LED current source problem

Hi All,

Instead of using commercial variable power supplies, I tried to build a simple variable current source for a LED. However, LEDs kept burning out on me and left me puzzled. The circuit diagram is attached. Any hint will be much appreciated. Thanks in advance.

Problem seen:
When replacing the LED with simple amp meter, the current reads what I expected it to be. But when I just connected the LED with the BNC connector, there was an initial burst of light, and then ... the LED is dead.
Almost identical problem happened when I simply used a 18V supply and put a LED and a power resistor in series. Initial burst and that killed the LED, though the max current the circuit can draw is no where near the current rating.
Even a Schottky diodo, a resistor and a capacitor were added in parallel to the LED to protect from reverse voltage (if any), I still saw the initial deadly burst.
Again, any idea will be a great help. Thanks.

Parts:
LED: CREEXTE-W130 from LEDSupply, current rating 1500mA.

I don't think you have a variable current source, although Rs should limit the current to something less than 1.5A

Is the LED well heat-sinked?

Even if the transistor was shorted out C-E the LED would only pass 18V - 3V (typical Vf) / 40 = 0.375A so something else is wrong. I think kam1787 is on the right track, the reason for the brief flash is it takes that long to overheat. This kind of LED has to be kept cool and MUST be mounted on a suitable heat sink at that current. Most manufacturers would use an Al backed PCB but you might be able to get away with a large copper area or for test purposes, to bond it to a metal heat sink.

Brian.

Hi All,

Instead of using commercial variable power supplies, I tried to build a simple variable current source for a LED. However, LEDs kept burning out on me and left me puzzled. The circuit diagram is attached. Any hint will be much appreciated. Thanks in advance.

Problem seen:
When replacing the LED with simple amp meter, the current reads what I expected it to be. But when I just connected the LED with the BNC connector, there was an initial burst of light, and then ... the LED is dead.
Almost identical problem happened when I simply used a 18V supply and put a LED and a power resistor in series. Initial burst and that killed the LED, though the max current the circuit can draw is no where near the current rating.
Even a Schottky diodo, a resistor and a capacitor were added in parallel to the LED to protect from reverse voltage (if any), I still saw the initial deadly burst.
Again, any idea will be a great help. Thanks.

Parts:
LED: CREEXTE-W130 from LEDSupply, current rating 1500mA.

Click to expand...

Hi jfhsu
Congratulations ! you have made a circuit for cooking foods ! such a circuit will deal with a lot of dissipation and inaccuracy . furthermore , if you're attempting to use a current source , you can simply use a programmable current source using an op amp and a transistor as well .

May i ask you something ? why you don't wanna use a simple voltage source instead ? perhaps a simple SMPS to avoid dissipation ?

Best Wishes
Goldsmith

betwixt said:

Even if the transistor was shorted out C-E the LED would only pass 18V - 3V (typical Vf) / 40 = 0.375A so something else is wrong. I think kam1787 is on the right track, the reason for the brief flash is it takes that long to overheat. This kind of LED has to be kept cool and MUST be mounted on a suitable heat sink at that current. Most manufacturers would use an Al backed PCB but you might be able to get away with a large copper area or for test purposes, to bond it to a metal heat sink.

Brian.

Click to expand...

Thanks kam1718 and Brian.
The LED is always mounted on a pad, which is in turn on a aluminum block with heatsink paste. It works with regular power supply but not with my homemade circuit.

- - - Updated - - -

goldsmith said:

Hi jfhsu

May i ask you something ? why you don't wanna use a simple voltage source instead ? perhaps a simple SMPS to avoid dissipation ?

Best Wishes
Goldsmith

Click to expand...

Goldsmith,
I used a current source simply because when using commercial variable powersupply and driving the LED I used current limited mode. Without too much consideration, I went with a current source which I can calculated how much current that's flowing through the LED.
Thanks.

Goldsmith,
I used a current source simply because when using commercial variable powersupply and driving the LED I used current limited mode. Without too much consideration, I went with a current source which I can calculated how much current that's flowing through the LED.
Thanks.

Click to expand...

Hi again
I think you're dealing with some difficulties in design procedure of power supplies and that's why you have selected a way which you think it's simplest way .
I suggest you to make your problem like this : i've 5 LEDs ( for instance ) each one of them will require 100mA ( for example ) and each one require 5 volts ( again for example ) .
Solution :
1-What kind of arrangement is the best one to glow them in max level of light ? ( series or parallel or or complex ? )
2-then , is my project for fun or perhaps for my home using or perhaps it's gonna be a commercial project in wide range ?
If it's commercial , everything must be considered to decrease the cost as low as possible to make it economical . if it's for home or personal using , high precision in calculations and rough considerations of design are not required .
So which one ?
Arrangement your problem like this , then i can lead you through the best solution . ( however if you are only interested in current source i can guide you about that too although i don't offer that to you too . )
Best Wishes
Goldsmith