New to LDD Drivers

[tet024]

Hi Guys,


After frying a couple of leds I came to the below conclusion. Could someone please tell me if i'm on the right track or not? For this test i'm using an LDD Driver and PI to DIM the leds.


For example if i'm using the LDD-700 this would constantly output 700ma of current. Now if i have a string of 10 leds in which each led has a working 3.3v and working current of 700ma. The voltage drop across the leds would be that of round about 33v. Now if the input of the LDD drivers would be of 40v and PWM 4096 it would output 40v so I would need to limit the MAX PWM to 3000 for example to meet the 33v output.


I gave the following example because i shall be having different LED's channels which have different voltage drop so I my conclusion was to check with a multi-meter the output voltage of the LDD driver and limit the MAX PWM to that voltage.


Am i on the right track or not?


My setup is as follows maybe someone could help me out.




LED UV x 20 - DC forward Voltage 3.4 - 3.8 Current 700mA
LED Cyan x 10 - DC forward Voltage 3.4 - 3.6 Current 700mA
LED Blue x 20 - DC Voltage 3.4 - 3.8 Current 700mA
LED Royal Blue x 20 Cree XT-E Current 1500mA
LED White x 20 Cree XT-E Voltage 3.2 - 3.6 Current 1500mA
LED RED x 10 - DC forward Voltage 2.7 - 3.0 Current 700mA




I was thinking to split each channel into 10 thus having 2 channels for UV,BLUE,ROYALBLUE and White. Thus having 10 Channels in all. I was thinking to use a 38v 10AMP PSU to run these.

Any Suggestions?

Thanks,
Kindest Regads,
Et

 

[Gorgok]

The LDD is a constant current driver. It regulates current by varying voltage (within the range its capable of). Your PWM control limits the current, voltage is still controlled by the driver internally. No need to limit the PWM to anything, unless you want the current to never hit the 700mA max.

 

[Pny]

The Pwm dimming dos not change the current nor the voltage, it does only pulse its output and change the duty cycle to dim the LED.

 

[perkint]

As above (and my post on the UK forum you posted this one), you are not quite on the right track. Within reason, you can completely ignore the output voltage from the LDD. It will sort that out, you don't need to. So long as you use an LDD appropriate to the LEDs you are using (ie it is rated at or below the max drive current of the LEDs) then you are good to go. And if it is rated above the max drive current, it will probably blow them even on 1% PWM...

Tim

 

[tet024]

So today I did the experiment with one led. Blue which had a current rating 700m and vlt 3.4- 3.8. I connected it to a 700H ldd and when powering on with one LED it was outputting 4.15 Volts which is higher then what the LED support after 30min the led blew up practically it dimmed drastically. Could it be that I must use a driver which less outputs less current.. So if the led says 1000 i give them 700 if the led say 700 i give them 500 so on an so forth.

 

[Silent Mike]

So today I did the experiment with one led. Blue which had a current rating 700m and vlt 3.4- 3.8. I connected it to a 700H ldd and when powering on with one LED it was outputting 4.15 Volts which is higher then what the LED support after 30min the led blew up practically it dimmed drastically. Could it be that I must use a driver which less outputs less current.. So if the led says 1000 i give them 700 if the led say 700 i give them 500 so on an so forth.

What was your input voltage? It could be that the LDD can only regulate the voltage down so far.

 

[tet024]

12volts was the input voltage

 

[oreo57]

What was your input voltage? It could be that the LDD can only regulate the voltage down so far.

LDD-H's are listed at

2-52V

... so that shouldn't be a problem..
I vote for thermal run away at that current in (possibly) free air....

Or poorly spec-ed diode..
"Testing" 3W cheap LED's w/ a Lithium button cell.. and they heat really badly in "free air"..(4.1-ish volts, obviously no current regulation)

O/p mounted diode or not???

Second thing was the diode hooked to the LDD before powering up.. W/ some drivers if you hot swap the LED into the driver there is not enough time to "throttle down" the voltage..causing high voltage and high current draw..

And to reiterate some of the above.. "Dimming" at 50% means 700mA fed at 50% of the time..for most current drivers.. EVEN regardless of how the dim signal is presented.. Even a 0-10V signal at 5V will output 700mA at 50% of the on time..
some drivers are different (actually restricting current and shifting color output) BUT they would be drivers I wouldn't use..

after 30min the led blew up

I'm still leaning on thermal runway.. and 4.1V "is" the V(f) at 700mA (possible measurement error) for "that"diode.. got a spec sheet???

Actually Epistar LED's w./ 700mA max have a V(f) of over 4V..
http://www.wayjun.com/Datasheet/Led/3W High Power LED.pdf

see pg 10.. Their charts don't exactly match their tabled specs..

 

[perkint]

My money is also on thermal run away. Are the LEDs mounted onto a star and the star mounted to a heatsink? With thermal paste/adhesive? Because if they were not, I would expect to see exactly what you are describing!

And there are benefits to running an LED at 700mA even when it is rated to 1000mA, but not that you'd see in a 30minute test..

Tim