Unbalanced LED current in parallel strings. Let's assume you have two strings of perfectly identical, perfectly matched LEDs on identical heatsinks in the same environment, but running quite warm. Initially they will draw the same current. Let's assume that some factor leads to one heatsink being slightly less efficient for an instant. This will lead to one set of LEDs (or even just one LED) getting slightly warmer. This will cause it's Vf to drop a little, which will unbalance the current in the to strings. This will cause the already slightly warmer string to draw more current and the slightly cooler one to draw less. The process will continue until some equilibrium is reached. This may be one string using 100% of the current from a constant current source, or the rate of heat transmitted to the environment starts to rise faster than the rate of heat increase (heatsinks transfer more energy as the differential between it's and ambient temperature increases).
Sorry for putting that question on you, I totally agree with your scenario description.
My questions are out of interest in the general use of LED's and for specific need.
No invalidation, just playing the scenario.
There is a small variation in outcome if the string has separated LED's or they run parallel to their partner string sharing the same PCB and HeatSink. Not a real measure worth worrying about, but an effect.
This is my Scenario in General Current Mismatch:
Based on the Mismatch coming from the LED Characteristics. Your scenario is also valid.
Let’s say we started with known LED’s from the same batch with similar trends.
EG..
With 2 x 2 LED’s
Let’s say we have a Maximum of 10% for variation with about 33% chance of it occurring.
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With 50 x 2 LED’s
To match the 2x2 scenario we need 50 LED’s in 1 string trending 10% different then the 50 opposite.
Of course, these numbers and the extreme comparison are not real.
My question is, does this basic logic work or make any difference if we design the balancing circuit or choose resistors with consideration for required correction?
Based on some base line assumptions and odds.
Maximum possible variation in % of Total voltage?
Example: 1 LED’s value.
Percent of correction you would like to achieve in the worst case scenario?
Example: 75% Correction.
Does this make sense?
Anybody want to contribute thoughts on real numbers you use for the possible variation and % correction?
Knowing if there is any logic in this scenario would help with discussion for balancing.
I would also like to know how you would apply balancing to our 151v strings within or outside PWM circuit?
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Most semiconductors fail short circuit, so a MOSFET failure will result in loss of current control. The current can rise to high value and can damage the LEDs before the fuse blows (if you have one). Regardless of whether or not you have a fuse, that string is no longer functional. If the one mosfet controls
Multiple strings then they will all go out. Keeping all the strings independent at the cost of a few cents seems reasonable (unless you're Chinese
).
My question was in consideration for some type of spike with failure.
The fail short answers the question, so in this case we fall back to the maximum current of driver.
I'm not sure what your on about with power sharing and timers. It should not be an issue.
The driver is 150v and I was concerned about glitches such as occur in power from relay and Solenoid valves on the same power as 555. I will connect the 555 to a separate AC/DC supply, $10 or I may have one lying around.
I am looking at the possibility of a 100% switch at the 555/Mosfet interface.
if posible 100% would be default use, PWM would be used on rare occasion to experiment down to 50% duty or 500mA.
Perhaps just install the PWM and Mosfet if needed, makes sense
In which case any balancing components would have to be separate, perhaps a negative?
At some point I will be building this array and might not have internet for reference.
I am learning as much as I can now, please do not think you are being ignored.
I am listening very carefully to your advice and may not make up my mind for some time.
There are a lot of possibilities to leave open, and possibly 2 more of these arrays to build.
All Good?
