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Topic: Running high power led's without drivers. (Read 7 times) previous topic - next topic

dhenry

Quote
you can light ten in series and ten of those in parallel, now this is the compromise, one fails only ten go out but its not as expensive as all in parallel


That's the approach they use in those led-based traffic lights.

carl1864


Am I really doing this the wrong way?  Do they need to be in series instead?

Yes, by having only one constant current driver, you then have no defined control mechanism for equal current flow to each parallel LED, your just assuming somehow that each will take its equal share, and while that may be your wish and desire that doesn't make it so. Kirchhoff has defined how current and voltage works in series and parallel circuits and is worth a review: http://physics.about.com/od/electromagnetics/f/KirchhoffRule.htm

And if so, can anyone explain why they would need to be in series and parallel doesn't work?

Well according to one of Kirchhoff's rules in a series circuit the current flow is equal at all points in the circuit. So if three leds are wired in series and 700ma is the circuit current flow then of course 700ma is flowing into and out of each led, so they all operate at the same current. But of course as the desired LED current is 700ma for your LEDs, your constant current driver needs to run at 700ma output only, you can't use one that outputs a constant 2800ma output.


This whole explanation was very helpful, and a lot more things are starting to make sense now.  I think I must have been mistakenly assuming that there is current drop the same way there is voltage drop.  Mistakenly thinking I have to add up the current of each led in series, the same way you add up voltage, but I guess this is wrong.  If I understand correctly, there is voltage drop across each led, but no current drop, so if I have 700ma led's, and a 700ma driver, I can light up as many led's as I possibly want in series, as long as the driver is putting out enough voltage?  Example if i want 100 3.5v led's, I would need a driver that outputs 350V, but only 700ma of current?

So you say I cannot power a 700ma led with a 2800ma driver?  What would happen? I was thinking as long as the driver puts out equal or more that it is fine?  Same way you can use a 12v 1A ac adaptor to power things that only use 100ma, since they only take what they need.  Or how on RC motors I use, I can use a 30A speed controller on a motor that only draws 15A if I want, since the motor only takes what it needs.  Are you saying things work completely differently when it comes to led's and drivers?

Docedison

I can verify that I think I still have a defective green light... about 25% of the lights either flicker or are dead which would point to a series - parallel configuration.
It's a big bugger... about 12 - 14 inches across and has a 14 X 14 grid of 5 mm green LED's arranged in a circle. I saw a traffic maintenance guy replacing one so I waited until he dropped the cherry picker down and I asked him for the defective one. It has 2 PCB's one for the display and one for a direct ac mains powered (No Transformer)
single output CC driver. In series each "String" will see a constant current and the current multiplies as you add more "Strings" in parallel.
This device is marked as having been Mf'd in 2005. AC power is 117VAC @ 12.6 W. This would point to Very high efficiency LED's operating at ~5 - 10 mA. I didn't want to connect it up and work on it... when I first got it because I then didn't own a 110V 1 - 1 isolation transformer and now it's a conversation piece...

Bob
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Grumpy_Mike

#38
Oct 09, 2012, 05:08 am Last Edit: Oct 09, 2012, 05:12 am by Grumpy_Mike Reason: 1
Thanks, it sounded to me like you were advocating each LED was in parallel not the LED and current limiting device in parallel.

Quote
So you say I cannot power a 700ma led with a 2800ma driver?  What would happen? I was thinking as long as the driver puts out equal or more that it is fine?  Same way you can use a 12v 1A ac adaptor to power things that only use 100ma, since they only take what they need.  Or how on RC motors I use, I can use a 30A speed controller on a motor that only draws 15A if I want, since the motor only takes what it needs.  Are you saying things work completely differently when it comes to led's and drivers?


We are saying that there is a difference between a linear load and a non linear load. Also a constant current supply will keep upping the voltage until that current is reached. So running a 700mA LED from a 2500mA constant current supply will fry the LED by putting a lot more current down it than it can stand.

Things work differently with a constant current supply than a constant voltage supply.

retrolefty

#39
Oct 09, 2012, 06:26 am Last Edit: Oct 09, 2012, 06:43 am by retrolefty Reason: 1


Am I really doing this the wrong way?  Do they need to be in series instead?

Yes, by having only one constant current driver, you then have no defined control mechanism for equal current flow to each parallel LED, your just assuming somehow that each will take its equal share, and while that may be your wish and desire that doesn't make it so. Kirchhoff has defined how current and voltage works in series and parallel circuits and is worth a review: http://physics.about.com/od/electromagnetics/f/KirchhoffRule.htm

And if so, can anyone explain why they would need to be in series and parallel doesn't work?

Well according to one of Kirchhoff's rules in a series circuit the current flow is equal at all points in the circuit. So if three leds are wired in series and 700ma is the circuit current flow then of course 700ma is flowing into and out of each led, so they all operate at the same current. But of course as the desired LED current is 700ma for your LEDs, your constant current driver needs to run at 700ma output only, you can't use one that outputs a constant 2800ma output.


This whole explanation was very helpful, and a lot more things are starting to make sense now.  I think I must have been mistakenly assuming that there is current drop the same way there is voltage drop.  Mistakenly thinking I have to add up the current of each led in series, the same way you add up voltage, but I guess this is wrong.  If I understand correctly, there is voltage drop across each led, but no current drop, so if I have 700ma led's, and a 700ma driver, I can light up as many led's as I possibly want in series, as long as the driver is putting out enough voltage?  [font=Verdana][font=Verdana]You got it now.[/font][/font]

 Example if i want 100 3.5v led's, I would need a driver that outputs 350V, but only 700ma of current?

Almost, you need a driver that can raise or lower it's voltage to at least 350 volts while maintaining a constant current of 700ma. There may be a time (say at low temperature) where it actually has to lower the voltage some to maintain the same constant 700ma of flow. The current driver is really a current regulator, just as a voltage regulator works by maintaining a constant output voltage even with variable load resistance and or variable input voltage to the voltage regulator.


So you say I cannot power a 700ma led with a 2800ma driver?  What would happen?

The constant current driver will force 2800ma of current through the LED, but only for awhile as soon the led will melt or explode open. Remember an LED cannot by itself control the current flowing through it, it's not like a incandescent  lamp that has a fixed resistance which controls the current at a given voltage per ohm's law. LEDs don't obey Dr. Ohm. Once a led is forward biased by a voltage equal or greater then its Vf spec it acts like a direct short circuit and will self-destruct unless the current is controlled or limited by something external to the LED

I was thinking as long as the driver puts out equal or more that it is fine?
No, a [font=Verdana]constant current[/font] driver puts out only a single value all the time, its rated value of say 2800ma.
 Same way you can use a 12v 1A ac adaptor to power things that only use 100ma, since they only take what they need.
That applies to [font=Verdana]constant voltage[/font] sources, where the current flow is determined by only the load resistance per Dr. Ohm, up to the maximum current capacity of the voltage source.

 Or how on RC motors I use, I can use a 30A speed controller on a motor that only draws 15A if I want, since the motor only takes what it needs.  Are you saying things work completely differently when it comes to led's and drivers?

No I'm saying constant voltage sources are different then constant current sources. LEDs are current operated and controlled devices (not voltage controlled) so they are best controlled by using a constant current source. If we must use a constant voltage source to power a led we need to add something else to control/limit the current, which is normally a simple series resistor.

Lefty



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