Author Topic: LED strips.  (Read 10696 times)

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Offline ShadyK

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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

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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

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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

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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.
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Offline Melkor

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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

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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

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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.
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Offline Towger

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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

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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?
 

Offline Monkeh

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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

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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 »
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Offline Monkeh

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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

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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?
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Offline Monkeh

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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

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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?
 

Offline Monkeh

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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

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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?
 

Offline Monkeh

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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

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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?
 

Offline Monkeh

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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 »
 

Offline Monkeh

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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

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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.
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Offline Monkeh

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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

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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?
 

Offline Monkeh

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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?
 


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