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LED and Resistor Help Needed - Page 2

post #11 of 21
ok mate i can help you, because of the limited curent draw the Arduino can handle you should use a ULN2003AN, it in laymans term a swith that can turn another curcuit on and of very quickly (well thats my understanding) so because of that it can be used with PWN pins as well, ill up load a scematic that i made for my mod.
ALSO DO NOT OUT RUN THEM IN SERIES, run them in parallel with a resistor for each LED, i recomend that you use a resistor array/network (basicly 2 - 8 resistors in a package) you can get them in a SMD form or a DIL form.

post #12 of 21
Thread Starter 
thanks dude - but I'll be honest I don't understand any of that SMD, array, 5G1 stuff. XD

And I don't understand how exactly I can run them in parallel the way that I'm wanting to set them up.
I figured this would be a simple hook X resistor Y times to Z amount of LEDs.
    
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post #13 of 21
Quote:
Originally Posted by OmegaNemesis28 View Post
thanks dude - but I'll be honest I don't understand any of that SMD, array, 5G1 stuff. XD

And I don't understand how exactly I can run them in parallel the way that I'm wanting to set them up.
I figured this would be a simple hook X resistor Y times to Z amount of LEDs.
it not as simple as that with more than 5 or so LEDs.

once i get of work i will post more in detail.... or mayby at lunch time if you are lucky about in 2.5 hours time
post #14 of 21
double post for the guys how are subscribed.

Quote:
Originally Posted by OmegaNemesis28 View Post
thanks dude - but I'll be honest I don't understand any of that SMD, array, 5G1 stuff. XD

And I don't understand how exactly I can run them in parallel the way that I'm wanting to set them up.
I figured this would be a simple hook X resistor Y times to Z amount of LEDs.
just so you know wiki is a great place to check most of this stuff out

Also when i said DIL (Dual in line pins) networked resisistors i meant SIL (single in line pins). (wiki has some good info on this stuff DIL and SIL)

SMD (also called SMT) means that the componant is soldered onto the board rather than through holes (like you normal resistor). WIKI link

Array is just what i decided to call it (its a array of LEDs isnt it )

5G1 is the name i gave to the + voltage solder point for the 1st Green LED on fan 1.

also NEVER! run LEDs without a resistor, i might work for a while but it will go boom in 1 to 5 weeks.

as i said in one of our PMs the Board that i am designing would be perfect for you purpose. it has the capability to suport up to 12 LEDs on each channel, and there 8 Channels (running of 8 Arduino pins).

and finally a quick picture of what the LEDs should be arranged (did this in MS paint, i dont have the program at work)


post #15 of 21
Um... Where is this information for running LEDs in parallel coming from? Unless it is necessary due to voltage deficiencies it is ALWAYS best to run them in series.. Why? Every LED will have a slightly different forward voltages.. These forward voltages change in relation to temperature(the Vf drops as temperature increases)... Since current is directly related to the voltage in LEDs, the LEDs that have a higher Vf will draw less current than the more efficient LEDs with a lower Vf.. This will lead to the more effecient LEDs drawing more current until it goes into a thermal runaway and dies.. Now, all of the current that the dead LED used to consume gets split among the remaining LEDs and begins a domino effect..

This is a big issue with high powered LEDs (>1W) on seperate heatsinks.. If they are tightly spaced on a single heatsink then it's not so much of a problem.. But with 5mm LED's and such where each LED is completely thermally isolated from the rest, the problem is huge..

It's one of the main reasons poorly designed LED traffic lights and "shower head" style led household replacement bulbs have arrays off bulbs burning out so quickly.. One way to prevent this is to use a current limiting circuit is which described here: http://www.ledsmagazine.com/features/6/2/2

Of course, it's not like your LEDs really need to be wired for very long life and all.. So go for a parallel circuit if you would like.. I just wanted to make the point that parallel circuits are terrible for LEDs..
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post #16 of 21
Quote:
Originally Posted by silent_nightr34 View Post
Um... Where is this information for running LEDs in parallel coming from? Unless it is necessary due to voltage deficiencies it is ALWAYS best to run them in series.. Why? Every LED will have a slightly different forward voltages.. These forward voltages change in relation to temperature(the Vf drops as temperature increases)... Since current is directly related to the voltage in LEDs, the LEDs that have a higher Vf will draw less current than the more efficient LEDs with a lower Vf.. This will lead to the more effecient LEDs drawing more current until it goes into a thermal runaway and dies.. Now, all of the current that the dead LED used to consume gets split among the remaining LEDs and begins a domino effect..

This is a big issue with high powered LEDs (>1W) on seperate heatsinks.. If they are tightly spaced on a single heatsink then it's not so much of a problem.. But with 5mm LED's and such where each LED is completely thermally isolated from the rest, the problem is huge..

It's one of the main reasons poorly designed LED traffic lights and "shower head" style led household replacement bulbs have arrays off bulbs burning out so quickly.. One way to prevent this is to use a current limiting circuit is which described here: http://www.ledsmagazine.com/features/6/2/2

Of course, it's not like your LEDs really need to be wired for very long life and all.. So go for a parallel circuit if you would like.. I just wanted to make the point that parallel circuits are terrible for LEDs..
This.

