it failed.

My strips were advertised as 5W not 72W and they are 1,2 Meters long.
If life was perfect would it be boring?
 

Offline Towger

  • Super Contributor
  • ***
  • Posts: 1344
  • Country: ie
Re: LED strips.
« Reply #7 on: January 27, 2014, 09:38:15 pm »
Hi Electroshok,

What is the voltage across one LED on the strip?
Taking the first 5050 datasheet Google throws up, their white 5050s forward voltage should be between 2.8 and 3.6v. If you look at the graph of Forward Voltage vs Forward Voltage you will see that they draw 5 times more current at 3.4V than at 3.0V. At a guess I would say that your 12v transformer was outputting more than 12v and would not take much extra voltage to damage the LEDs.
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #8 on: January 27, 2014, 09:51:06 pm »
Sorry, I did not express myself clearly, the LED's are OK, after hooking them to an LED Driver they work at full brightness the wall charger was the one that gave up not the LED's.

My strips stay on all the time, and the wall charger did not take it.

If you don't know an LED only draws the current that it needs, and as it heats up it needs more current, and then it heats up some more, and needs even more current, and so on till one of the components can't take it any more, in my case it was the wall charger.
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #9 on: January 27, 2014, 09:53:29 pm »
If you don't know an LED only draws the current that it needs

That's not how diodes work.
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #10 on: January 27, 2014, 10:58:10 pm »
You should talk to Wikipedia to modify their Diode page.

"As temperature rises, the saturation current I_S  rises"

And just for kicks, I've put an XM-L T6 to a 5v 6A PSU and the current draw is 3.1A and rising with the temperature. The voltage is 3.6V, no resistor no nothing, PSU and LED.

A conventional power supply is voltage limited and since LED's are current driven you can't use a voltage regulated PSU and you need a Current regulated one.

Perhaps I'm wrong, Wikipedia could be wrong to, but from experience, it does no work with simple wall chargers, they fail in short time and you end up putting an LED driver. So if the strips will be installed for long term use you need an LED driver. Or you can buy wall chargers once every few weeks, this way you risk damaging the strip itself.
« Last Edit: January 27, 2014, 11:02:04 pm by elektroshok »
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #11 on: January 27, 2014, 11:06:04 pm »
You should talk to Wikipedia to modify their Diode page.

"As temperature rises, the saturation current I_S  rises"

A conventional power supply is voltage limited and since LED's are current driven you can't use a voltage regulated PSU and you need a Current regulated one.

Perhaps I'm wrong, Wikipedia could be wrong to, but from experience, it does no work with simple wall chargers, they fail in short time and you end up putting an LED driver. So if the strips will be installed for long term use you need an LED driver. Or you can buy wall chargers once every few weeks, this way you risk damaging the strip itself.

*sigh*

Reverse saturation current is not the same thing as forward current.

The resistors aren't for show.

And just for kicks, I've put an XM-L T6 to a 5v 6A PSU and the current draw is 3.1A and rising with the temperature. The voltage is 3.6V, no resistor no nothing, PSU and LED.

Put a 12V 150W supply across it. I suggest wearing safety glasses. ::)
« Last Edit: January 27, 2014, 11:18:52 pm by Monkeh »
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #12 on: January 27, 2014, 11:18:58 pm »
Ok, so explain to me why the T6 that is rated at 3A is drawing 3.1A and rising with the temperature from a 6A PSU? If that is not how an LED works?

Could it be magic?
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #13 on: January 27, 2014, 11:21:57 pm »
Ok, so explain to me why the T6 that is rated at 3A is drawing 3.1A and rising with the temperature from a 6A PSU? If that is not how an LED works?

Could it be magic?

As I said, put a 12V supply across it, enjoy the smoke. Also, you're outside the maximum specs for that LED, so..

For another experiment, connect a normal 5mm LED across your 5V supply, watch it die.
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #14 on: January 27, 2014, 11:31:06 pm »
The LED strip I have is rated at 12v 0.5A. The LED will die in my 5V 6A PSU because of the current, not because of the voltage. If I regulate the amperage at 70mA the LED will be OK.

If you dont have enough voltage to jump that gap, it wont operate to draw the current required.

