hello
i am in the process of building a 5 x 5 x 5 led matrix using some high power LED's - the ones i am keen on using have a fairly high forward current of 300mA and 3.0v DC forward voltage. I have been told on this forum that i will not be able to use a series resistor and instead need a constant current drive. I was hoping to get some advice regarding this:
could I get just one fairly powerful constant current driver for all of the LED's? if so - has anyone had much luck with this? I need something that can power 125 of these high power LED's. or do i have to get a number of them and then wire up the LED's in groups. I have found that using a 3w (33ohm) resistor gets good results but the resistor gets so hot that this is clearly not the solution especially if i want to build all of the circuit board into a fairly compact board.
thanks for your time
3-terminal regulators can be used to create constant current sources.
Check the datasheet for an LM7805.
thanks for that - am i correct in thinking i will need one of these per LED?
If your resistor gets hot and you replace it with a constant current device, this device will also get hot since it will be doing exactly the same job as the resistor , the only difference being that it's an active control device as against a passive one. You will therefore need to supply heat sinks for each controller.
You can group several LEDs in series and drive from a single constant current source. Do not put the LEDs in parallel since there is no control over which LEDs the current flows through.
125 x 3 x .3 = 112 watts wow !!
I'd suggest 20 strings of 6 LEDs per string, each string being driven by its own constant current controller from by a single 24 volt supply (rated at 10 amps or so)
Dropping say 6 to 8 volts at 0.3 amps, each controller will be dissipating around 2.5 watts so you should use devices rated for 5 watts minimum. (Not good practice to drive components at 100% of rating)
jack
could I get just one fairly powerful constant current driver for all of the LED's?
basically no if the LEDs are not going to be on at the same time. It's constant current so say it is 1 amp, going through 5 LEDs (even if they share) there will be 200mA per LED but with one LED there will be 1 Amp going through it. .... Bang.
thanks for that advice mike -
i was thinking about the possibility of using something like this:
as an led driver for the led's whilst doing the actual switching through the max7219 chips. the rgb led drivers can handle up to 5amp per channel - which is equal to 16 of the LED's I want to use. I realised that since i am using a max7219 chip there is not any point at which more than 8 are turned on - even to turn all of them on, the wiring library just cycles really fast through all the pins creating the illusion that they are all on. this should mean that one of those rgb drivers should be enough for the entire matrix.
if i just wire up the + of the led's to the mosfet driver and then control them through the GND - will this work? the pic board comes pre programmed with a load of different preset sequences but they should not matter as the driver does the switching through the GND which i hope to plug into the multiplexing chip instead, providing the driver is constantly outputting the required current for the led's.
could this work?