unbalanced leds

[Bigbibi]

Hi.

In my DIY light I have one ELN-60-24D, 3-24 vdc, 2.5 A, which powers 3 strings of 5 leds, connected in parallel. The leds are:
Cree Xp-G
Forward voltage (@ 350 mA, 25 °C) V 2.9 3.25
Forward voltage (@ 700 mA, 25 °C) V 3.05
Forward voltage (@ 1000 mA, 25 °C) V 3.15
Forward voltage (@ 1500 mA, 25 °C) V 3.25

, so 5 leds per string should pull 0,83A, with 15+VDC

The problem is that the light comming from each string is of different intensity, so I need a simple solution in order to balance the current between the 3 strings.
Note that the strings are being used dimmed , between 1% and 40% power.

Thank you.
Claudiu

 

[mcgyvr]

Exactly why you don't do LEDs in parallel.. Despite what their mommy taught them.. They don't share nicely..

A current mirror circuit can be used..
Or better yet have a driver for each series string..

 

[Bigbibi]

Thanks for the input.
I've read about current mirror circuits, but the amount of information is too much for my skill level.

Could you be more specific and maybe share a link where I can buy such a circuit?

Thanks,
Claudiu

 

[mcgyvr]

Thanks for the input.
I've read about current mirror circuits, but the amount of information is too much for my skill level.

Then I suggest you go with an LDD driver or similar per string..

 

[Bigbibi]

Question:
If I dim the power supply (between 0% and 50%, lets say), will the ldd behave the same?
I need to dim the light...

Thanks

 

[mcgyvr]

Question:
If I dim the power supply (between 0% and 50%, lets say), will the ldd behave the same?
I need to dim the light...

Thanks

When you put led strings in parallel all bets are off..
Slight changes in the LEDs forward voltage can effect the current per string and depending on many factors the forward voltage can change unequally per string..

Worst case is thermal runaway and a fire starts...

The cheapest solution for you may be to just put them all in one series string and get a 48-52V driver in the 750mA range..
You may need to loose an LED to stay under the output voltage max of the driver..

 

[oreo57]

https://youtu.be/HU0caBfvlLI