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61  Forum 2005-2010 (read only) / Interfacing / Re: constant current driver advice sought on: May 05, 2010, 11:10:04 am
thanks for that advice mike -
i was thinking about the possibility of using something like this:

http://picprojects.org/projects/bigmosfetrgb/index.htm
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?

62  Forum 2005-2010 (read only) / Interfacing / Re: constant current driver advice sought on: May 05, 2010, 05:59:22 am
thanks for that - am i correct in thinking i will need one of these per LED?
63  Forum 2005-2010 (read only) / Interfacing / constant current driver advice sought on: May 05, 2010, 05:44:56 am
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
64  Forum 2005-2010 (read only) / Interfacing / Re: quick question re. correct resistor on: March 31, 2010, 04:17:16 pm
just had a look at the spec sheet - and it has a maximum continuous collector current rating of 600mA, so should be ok.
65  Forum 2005-2010 (read only) / Interfacing / Re: quick question re. correct resistor on: March 31, 2010, 12:51:51 pm
hi mike

thanks for your reply.
I did not know that 300mA LEDs needed a constant current drive - what is the maximum forward current that you can use a series resistor for? i am keen to use the LEDs i have mentioned as they are really bright and will be just right for the job + they are fairly cheap (I found 150 of them for $70).
With regards to the MAX7219 chip - i have used it in the past to control 64  LEDs with an external power supply, the chip itself got power through the arduino but the LEDs were powered externally. I have attached a schematic of how I did it before (with great results), however I was using not as powerful LEDs.
with regards to the DC forward Voltage - the minimum is 3.0v, typical 3.4v and max 3.6v.

How would I go about providing the LEDs with a constant current drive?

thanks

66  Forum 2005-2010 (read only) / Interfacing / quick question re. correct resistor on: March 31, 2010, 06:02:56 am
hello

just a very quick question: i am building a large scale led matrix (3 x 3 x 3 metres) and just wanted to make sure i have my calculations correct in terms of the necessary resistors.
I will be using an external 12v power supply for the 125 LEDs, the matrix itself will be driven by an arduino and multiplexed through two MAX7219 chips. The LEDs i intend to use have the following spec:

Luminous Intensity-MCD: Typ: 230,000 mcd
Reverse Voltage:5.0 V
DC Forward Voltage: Typical:
DC Forward Current:300mA

so am I correct in thinking i need to order :
3.0 Watt capable or greater 33 ohm resistor for each led?
I will be using 2N2222A transistors for each led as the arduino does not put out enough current to drive so many of them.

been putting off working on this for a while but all the parts have finally arrived and there are no excuses now.
thank you for your time

67  Forum 2005-2010 (read only) / Interfacing / Re: help with multiplexing and transistors please on: December 18, 2009, 06:56:43 am
thanks for the replies - i have managed to get it to work correctly now. i needed to make the arduino and the external power supply share a common GND. Once i did that and stopped the base going to ground it all works perfectly now.
thanks again for the help
68  Forum 2005-2010 (read only) / Interfacing / Re: help with multiplexing and transistors please on: December 17, 2009, 01:01:28 pm
@jluciani

thanks for your reply - my bad on the schematic (been looking at too many of them lately) - changed it to the correct labeling. I have just tried it with your suggestions - but although the LED is dimmer than it was before (means i must have been sending too much current through them) the problem still persists. I cannot plug in more than 1 row at a time - which makes the multiplexer slightly redundant.  The Arduino + is the signal coming from the SEG pin of the multiplexing chip and the Arduino GND is the signal coming from the DIG of the chip.
69  Forum 2005-2010 (read only) / Interfacing / help with multiplexing and transistors please on: December 17, 2009, 12:29:03 pm
hello

I am currently working on a project which involves controlling a matrix of 64 LED's I am using the MAX7219 multiplexing chip. I have managed to program it correctly and have now started the task of wiring up a breadboard to plug in the 64 LED's that I want to use. I was not getting enough power from the chip itself so have decided to use an external power supply and some transistors to allow me to use some brighter LED's. I am using the 2N2222A transistors with these LED's:

Luminous Intensity: 25000mcd
Reverse Voltage:5.0 V
DC Forward Voltage: Typical:3.2 V Max: 3.6V
DC Forward Current:20mA

when i wire up the first row of the matrix it all works fine, but as soon as i wire in the second row none of the led's will light up. I have tried so many variations but i just do not know enough about electronics at the moment. I am convinced it is something to do with the GND. I have the MX7219 GND and the + coming off the arduino and the LED's getting their + and GND off an external power supply.
the way i actually plugged in the transistors at the moment is contrary to the way I have seen it online but it works well. however it is this problem of plugging in more that one row of the matrix that i have come up against.

