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[W-Gaming5]

 

[Modded Matt]

I need the supply voltage, and forward voltage on your leds..purple dosnt tell me anything. help me help you

[802Chives]

since purple leds tend to have a Vf= 2.8-3.8V as a guess I would use 3.2 for Vf(you can find out for sure by consulting the specification sheets for your LED)

PSP phat has a 5V supply.   And all of your LEDs will be in parallel.  Probably should run them at about 20mA

so..... using this LED array calculator
with these values:
Source Voltage = 5 V
diode forward voltage = 3.2 V
diode forward current (mA) = 20 mA
number of LEDs in your array = 8

It tells me you need 100ohm 1/4W resistors for each LED.  If you are looking to have them as bright as possible then you need to up the diode forward current.  If you consult your spec sheet for the diode you will see the max diode forward current that you can run them with.(Usually 25 mA is the max continous, ignore pulsed specs as they are usually for 1/100th of a second which is not what you are doing)

Another trick I use a lot is to take a 1kohm potentiometer and tie the wiper to the negative LED lead and one of the outside pins to ground, and with the positive side of the LED hooked up to your target voltage(5V) and turn the potentiometer down from 1k till you get what you want for you LED brightness.(you can also measure current in line whilst doing it to make sure you dont exceed the maximum current specs) This is the best way to tune your LED brightness, as calculators dont take the output into account.

and remember brighter isnt always better, depends on what you are trying to achieve.

Edit: LOL matt, I have a handy little graph that helps get ballpark Vf for color diodes:
http://www.oksolar.com/led/led_color_chart.htm

[Modded Matt]

I was wondering where you got the VF from?? good to know. how do you store all this information? I think you have like ten super computers under your desk or something LOL

[802Chives]

hahahaha, no need to store information, google does it for me.

I guess that is the best skill I learned in college, above all this electronix crap, is that I can find information very fast, and without sifting through a bunch of grabage.  It is all about the keywords to seach for, and sometimes more importantly, the keywords you dont want included in the search.

[W-Gaming5]

 

[802Chives]

righto, them will do.  Should probably double check my numbers and re-calculate with the exact numbers you have to make sure they will work.

being in the same ballpark is usually good enough for LED limiting resistors.  Remeber if the LEDs are too bright up the resistance to tone them down about.

[W-Gaming5]

 

[802Chives]

more resistance = dimmer

less resistance = brighter

the best way to make sure you have the right brightness is to use a potentiometer to find the correct brightness, and then measure the resistance and use that.

[W-Gaming5]

 

[blackwid1]

do you have to use a resistor for every led or can you put in a resistor from the source voltage and then from the resistor to all the other led's

[Jumbo]

do you have to use a resistor for every led or can you put in a resistor from the source voltage and then from the resistor to all the other led's

That works.

[802Chives]

do you have to use a resistor for every led or can you put in a resistor from the source voltage and then from the resistor to all the other led's

according to many people it works.......... according to me, it can work but it can and will fail and ultimately cause all your LEDs to blow.  The reason being is that there are tiny variations in manufacturing of LEDs that will make them different enough so that a weak one will draw more current then another thus creating a path of less resistance which will ultimately cause that weak LED to fail.  Once one LED fails it will cause a domino effect cuz the current that was going through that LED will be distributed to the rest which causes the next weakest LED to fail and so fourth and so on til eventually they all fail.

In order to prevent that from happening, each LED must have its own limiting resistor so each individual LED will be current limited by its own resistor.  that way if one blows it will not have any affect on the rest as they will all be limited individually.

The reason why people can get away with it is that LED manufacturers are doing a good job making very similar devices, as well as most of the time people use them with batteries and the batteries internal resistance helps the situation.  Using one resistor for different color/makes of LED will definately cause premature failure in your LEDs.

In conclusion, ya it may work, but if you want to do it properly, you have to limit each LED with its own resistor.