If the led was a voltage controlled device it would be brigher with 24v @ 20ma then with 9v @ 20ma.

as long as the forward voltage is satisfied, it is a current controlled device. You change the current to control the brightness.
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #15 on: January 27, 2014, 11:36:03 pm »
The LED strip I have is rated at 12v 0.5A. The LED will die in my 5V 6A PSU because of the current, not because of the voltage. If I regulate the amperage at 70mA the LED will be OK.

If you dont have enough voltage to jump that gap, it wont operate to draw the current required.

If the led was a voltage controlled device it would be brigher with 24v @ 20ma then with 9v @ 20ma.

as long as the forward voltage is satisfied, it is a current controlled device. You change the current to control the brightness.

Yes, and the current doesn't regulate itself..

Place a 12V supply capable of 5A+ across your bare XM-L T6. It will die.
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #16 on: January 28, 2014, 12:09:09 am »
I did exactly that, I just told you that, bare XML-T6 directly on the 5V PSU, and the readings are ~3.1A and rising as the temperature rises. That is why an LED dies on high amperage PSU's, the LED gets it's required A and then heats up, gets more Amps and heats up a little more, and so on until it fails.
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #17 on: January 28, 2014, 12:17:22 am »
I did exactly that, I just told you that, bare XML-T6 directly on the 5V PSU, and the readings are ~3.1A and rising as the temperature rises.

No, you didn't. The numbers '5' and '12' are not equal. With a 5V supply and a Vf of 3.6A, you only need ~450milliohms stray resistance to be limiting the current.

Quote
That is why an LED dies on high amperage PSU's, the LED gets it's required A and then heats up, gets more Amps and heats up a little more, and so on until it fails.

No, it's not. LEDs do not only draw the current they need. They simply do not work like that.
« Last Edit: January 28, 2014, 12:26:11 am by Monkeh »
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #18 on: January 28, 2014, 01:32:36 am »
So it's magic right?

I've hooked up a 70mA LED at a Nokia charger 5v 1000mA and the current draw is ~85mA and climbing slowly, no resistor in between or any other component, just LED and charger. Why is that? If you give me an explanation for this behavior I'll believe you.

Or perhaps my multimeter is broken...At the same charger a Nokia Phone draws ~900mA.
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #19 on: January 28, 2014, 02:20:26 am »
If you give me one, perhaps I'll believe you. Until then, you've got some random experiments I can't see and you refuse to try one I suggest (probably because you know what will happen).

You could, of course, also ask yourself: What limits the current on a 1N4001? Because it's really the same thing as a LED.

Meanwhile, I'm going to go quickly pop a LED, and I'll even show you.
« Last Edit: January 28, 2014, 02:22:46 am by Monkeh »
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #20 on: January 28, 2014, 02:49:17 am »


So much for that idea. I took a nice, stinky one of a white LED sizzling away, but my phone crashed and it didn't save.

Yes, some LEDs will have an upper bound on their current due to physical limitations of the die. No, they don't 'only draw the current they need'. They behave like any other diode, except more delicate.
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #21 on: January 28, 2014, 04:42:36 am »
Test that with a wall charger not with a current regulated PSU.
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #22 on: January 28, 2014, 04:43:26 am »
Test that with a wall charger not with a current regulated PSU.

....

The PSU was in CV mode. I was drawing the current through a dummy load. It would not behave any differently if simply placed across a voltage source (the 12V supplies I have would simply instantly pop it).
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #23 on: January 28, 2014, 05:03:26 am »
Here, LED, no resistor, powered from PC PSU at 5v 30A and the LED only draws 32mA.

If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #24 on: January 28, 2014, 05:05:41 am »
I can't see the display clearly, but that says 0.32A?
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #25 on: January 28, 2014, 05:11:44 am »
Test it yourself, on a PC Power Supply, or a wall phone charger and a multimeter and you will see, that test you did is just forced a regulated current into an LED, my multimeter only shows numbers, it does not display a big A for you to see it, I just set it to 10A and it displays in digits what it measures, in my case 0.32.

Test it and then we talk.
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #26 on: January 28, 2014, 05:12:28 am »
Test it yourself, on a PC Power Supply, or a wall phone charger and a multimeter and you will see, that test you did is just forced a regulated current into an LED, my multimeter only shows numbers, it does not display a big A for you to see it, I just set it to 10A and it displays in digits what it measures, in my case 0.32.

Test it and than we talk.

0.32A is not 32mA. It is 320mA.

