Want to run CXA3070 at 80w is that Safe, What driver?

iPot

iPot

Well-Known Member
alesh said:
You need to understand that if you apply a voltage over a diode, specific current flows through. Since this current is heavily temperature-dependent, you need something that supplies rather specific current than voltage. CC drivers are basically voltage sources that tune the voltage precisely to a value at which rated current flows through.

If you ran a CXA3070 at 42V, you'd pushing about 3A through it which would result in heavy temperature rise and much more current flowing through.

So, yes. You would probably shorten a CXA's life to few seconds.
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So even if the driver supplies 42v and 1.8A. 3A would still be going through the led?
 
Positivity

Positivity

Well-Known Member
epicfail

epicfail

Well-Known Member
@iPot , the graph Pos posted the first page of this thread shows it. If the CXA has a Tc of 55C and you use a Constant Current driver that outputs 1.4A the CXA will draw 37V at 700mA the CXA will draw about 34.5V and at 2.8A just about 41 volts.

while I was typing he beat me too it but you need to do some more research I think before you start.
 
iPot

iPot

Well-Known Member
One more question before I think I will have a full understanding of it.
Lets say the driver supplies 2.2A and 36v. On the graph, at that current, the LED wants to draw more volts then whats available so does the LED still draw the 2.2A and 36v. Or does it draw less current to match the available voltage, 1.2A and 36v.

Im pretty sure the latter is correct.
 
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churchhaze

Well-Known Member
It draws less current to match the voltage across the diode. The diodes characteristic curve decides that.

The driver has a range of output voltages, and will vary the voltage in attempt to get a certain current flowing at a constant.

The reason you want a current source like this over a voltage source is because diodes don't follow ohms law. Instead, they follow an exponential relationship modeled by the characteristic curve on the datasheet (positivity posted it on the first page)
 
iPot

iPot

Well-Known Member
This is what I'm thinking about now. A HLG-320H-42B 7.65A wired in parallel with 4 cxa3070. That would allow 1.92A for each LED drawing about 38v. I will put a 2.2 ohm resistor on each parallel circuit to match them. Im still looking at what to do if one led blows out. I have looked at other peoples designs and this way is the cheapest way to get 288 watts of diode dissipation. How does that sound/look?
 

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churchhaze

Well-Known Member
Parallel designs are possible (Cobs themself are series strings in parallel), but then you have to worry about slight variations in the diodes forward voltages at the desired current.

The 4 cobs will split the 7.65A, but nothing guarantees it to be evenly divided 4 ways. @MrFlux uses 5 parallel strings of 5 vero 10, but has to carefully puzzle it together so the forward voltage of each string at the desired current is equal. This might be hard to do with cxa3070 since mrflux was dealing with strings of 5 whereas you'd be dealing with "strings" of 1, which means less combinations to try to get forward voltages equal..

The advantage to 1 long string is that it's very easy to regulate the current. (it's the same throughout the string). Anything on the same string is guaranteed to have the same current.

7.65A would also require very thick gauge wiring where the 4 cobs meet the supply to handle all the current. The higher voltage solution would require better wire insulation and care whereas the higher current version is harder to regulate and needs fatter wires.
 
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