Series over parallel not only ensures that each LED has the same current draw, it also cuts down your total current draw. Say each LED's draws 15mA. Putting 5 in series will draw 15mA from the source, but 5 in parallel will draw 75mA (with the extra power being dissipated in each of the branch resistors). If you run them all in series ( or two series branches in parallel depending on rated voltage), you wouldn't need any sort of external drive circuitry (ie. ULN2003AN or TIP122).
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post #17 of 21
Du-z nailed it. You never want to string LEDs one after the other, always have them on separate currents with a single resistor for each LED.

Oh and use a solderless breadboard to test your current. You can pick one up at most electronic stores pretty cheap.
post #18 of 21
Quote:
Originally Posted by SilverPotato View Post
Du-z nailed it. You never want to string LEDs one after the other, always have them on separate currents with a single resistor for each LED.
Seriously?

Can someone please back this up with some references or a legit site that says this?

Here are some references I found here to back up my idea:

# http://members.misty.com/don/ledd.html
Do not put LEDs in parallel with each other. Although this usually works, it is not reliable. LEDs become more conductive as they warm up, which may lead to unstable current distribution through paralleled LEDs. LEDs in parallel need their own individual dropping resistors. Series strings can be paralleled if each string has its own dropping resistor.

# http://www.bivar.com/eLetter/driving-la.htm
You can also connect LEDs in parallel. However, variations in the forward voltage requirements of individual LEDs will result in non-uniform current distribution, and non-uniform current distribution results in non-uniform brightness.

# http://www.kpsec.freeuk.com/components/led.htm
Avoid connecting LEDs in parallel! Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea. If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor.

# http://www.otherpower.com/otherpower_lighting_leds.html
It's important that each string has its own resistor.... putting them in parallel with a single resistor is bad practice. We didn't know this when this article was first written....thanks to all the folks that pointed this out!

# http://home.nikocity.de/andymon/hfg/ledlamps.htm
If you want to operate LEDs in parallel with higher power levels, you must select them for equal voltage/current characteristics! It won't do to buy four LEDs if you want to build a four-LED-Lamp! Buy ten and measure the voltage of each LED at 20mA.

# http://www.help.ip3.com/s.e.design/C...ng-soLip.shtml
{both sides of an argument on this subject}

So unless you want to individually test the Vf of each LED and get an appropriate resistor, series is the way to go for long life.. But since I doubt you are trying to get 50k hours out of it, parallel may be fine as long as you aren't running at the LED's limits.. I wouldn't even recommend running no-name brand LEDs at 20mA in series.. The thermal management of most of them are TERRIBLE unless you go with a brand like Nichia, Cree, etc who use special epoxies that are more thermally conductive as well as dies that can handle the heat and current..
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post #19 of 21
mate the LEDs he want to use arnt that high power so they will hardly heat up. no were near the 350mA in your reference

He would need to use approximately 12 volts to power 3 LEDs in a string (5v should only used with a arduino) (the nail in the coffin for the series argument)

using a 68 ohm resistor on each LED will lessen that chance of the power not being distributed properly. (as shown in some of you references, the 3rd and 4th one)

Quote:
Originally Posted by C-bro View Post
This.

Series over parallel not only ensures that each LED has the same current draw, it also cuts down your total current draw. Say each LED's draws 15mA. Putting 5 in series will draw 15mA from the source, but 5 in parallel will draw 75mA (with the extra power being dissipated in each of the branch resistors). If you run them all in series ( or two series branches in parallel depending on rated voltage), you wouldn't need any sort of external drive circuitry (ie. ULN2003AN or TIP122).
the ULN2003AN is still needed as he wants to use PWM to control the Dimming. there are 6 PWM pins. and there is a max draw of 40mA per pin on the arduino and he wants to use up to 3 LEDs per pin. (approximately 60mA)

http://arduino.cc/en/Main/ArduinoBoardDuemilanove
Edited by Du-z - 5/23/09 at 2:49am
post #20 of 21
Quote:
Originally Posted by Du-z View Post
mate the LEDs he want to use arnt that high power so they will hardly heat up. no were near the 350mA in your reference

He would need to use approximately 12 volts to power 3 LEDs in a string (5v should only used with a arduino) (the nail in the coffin for the series argument)

using a 68 ohm resistor on each LED will lessen that chance of the power not being distributed properly. (as shown in some of you references, the 3rd and 4th one)
5mm LEDs do heat up pretty hot at 20mA.. That's due to the current density of the small die being so high. If you can't get the heat out fast enough then the die will start to degrade and reduce in output. Since most 5mm LEDs use crap epoxy that's acts like an insulator the die temps can get very high.. That's why I always recommend running cheap 5mm LEDs for long life applications at less than 20mA... Here is some proof of 5mm LEDs running crap life times due to heat.

Just to make it clear I'm not trying to say LEDs should NEVER be run in parallel, I just mean that it's much better to run in series unless driver issues or inadequate voltages for you to run in parallel. In this case I don't think there will be much issue running in parallel since it's not meant for general lighting and long life(relatively). The only reason I came into this thread was because of the claims of parallel being better than series for LEDs.

You still have not provide any evidence as to why parallel is better than series.
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