I have included a little schematic below which shows how i have wired it up currently (the arduino + and GND is the SEG and DIG coming out of the chip)


I have plugged in the GND from the external power supply into the base pin alongside the Arduino +. I know it must be wrong but weirdly it works in terms of lighting up the correct led.
does anyone have any experience wiring up the MX7219 chip with transistors to run 64 LED's - any help would be greatly appreciated as i am working against a very tight deadline.
70  Forum 2005-2010 (read only) / Interfacing / Re: multiplexing and 1W LED's on: November 30, 2009, 06:27:58 pm
apologies - i hope to get some results soon, this seems like it is going to be a pretty hard challenge getting this space ship lit up , but i think it will be worth it.
thanks for your help mike - will make sure to read the manual in full next time.
71  Forum 2005-2010 (read only) / Interfacing / Re: multiplexing and 1W LED's on: November 30, 2009, 09:32:07 am
ok - so would the following calculation be correct: 5(voltage supplied) / 500 (500mA of current required to drive 25 led drawing 20mA each) = 0.01K

i was hoping to drive more off each chip but since it has a 500mA peak current and each led requires 20mA - i figured that only 25 could be wired up.
72  Forum 2005-2010 (read only) / Interfacing / Re: multiplexing and 1W LED's on: November 30, 2009, 07:42:35 am
just a quick question - I am just looking at the schematic (http://www.arduino.cc/playground/uploads/Main/MAX72XX_Schematic.jpg) and noticed that the resistor rating is not defined, just called RSet. could anyone explain what this means?
73  Forum 2005-2010 (read only) / Interfacing / Re: multiplexing and 1W LED's on: November 24, 2009, 08:54:24 am
just starting to get all the different parts in the post and looking at the schematic found over on the arduino playground for multiplexing using the MAX7219 chips (http://www.arduino.cc/playground/uploads/Main/MAX72XX_Schematic.jpg)
i am not sure how i would wire this up with an external power supply necessary for all these LEDs ( i am not going to be driving 48 on each chip). has anyone used these chips in conjunction with an external power supply?
74  Forum 2005-2010 (read only) / Interfacing / Re: multiplexing and 1W LED's on: November 20, 2009, 09:43:46 am
I did not know about that issue regarding multiplexing -

The LEDs I have been looking at have the following information provided:
Power Dissipation      PD      100      mW
Forward Current(DC)      IF      20      mA
Peak Forward Current*      IFP      30      mA

just trying to calculate a solution; if I have 64 LED's and the current is set to 30mA am I right in thinking that I would need to provide a 12V supply as that would give me a 480mA of total current drawn, and not exceed the maximum current possible through the MAX7219 chips?
 


75  Forum 2005-2010 (read only) / Interfacing / Re: multiplexing and 1W LED's on: November 20, 2009, 08:25:48 am
thanks Mike -
been looking around for a work around. One of the options is of course to use less powerful LEDs - I have come across 0.5w ones that have a much lower power consumption and have a comparable brightness (instead of 60-70Lm they are 47Lm - which is a compromise i can work with).
their specifications are as follows:

Emitting Colour:WARM WHITE
Luminous Intensity: 15000mcd
Color Temperature: 3000K
Reverse Voltage:5.0 V
DC Forward Voltage: Typical:3.2 V Max: 3.6V
DC Forward Current:20mA
Viewing Angle:120±6 degree
Lead Soldering Temp:260°C for 5 seconds

would their total power dissipation just be 20mA x 64 (number of LED's I am hoping to drive)?
if not, could you please advise how to calculate it correctly

many thanks
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