Basic question, and I expect I know the answer, but I'm terrible at semiconductors and would like confirmation as I try to troubleshoot my problem.
Say I have 4 LEDs in parallel, with a fixed current at roughly and voltage supply that. Will current be shared evenly between the LEDs, but be dimmer or will some turn on and others stay off based on internal characteristics of the circuit and diode? Or will this depend on if there is enough current to turn on each LED?
Basically, in my circuit, I have 4 RGB LEDs which each share a single current limiting resistor per color. When I apply power to all 4 as a test, only 2 light up, but as far as I can tell, all 4 should be a complete circuit. The intention was to light up only one LED at a time, so I can use that to test, but I'm trying to understand why all 4 don't light up dimly (And it would be a nice self-test).
Each LED requires it's own current limiting resistor.
If not then the one that turns on first takes all the current and does not let the voltage rise enough to turn the others on.
Ah yes - this brings back fond memories. My first LED circuit with a single RGB. Being used to mathematics and programming where factoring out a common component is a correct and good practice, I applied the same to all those resistors I had on all those LEDs down to a single resistor.
I then spent a number of hours trying to understand why I could not get the RGB colour rainbow code to work, thinking I had a software problem.
The cause - you can NOT factor the resistors round that way. Each LED its own resistor. Lesson learned.
Paul__B:
Unless of course, you obtain al the LEDs from the same manufacturing batch.
No you still can't use a single resistor foe LEDs in parallel. For one thing they age at different rates. For another even the same batch has a spread in Vf.
For the sake of a resistor a $0.001 component is it worth it?
Grumpy_Mike:
No you still can't use a single resistor foe LEDs in parallel. For one thing they age at different rates. For another even the same batch has a spread in Vf.
Grumpy_Mike:
No you still can't use a single resistor foe LEDs in parallel. For one thing they age at different rates. For another even the same batch has a spread in Vf.
How then do they manufacture 100W LEDs?
I have not idea how that question relates to the current discussion.
You can make the individual LED brighter, and you can connect lots of LED in SERIES. If you have enough of them you can have 100V and still only pass 20mA and every LED is bright.
Lastly there are active current regulation circuits - there is one in every "mains voltage" LED lamp.
Actually, my problem resulted from having the JTAGEN fuse set (I'm not trying to do Arduino, but it seemed like a good place to ask anyways).
My intention was to never light up all 4 at once, just one at a time, but it lead me to the question since my initial test was, of course, to light them all up at once, because I am smart like that. The other concern I had was board space/assembly/respin concerns. The price of a few resistors isn't high, but I had already ordered the board and don't really want to wait 3 weeks for another from OSHPark, and board space in that area was limited and working with smaller parts gets difficult when building by hand. I am also limited in pins I can use, since I really don't want to add in a shift register. Since color will always be consistent, I was going to leave the color the same, and change the pin, similar to say a 7-segment display.
What is the proper way to hook up an Common cathode RGB LED at full brightness? I'm working off the assumption that each channel of the RGB can handle say 20mA. So this means the total current could be 60mA. But a single resistor for 60mA would be too much for a single channel. Should there just be a 20mA limiting resistor per channel per LED?
Should there just be a 20mA limiting resistor per channel per LED?
Yes.
but I had already ordered the board and don't really want to wait 3 weeks for another from OSHPark, and board space in that area was limited and working with smaller parts gets difficult when building by hand.
Do what the professionals do. Get a scalpel, cut the track and solder a surface mount resistor across the track. No extra space needed.
But a single resistor for 60mA would be too much for a single channel.
I thought we have been trough that, if it is to be 60mA then the LEDs would all be on except they wouldn't and the red one would get all the current, probably being too much for it.