And no, I didn't force a current into the LED. I pulled one through it. You'll note the current went up when I increased the voltage, the dummy load could not pull more through at 3V, and it was not the dummy load limiting that (it has a 100milliohm shunt, it can pass the full range 12A at 1.5V).
« Last Edit: January 28, 2014, 05:15:28 am by Monkeh »
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #27 on: January 28, 2014, 05:15:24 am »
Ok sorry, my mistake, 320mA, but not 30A that the PSU is rated at. So why doesn't the LED go to 30A.
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #28 on: January 28, 2014, 05:17:11 am »
Ok sorry, my mistake, 320mA, but not 30A that the PSU is rated at. So why doesn't the LED go to 30A.

Lead impedance, shunt impedance (your meter), physical limitations of the junction. There is no intentional current limiting going on there. Remember I mentioned a 450milliohm figure for your big Cree LED? Your meter is likely 100milliohms of that in series. Add contact resistance and lead resistance, along with heating of the bond wires and pads..

It is way outside even the pulse specification for the average 5mm LED. It would explode if 30A passed through it, and it will almost certainly just fail if you try that on the 12V rail, as I've told you to several times now. 12V vs a random white LED I had around resulted in an internal arcover after the junction breakdown and one foul smell, unfortunately, as I said, my phone crashed during recording.
« Last Edit: January 28, 2014, 05:24:28 am by Monkeh »
 

Offline Mr Smiley

  • Frequent Contributor
  • **
  • Posts: 324
  • Country: gb
Re: LED strips.
« Reply #29 on: January 28, 2014, 05:24:22 am »
And just for kicks, I've put an XM-L T6 to a 5v 6A PSU and the current draw is 3.1A and rising with the temperature. The voltage is 3.6V, no resistor no nothing, PSU and LED.

So the output impedance of your supposedly 5v 6A PSU is dropping the other 1.4v at 3.1A  :--

I'd get another PSU if it can't keep up 5v at 3.1A  :-DD

 :)
There is enough on this planet to sustain mans needs. There will never be enough on this planet to sustain mans greed.
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #30 on: January 28, 2014, 05:26:36 am »
There is no point in arguing with you, just test the damn thing.

Just hook up an LED to the molex connector of a PC PSU at the red and black wires and see if it blows, after you do that measure the Amp Draw on it and tell me what is the result.
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #31 on: January 28, 2014, 05:28:08 am »
There is no point in arguing with you, just test the damn thing.

Just hook up an LED to the molex connector of a PC PSU and see if it blows, after you do that measure the Amp Draw on it and tell me what is the result.

I already tested for you, if you can't see that it was a valid test, there's not much hope. But if you insist, I'll demonstrate +12V with no current limit and not even the burden voltage of a meter (I like my fuses intact).
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #32 on: January 28, 2014, 05:29:24 am »
I put the LED on 5v not 12v, why do you twist everything ?

Test it on the RED and BLACK wires (5v).
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #33 on: January 28, 2014, 05:31:56 am »
I put the LED on 5v not 12v, why do you twist everything ?

I know you did, and I've told you repeatedly to try it with +12V and see what happens. The lack of current limiting will prove instantly fatal.

Quote
Test it on the RED and BLACK wires (5v).

I don't need to, you've already handily demonstrated (as has my original video!) how stray resistance and physical limitations of the die can prevent immediate failure by limiting the current at that voltage (although my red LED popped very happily at 5V!).
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #34 on: January 28, 2014, 05:44:44 am »
Two more red LEDs have died a violent death at the hands of a cheap, nasty power supply.

 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #35 on: January 28, 2014, 05:49:48 am »
Not really, I can't see any PC PSU, any multimetr, just an LED with two leads. From what I know you just did the same test you did before but not gradually, you left the Amps at 300mA and blown those LED's. But I don't care anymore, I've tested with 3 PSU's and 5 types of LED's and none died even on a 30A PSU they lived so perhaps I've got magical LED's and you have real ones.
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #36 on: January 28, 2014, 05:51:18 am »
Not really, I can't see any PC PSU, any multimetr, just an LED with two leads. From what I know you just did the same test you did before but not gradually, you left the Amps at 300mA and blown those LED's. But I don't care anymore, I've tested with 3 PSU's and 5 types of LED's and none died even on a 30A PSU they lived so perhaps I've got magical LED's and you have real ones.

You really want me to pull back and show you the mess that is my bench while I destroy yet more LEDs to prove a point? Fine, let me go get the vice to hold the ATX connector, and I'll insert them directly.

