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LED Drivers - what are they, really?

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jumper2high

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I've been looking around for a while now. Got a few datasheets, and they describe what the pins are named, and how are they wired into a circuit, but I could never find a good article/faq or anything else actually describing what the point of a 'driver' is.

So, could someone be kind enough to explain (or point me to a good place to find an explanation) about what do LED drivers really do. What are they used for, how are they regulated/controlled/programmed?
 

ark5230

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Simple explanation is that insted of connecting a LED to a pin in a circuit (like microcontroller or a logic IC), the LED driver pin is connected to the circuit and the driver is something like darlington pair of transistors (as in ULN 2003, ULN2803). These drivers draw current from the supply and feed it to the LED's.
 

haker_fox

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jumper2high said:
So, could someone be kind enough to explain (or point me to a good place to find an explanation) about what do LED drivers really do.
The main task of the driver is to stabilize suitable current through the LED.
 

tlihu

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So the LED drivers look like current DAC, right?
 

CMOS

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LED drivers are nothing but constant-voltage constant-current power supplies.
 

pranam77

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Thats a stright forward, simple and effective answer by "CMOS". Cheers
 

ark5230

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I am surperised! Does a constant voltage constant current power really exist. What loads such supplies could handle. Can any body give a feel or a reliable reference please.
Raoof
 

eem2am

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ITS BASICALLY A CHIP THAT ALLOWS YOU TO CONTROL (OR "REGULATE") THE LED CURRENT

(woops......sorry about capitals!)
 

CMOS

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ark5230 said:
I am surperised! Does a constant voltage constant current power really exist. What loads such supplies could handle. Can any body give a feel or a reliable reference please.
Raoof
Why do you doubt its existence ? Here is one of them
https://www.dealextreme.com/details.dx/sku.20330
There are lot more like this, do a Google search!
 

snafflekid

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I think ark thinks the power source is constant voltage and constant current at the same time! That circuit switches between modes depending on the load.

the LED drivers are constant current sources. That's because the forward voltage on LEDs vary a lot over process and if you tried to drive them with the same constant voltage, your LEDs would come out with different brightness levels.
 

CMOS

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snafflekid said:
the LED drivers are constant current sources. That's because the forward voltage on LEDs vary a lot over process and if you tried to drive them with the same constant voltage, your LEDs would come out with different brightness levels.
So lets say I have ten 3.3V 500mA LEDs conected in series and I have a 36V 500mA constant current source (no voltage regulation).
LEDs string here requires 3.3x10 = 33V to turn-on fully but since my power supply is 36V where do you think the additional 3V will go ? That will be lost in the form of heat and will also cause extra stress on the LEDs. And since supply is not voltage regulated, fluctuations may cause more power dissipation and extra stress.
So to run at maximum efficiency and for longer LED life, we use constant-voltage constant-current power supply. (33V 500mA in this case)
 

ark5230

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It seems CMOS does not have hands on experience with constant voltage and constant current sources. The recent post invites lot of questions that may go outside the interest of the basic post.
Also no such thing like constant voltage and constant current power supply is fund at the given link ( even no reference to such a thing).
Raoof
 

CMOS

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ark5230 said:
It seems CMOS does not have hands on experience with constant voltage and constant current sources. The recent post invites lot of questions that may go outside the interest of the basic post.
Also no such thing like constant voltage and constant current power supply is fund at the given link ( even no reference to such a thing).
Raoof
Seems like you have enough "hands-on" experience. Why don't you enlighten all of us here and I am sure it will still be in the interest of the main topic.
You might want to read some stuff online if you like. Here is one such example of a supply which you think does not exist.
https://freecircuitdiagram.com/2009...nstant-voltage-smps-switch-mode-power-supply/
Have you ever heard of CC-CV battery chargers? What do you think are those? Only CC supplies with CV just being marketing gimmick?? Because according to you CC-CV supplies do not exist right?

I would like to know why can't a CC-CV supply exist.
 

snafflekid

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CMOS said:
So lets say I have ten 3.3V 500mA LEDs conected in series and I have a 36V 500mA constant current source (no voltage regulation).
LEDs string here requires 3.3x10 = 33V to turn-on fully but since my power supply is 36V where do you think the additional 3V will go ? That will be lost in the form of heat and will also cause extra stress on the LEDs. And since supply is not voltage regulated, fluctuations may cause more power dissipation and extra stress.
So to run at maximum efficiency and for longer LED life, we use constant-voltage constant-current power supply. (33V 500mA in this case)

They are typically switching supplies where the voltage drop is across an inductor that dissipates power only in its DCR.
 

CMOS

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snafflekid said:
CMOS said:
So lets say I have ten 3.3V 500mA LEDs conected in series and I have a 36V 500mA constant current source (no voltage regulation).
LEDs string here requires 3.3x10 = 33V to turn-on fully but since my power supply is 36V where do you think the additional 3V will go ? That will be lost in the form of heat and will also cause extra stress on the LEDs. And since supply is not voltage regulated, fluctuations may cause more power dissipation and extra stress.
So to run at maximum efficiency and for longer LED life, we use constant-voltage constant-current power supply. (33V 500mA in this case)

They are typically switching supplies where the voltage drop is across an inductor that dissipates power only in its DCR.
Indeed; they are switching supplies. But voltage regulation is required. Basically the idea is to output exact voltage required to tun-on an LED and maintain it at specified current, thereby reducing power loss.
Linear supply would dissipate crazy amount of heat depending on input-output voltage difference and output current defeating the purpose of using LEDs (low power)
 

ark5230

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Dear CMOS
Please dont feel offended, I do have hands on experience in constant voltage or constant current power supplies.
I wish to understand (if you feel like) a 5V (constant voltage) and 1 A(constant current) power supply is connected to 5 ohm and then to 5 K ohm resistance.
What would be the curent through the two? Constant current!
Losely speaking every power supply is a constatnt voltage and constant curretn supply while supplying to a given fixed resistive load.
Raoof
 

CMOS

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ark5230 said:
Dear CMOS
Please dont feel offended, I do have hands on experience in constant voltage or constant current power supplies.
I wish to understand (if you feel like) a 5V (constant voltage) and 1 A(constant current) power supply is connected to 5 ohm and then to 5 K ohm resistance.
What would be the curent through the two? Constant current!
Losely speaking every power supply is a constatnt voltage and constant curretn supply while supplying to a given fixed resistive load.
Raoof
I think you are misinterpreting the concept of Constant Current Source. It nowhere means it will try to push specified current into any load!! But it does tell you that no matter what load you use, it will not deliver more than the specified current.
The load current will increase with decrease in load resistance till the current limit point is reached. Thereafter current is constant.

Applying Ohms law for your case,
Current through 5Ω resistor should be I = V/R = 5 / 5 = 1A
And Current through 5KΩ resistor should be I = V/R = 5 / 5000 = 1mA
In neither of the two cases current limiting will come into picture since power supply will happily deliver 1A.
Lets consider a lower resistor value of 1Ω.
Current through 1Ω resistor will be = 5 / 1 = 5A (Ideally) !!!
Now the constant current part of the power supply comes into picture since your supply is limited to 1A, only 1A will flow through the resistor.

This same thing is used in powering HB LEDs. If your supply is not current limited, LEDs will draw more current than their rated capacity and burn out. And if the supply voltage is not regulated, you are wasting power as I mentioned earlier and also reducing the life of LEDs.
Most important thing to keep in mind here is that LEDs are non-linear devices. They do not follow ohms law beyond threshold voltage so above calculations do not apply to them.

I hope you got the point.
 

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