Author Topic: LED strips.  (Read 11521 times)

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Offline elektroshok

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Re: LED strips.
« Reply #25 on: January 27, 2014, 06:11:44 pm »
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.
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Offline Monkeh

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Re: LED strips.
« Reply #26 on: January 27, 2014, 06:12:28 pm »
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 27, 2014, 06:15:28 pm by Monkeh »
 

Offline elektroshok

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Re: LED strips.
« Reply #27 on: January 27, 2014, 06:15:24 pm »
Ok sorry, my mistake, 320mA, but not 30A that the PSU is rated at. So why doesn't the LED go to 30A.
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Offline Monkeh

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Re: LED strips.
« Reply #28 on: January 27, 2014, 06:17:11 pm »
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 27, 2014, 06:24:28 pm by Monkeh »
 

Offline Mr Smiley

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Re: LED strips.
« Reply #29 on: January 27, 2014, 06:24:22 pm »
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

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Offline elektroshok

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Re: LED strips.
« Reply #30 on: January 27, 2014, 06:26:36 pm »
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.
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Offline Monkeh

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Re: LED strips.
« Reply #31 on: January 27, 2014, 06:28:08 pm »
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

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Re: LED strips.
« Reply #32 on: January 27, 2014, 06:29:24 pm »
I put the LED on 5v not 12v, why do you twist everything ?

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

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Re: LED strips.
« Reply #33 on: January 27, 2014, 06:31:56 pm »
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!).
 

Offline Monkeh

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Re: LED strips.
« Reply #34 on: January 27, 2014, 06:44:44 pm »
Two more red LEDs have died a violent death at the hands of a cheap, nasty power supply.

 

Offline elektroshok

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Re: LED strips.
« Reply #35 on: January 27, 2014, 06:49:48 pm »
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.
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Offline Monkeh

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Re: LED strips.
« Reply #36 on: January 27, 2014, 06:51:18 pm »
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

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Re: LED strips.
« Reply #37 on: January 27, 2014, 06:55:18 pm »
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).
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Offline Monkeh

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Re: LED strips.
« Reply #38 on: January 27, 2014, 06:59:58 pm »


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

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Re: LED strips.
« Reply #39 on: January 27, 2014, 07:20:43 pm »
I find it really painfull to read this thread... So much fail  :palm:
 

Offline elektroshok

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Re: LED strips.
« Reply #40 on: January 27, 2014, 07:35:34 pm »
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Offline Monkeh

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Re: LED strips.
« Reply #41 on: January 27, 2014, 07:37:49 pm »
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 27, 2014, 07:47:58 pm by Monkeh »
 

Offline elektroshok

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Re: LED strips.
« Reply #42 on: January 27, 2014, 07:42:08 pm »
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?
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Offline Monkeh

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Re: LED strips.
« Reply #43 on: January 27, 2014, 07:43:54 pm »
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 27, 2014, 07:50:13 pm by Monkeh »
 

Offline elektroshok

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Re: LED strips.
« Reply #44 on: January 27, 2014, 07:51:00 pm »
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...
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Offline Monkeh

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Re: LED strips.
« Reply #45 on: January 27, 2014, 07:54:18 pm »
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

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Re: LED strips.
« Reply #46 on: January 27, 2014, 07:55:44 pm »
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 27, 2014, 08:05:00 pm by c4757p »
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Offline mrkev

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Re: LED strips.
« Reply #47 on: January 27, 2014, 08:33:45 pm »
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

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Re: LED strips.
« Reply #48 on: January 27, 2014, 10:09:05 pm »
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).
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