And no, there is no multimeter. You want the multimeter involved, you pay for the HRC fuses in it. I am not risking popping those to prove a point, I don't have the money.
 

Offline nanofrog

  • Super Contributor
  • ***
  • Posts: 5420
  • Country: us
Re: LED strips.
« Reply #37 on: January 28, 2014, 05:55:18 am »
The PSU was in CV mode. I was drawing the current through a dummy load. It would not behave any differently if simply placed across a voltage source (the 12V supplies I have would simply instantly pop it).
Brilliant. Particularly liked the humor.  :)
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #38 on: January 28, 2014, 05:59:58 am »


How many LEDs do I have to kill, here? Seriously, I'm running low on red, and that's all I had spares of.
 

Offline mrkev

  • Regular Contributor
  • *
  • Posts: 212
  • Country: cz
Re: LED strips.
« Reply #39 on: January 28, 2014, 06:20:43 am »
I find it really painfull to read this thread... So much fail  :palm:
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #40 on: January 28, 2014, 06:35:34 am »
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #41 on: January 28, 2014, 06:37:49 am »
Try a red one. Or put one of those on 12V as I asked you to.

And at no point have I said they are voltage driven, but the voltage matters. Look at the formulas.
« Last Edit: January 28, 2014, 06:47:58 am by Monkeh »
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #42 on: January 28, 2014, 06:42:08 am »
I don't have red LED's.

I assumed we are talking about current not voltage, at 12v it will blow, but we were talking about LED's drawing current not voltage. Now tell me how a small LED is able to run on a PC PSU rated at 30A without any resistors if they are not drawing just what they need?
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #43 on: January 28, 2014, 06:43:54 am »
I assumed we are talking about current not voltage, at 12v it will blow, but we were talking about LED's drawing current not voltage. Now tell me how a small LED is able to run on a PC PSU rated at 30A without any resistors if they are not drawing just what they need?

Stray resistance, the high Vf of white LEDs, incidental current limiting due to physical factors, and probably some diode physics I'm not overly familiar with playing a part.

If you apply 12V, much more current will flow, and the small LED should quite happily pop. Bigger one should, too. It's also entirely possible you have LEDs with built in dropper resistors, they exist! They're also hugely inflexible in design (great for things like that lighter, though, because fewer parts).

And yes, my LEDs (I'm still watching the video) are pretty crap, a friend of mine just managed to get a better red one to survive 5V for an extended duration (at three times the maximum pulse current, and it taped off as the junction burnt up). Still won't take 12V without current limiting.

Please, please, do some more reading on diode behaviour. The Shockley ideal diode equation should prove interesting..
« Last Edit: January 28, 2014, 06:50:13 am by Monkeh »
 

Offline elektroshok

  • Contributor
  • Posts: 25
  • Country: ro
Re: LED strips.
« Reply #44 on: January 28, 2014, 06:51:00 am »
Even at 10mA the LED will pop at 12v, stray resistance in the order of 10's of amps, are you serious? What physical factors? The wires acting as 1000Ohms resistors and LED pins acting as current transformers?

You are telling me that 30A are going to the LED and from stray resistance and physical factors those 30A turn into 350mA?

I'm lost...so ok, you are right, it's just an invisible resistor made by a tooth fairy and the Easter bunny. Or they just stole 29A from the PSU...
If life was perfect would it be boring?
 

Online Monkeh

  • Super Contributor
  • ***
  • Posts: 5535
  • Country: gb
Re: LED strips.
« Reply #45 on: January 28, 2014, 06:54:18 am »
Even at 10mA the LED will pop at 12v

No, that's not what I said. If you forward bias a LED with 12V, one hell of a lot more current than 10mA flows. Please look into how dropper resistors work, and forward voltage vs forward current plots.

Quote
stray resistance in the order of 10's of amps, are you serious? What physical factors? The wires acting as 1000Ohms resistors and LED pins acting as current transformers?

Slow down here and think about what you're saying. Resistance isn't measured in amps, and current transformers have nothing to do with the subject..

Quote
You are telling me that 30A are going to the LED and from stray resistance and physical factors those 30A turn into 350mA?

No, that's not how current and resistance works..

Quote
I'm lost...so ok, you are right, it's just an invisible resistor made by a tooth fairy and the Easter bunny. Or they just stole 29A from the PSU...

Please, put those two on 12V and see what happens. Put the Cree one on, too, but it's expensive..
 

