Team,
I am testing blue leds for a cube project; all are in parallel fed with 5V via a common 100 ohm resistor to drain.
All light up, even with up to 15 in the circuit... but as soon as I place an alternate manufacture red, green, or yellow led in the path, all the blue leds turn off, and only the newly placed led lights....
I would figure the voltage drain of the red would not be an issue, so am really confused and bewildered....
Any feedback would be appreciated.
Joe M
Different colour LEDs use different semiconductor materials with different bandgaps, so the forward voltages will be different.
Place a red and blue LED in parallel and the red LED will start conducting at its forward voltage (1.7..1.9V or so), and the blue one won't ever see enough voltage to start conducting at all...
With well-matched LEDs you can get away with running in parallel, but its not a good idea unless all the same manufacturer and type. Certainly you shouldn't expect this to work with different devices.
You could be unlucky even with LEDs of the same type - different manufacturing batches or production lines might give variations in forward voltage.
In general one drives LEDs with a current, not a voltage, since the brightness depends on the current, the forward voltage depends on manufacture and temperature as well as the current.
Thanks Mark.
Been messing with electronics for years and still this voltage vs current thing confuses me at times.
I was just testing blue leds for a project, to ensure relative brightness, then received a led multipack and wanted to see what I received, so placed them in my circuit to see, and found that every color, size and shape would kill the five blue leds that were in parallel.
R,
Joe
all are in parallel fed with 5V via a common 100 ohm resistor to drain.
Each LED should have its own limiting resistor. If the connected LED's have different forward voltage drops, choose a suitable resistor value for each type. This should work.