I was looking at the xitanium constant current PSU data sheets.
On bottom of page 2 (please look!) they have the reccommended wiring schematics, where they utilise parallel and series combined. This way you can power lots of LEDs from a single PSU, which is cool.
Now lets take the following arrangement as our example:
They have models (not shown on the above datasheet) that pump out 4A, which lets say we use to power 24 crees, in 4 parrallel legs of 6 leds in series per leg (as per data sheet diagrams). This way each leg recieves 1A.
All pretty standard so far, however whats got me thinking is this, hope I can explain clearly: The circuit is up and running as per above. If one LED in any leg fails (or a connection anywhere becomes loose), that whole leg is taken out of the equation, and suddenly the other legs are getting 1.3A each. The remaining three legs are now recieving 30% more current and are in effect overdriven (if we designed the fixture, sinking etc for 1A operation) and this will eventually cause another LED somewhere to blow soonish. Then the two remaining legs are pushed at 2A and surely will fail very very soon, if not instantly. So in effect failure of just one LED can lead to a cascading blowout of many more LEDs; which can end up expensive and total darkness to boot too! This is not what I would assume is a desirable thing!
So question is how do we insure ourselves against such a possibility?
Would resistors work as a current limiter? In my thinking and comprehension I only see resistors used when the supply voltage is above the LED Vf, ie to set up the circuit, not as an insurance, especially when the dynamics of the circuit change as in this case. Also if we put a big enough resistor in there to prevent a blow out, what does this resitor do to the circuit during normal operation?
I can also think of maybe having a 1A fuse in each leg, but this would mean that if an LED fails, then 3 fuses will surely blow, which is better than instaflashing 18 LEDs though!
I can also think of a LM317 (in current regualtor type thing on every leg, but as with the resistors I wonder if its desirable to have it (in terms of heat and wasted energy during normal running).
I have given the above numbers only as an example here; whether crees can take 1.3A or 2A etc is not the question, finding the correct concept is the aim of this thread.
So what is the trick for preventing cascading failures in parrallel circuits?
PS: Also I just realised, that the same question applies to setting up and first time testing of your circuit as well. If you have a bad connector somewhere, or wired even one LED wrong, just powering up the circuit for the first time can have the same effect! How would you test and power up your setup safely for the first time?
On bottom of page 2 (please look!) they have the reccommended wiring schematics, where they utilise parallel and series combined. This way you can power lots of LEDs from a single PSU, which is cool.
Now lets take the following arrangement as our example:
They have models (not shown on the above datasheet) that pump out 4A, which lets say we use to power 24 crees, in 4 parrallel legs of 6 leds in series per leg (as per data sheet diagrams). This way each leg recieves 1A.
All pretty standard so far, however whats got me thinking is this, hope I can explain clearly: The circuit is up and running as per above. If one LED in any leg fails (or a connection anywhere becomes loose), that whole leg is taken out of the equation, and suddenly the other legs are getting 1.3A each. The remaining three legs are now recieving 30% more current and are in effect overdriven (if we designed the fixture, sinking etc for 1A operation) and this will eventually cause another LED somewhere to blow soonish. Then the two remaining legs are pushed at 2A and surely will fail very very soon, if not instantly. So in effect failure of just one LED can lead to a cascading blowout of many more LEDs; which can end up expensive and total darkness to boot too! This is not what I would assume is a desirable thing!
So question is how do we insure ourselves against such a possibility?
Would resistors work as a current limiter? In my thinking and comprehension I only see resistors used when the supply voltage is above the LED Vf, ie to set up the circuit, not as an insurance, especially when the dynamics of the circuit change as in this case. Also if we put a big enough resistor in there to prevent a blow out, what does this resitor do to the circuit during normal operation?
I can also think of maybe having a 1A fuse in each leg, but this would mean that if an LED fails, then 3 fuses will surely blow, which is better than instaflashing 18 LEDs though!
I can also think of a LM317 (in current regualtor type thing on every leg, but as with the resistors I wonder if its desirable to have it (in terms of heat and wasted energy during normal running).
I have given the above numbers only as an example here; whether crees can take 1.3A or 2A etc is not the question, finding the correct concept is the aim of this thread.
So what is the trick for preventing cascading failures in parrallel circuits?
PS: Also I just realised, that the same question applies to setting up and first time testing of your circuit as well. If you have a bad connector somewhere, or wired even one LED wrong, just powering up the circuit for the first time can have the same effect! How would you test and power up your setup safely for the first time?
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thinking of wire gauges?