Offline c4757p

  • Super Contributor
  • ***
  • Posts: 7805
  • Country: us
  • adieu
Re: LED strips.
« Reply #46 on: January 28, 2014, 06:55:44 am »
The wires acting as 1000Ohms resistors and LED pins acting as current transformers?

Ohm's Law. Ohm's Law Ohm's Law Ohm's Law. PLEASE!

12V / 0.35A = 34 ohms. That's all you need to keep the current at 350mA.

Quote
I'm lost...so ok, you are right, it's just an invisible resistor made by a tooth fairy and the Easter bunny. Or they just stole 29A from the PSU...

The world is full of invisible resistors, capacitors and inductors made by the Parasitics Fairy.

As for "stealing 29A", remember that the 30A rating is just that: a maximum rating.

Some LEDs actually do have stunningly high "internal impedance" (the apparent resistance you'd get if you smooth out the exponential IV curve to look like a straight line - bit of a simplification). Look at this I/V curve from the NSSW108T datasheet:
« Last Edit: January 28, 2014, 07:05:00 am by c4757p »
No longer active here - try the IRC channel if you just can't be without me :)
 

Offline mrkev

  • Regular Contributor
  • *
  • Posts: 212
  • Country: cz
Re: LED strips.
« Reply #47 on: January 28, 2014, 07:33:45 am »
To say something about this problem and not just moralize.

Led's are usually driven in current mode, as they have low dynamic resistance when open. You can use aproximation circuit which is made from voltage source and resistor. I draw a picture, the graph on left is from datasheet of standard super bright red LED, I added an aprox. line. As you can see, slight change in voltage is gonna make much bigger change in current. To drive it by voltage is not therefore wise, however you can still do it in some cases (f.e. from battery that you know will not exceed certain voltage). Anyway, current is ALWAYS directly deppending on voltage, you can't have 20mA running through LED at 12V, ever...

There are several types of LED-strips on market. You can get current driven strips, that has no resistors and are a bit more efficient, but you need special current source for it. Standard 1210 strips has 3 diodes per segment and build in resistor (usually about 3V per diode and 3V on resistor). This resistor is all you need to drive them by voltage. F. e. in this case, 13V at input will give you current about 26mA, which is still pretty safe (with just a bit lower life expectancy). Lenght of segment is usually 5 or 2,5cm, 20mA per segment, so about 400mA or 800mA per meter.
Strips with 5050 diodes are a bit different as those diodes are in fact tripple LEDs (they have literarry three pn junctions in one device). Making the current about 1,2A or 2,4A per meter.
 

Offline Corporate666

  • Supporter
  • ****
  • Posts: 1983
  • Country: us
  • Remember, you are unique, just like everybody else
Re: LED strips.
« Reply #48 on: January 28, 2014, 09:09:05 am »
Even at 10mA the LED will pop at 12v, stray resistance in the order of 10's of amps, are you serious? What physical factors? The wires acting as 1000Ohms resistors and LED pins acting as current transformers?

You are telling me that 30A are going to the LED and from stray resistance and physical factors those 30A turn into 350mA?

I'm lost...so ok, you are right, it's just an invisible resistor made by a tooth fairy and the Easter bunny. Or they just stole 29A from the PSU...

Voltage and current are essentially the same thing (Ohms law).  More voltage = more current.

You are running a white LED which has a higher forward voltage than the red ones in the video...so less current is going to flow anyway, because more voltage is required to get the current flowing in the first place.

LED's do not draw only what they need - they draw whatever current is available at the voltage across them.  I wonder if you aren't (perhaps mistakenly) powering your white LED from the 3.3V rail?  3.3V would be right around (slightly below) the max forward voltage of the LED, so it will light up and run forever.

At higher currents, there will be more noticeable resistance in your wires, and if your LED is 3.5V and your PSU is 5V, then only 4.3 ohms of resistance is required to cause your LED to run at 350mA.  If your LED forward voltage is 3.7 and your PSU is putting out 4.9, then it's 3.4 ohms of resistance required.

As Monkeh said, if you believe an LED only draws whatever current it needs, put one across the 12V rail and see what happens.  It will pop instantly.  I'm surprised it didn't pop on the 5V rail, so I have to think there is something else at play (either it's not 5 volts or there is something causing it not to deliver 5V into that LED).
It's not always the most popular person who gets the job done.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf