I want to drive and control the intensity of 4 LEDs (rated at 2.2 V , 0.5A)
My two approaches are
1: from a power supply i drive the LEDs through PWM. Problem is as LEDs get hotter their resistance changes and they get burned after a while despite me keeping PWM steady
so i need to go to second approach
2. From a power supply i drive the LEDs via a current regulator. But now i need somehow to
A: monitor the current via the arduino
B: A feedback loop so i can control the current regulator from arduino.
Are there any nice solutions for my second approach? i need this to be robust because these LEDs are quite expensive and cant afford to burn more of them:P
How about a schematic? if you have 4 of these in series, the voltage drop is 8.8 V which is more than an arduino can handle. Are you using a transistor or MOSFET to control it?
High power LEDs (1W or more) are usually driven by a [u]constant current source[/u] (or "controlled current" for dimming). The current is fixed and the voltage "magically" falls into place. Typically, they have "extra" voltage available so you can connect multiple LEDs in series.
A switchmode constant-current power supply (or any switchmode power supply) is not an easy thing to build yourself...
Some of these LED drivers/power supplies can be controlled with PWM (10V PWM is standard for this) but the current through the LED is usually DC.
Regular little LEDs use a resistor for current limiting and PWM can be used to make the LED appear dim, but PWM does not reduce the peak current (or voltage). You can use the same approach with high-power LEDs but you need a power resistor and often as-much, or more, power is wasted in the resistor as-is used by the LED so it's inefficient and it's a "Mickey Mouse" design.
From a power supply i drive the LEDs via a current regulator. But now i need somehow to
A: monitor the current via the arduino
B: A feedback loop so i can control the current regulator from arduino.
Just like with a regular (constant-voltage) power supply, the regulation-feedback is normally built-into the hardware. If there is a PWM control-input, you'll get 50% current at 50% PWM, etc.
Low voltage constant current LED drivers, like the FemtoBuck (single LED string) and PicoBuck (three strings) from Sparkfun are designed for this. If know what you're doing, then you can find similar/clones on ebay.
On these boards you find one (or two) low value current sense resistor.
On the above boards, 0.3 ohm (x2).
The formula for that is "100/sense resistor in mA", so 100/0.3= 333mA (= ~1watt for a white LED with a Vf of ~3.3volt)
The second resistor on those boards can be selected with a solder bridge, to increase current to 2*333mA (666mA)
If you need exactly 500mA, then you must replace the resistors with a 0.2 ohm resistor (100/0.2= 500mA).
You can of course still PWM these boards, to dim and/or lower the average LED current.
But it will never go over the set max current.
These boards can also be 'voltage' dimmed.
The ramp is usually between about 0.25 and 2.5volt (see the datasheet of the used chip).
LED don't usually 'burn' from over-current. They only 'age' faster. They usually fail when getting too hot.
A Vf of 2.2volt at 500mA is 1.1watt, and that NEEDS a heatsink.
Leo..
PS
There are also ready made 500mA modules, like the Meanwell LDD-500
nsansen:
I want to drive and control the intensity of 4 LEDs (rated at 2.2 V , 0.5A)
Sorry, totally inadequate and useless description.
We need to know:
Web link giving specifications. datasheet of these LEDs.
The purpose for which you propose to use them.
The range of dimming you require.
What Arduino you propose to use?
nsansen:
from a power supply i drive the LEDs through PWM. Problem is as LEDs get hotter their resistance changes and they get burned after a while despite me keeping PWM steady
Are you saying you actually did this, having no idea whatsoever what you were doing?
You mean to say that OP is trying to control current with PWM only,
without any current limiting resistor for the LEDs?
Nahh, can't be that, can it?
:o
A Vf of 2.2volt seems too high for IR, and too low for a 1watt red LED.
We indeed need to see the datasheet.
And a circuit diagram.
Leo..
Wawa:
You mean to say that OP is trying to control current with PWM only, without any current limiting resistor for the LEDs?
Nahh, can't be that, can it?
Sounds like it!
Red LEDs?
Good point. I assumed he would be wanting white LEDs, and yes, 2.2 V is not it.
I thought for a moment that asking for the datasheet for a full specification might just be a bit excessive. I was clearly wrong, wasn't I?
THESE leds are different than usual Leds (have three pin out) and i dont think you can install a heatsink
The DC power supply for each of the string of four leds in series will be powered by external adjustable power supply of 24V.
Range of dimming is ideally from 0%-100% but it is ok if i can have 60%-10%.
My plan is to control the constant current source only for dimming and switching on-off so i don't need to use mosfets as seen in an old school schematic
Power supply 24V-------------------Constant current driver---------LED1---LED2---LED3---LED4
So just a narrow beam blue and IR LED in a TO-39 case.
Assuming you use them for illumination, not pulsed/data/remote control.
They won't survive that (continuous) current without heatsink.
You must find a way to cool them. Google "TO-39 heatsink".
Note that one of the LED terminals is connected to the case (can't use a single heatsink for four).
Four LEDs in series on a 24volt supply is too much for linear current limiting (mosfet/resistor).
Must use switching constant current LED drivers, as explained.
Meanwell modules (350mA and 500mA) are an easy solution, and can be controlled (PWM) with a Nano.
Leo..
Thank you very much LEO, i will look for 350ma and 500ma Connstant current drivers .. i am getting slightly confused on if they are pwm compatible with 5V (as in arduino) . some of them indicate pwm with 10V so i am trying to find a suitable one .
nsansen:
Thank you very much LEO, i will look for 350ma and 500ma Connstant current drivers .. i am getting slightly confused on if they are pwm compatible with 5V (as in arduino) . some of them indicate pwm with 10V so i am trying to find a suitable one .
If they are 10V PWM, then just add another transistor, connected to your 24V PS through a voltage divider to drive the current source at 10V while the arduino drives the transistor at 5V
The drivers I linked to and the LDD-xxx modules are 5volt compatible.
Industrial and mains powered drivers could be 10volt PWM.
Ask (with a link to the product) before you order.
Leo.
P.S.
If I may ask, what are you trying to do with these odd LEDs.
I can understand narrow-beam IR for things like very long distance beam-break (>50m),
but narrow-beam blue??
Thank you very much again. I am trying to make a Luminescence dating device. Not sure if i will succeed but i will give it a try. (since i already have necessary leds and some other instruments).
