yet another LED question. series or parallel?

[JimChud]

Ive already read a few tutorials and to be honest im unsure where to go at current as i was tempted to run 2 series of 4 in parallel, though im not sure.

I think running them in parrallel would be better but wasnt sure though it would make more sense.

i have 8 Red 2.5 volt LEDs which would run at 20mA.

the way i saw it was 1 of 3 ways.
* 2 series of 4 LEDs in parallel with a 120 ohm resistor at 1/4w on each series.
* 1 parallel circuit with a 68ohm resistor at 2w at the beginning of the series.
* 1 parallel with a resistor on each LED however i have no idea how to work out what ohm / w i need for that.

anyone got any golden advise on which way i should potentially go?

[crazybillybob]

The voltage that you want to Run the LEDS on would help us out

As For you resistor for each LED in Parallel It would be 68ohms 2W for each.

Most of the Time I would go with 9 LEDS. And Run of 3 LEDS in series
This would give you 3 Groups that would run in Parallel to power.
Drop your numbers into this Calc. Select Schematic and it will Draw what you need (tells you Resistor wattage and every thing) Use Solution 1! You'll need 3 X 270 ohm 1/4W Resistors and 9 2.5v 20MA LEDS. Why 9, these are not real bright LEDs so an extra will make it brighter.

If you have to use 8 (it's a pattern or so forth)
Go with the 2 groups of 4 LEDS in series with 100 ohm 1/4W resistors.
Again Drop your numbers into this Calc. It will even give you the wiring diagram if you ask it!

By the way I assumed 12Vdc based on your numbers

Hope this Helps!

CrazyBillyBob

[JimChud]

The voltage that you want to Run the LEDS on would help us out

As For you resistor for each LED in Parallel It would be 68ohms 2W for each.

Most of the Time I would go with 9 LEDS. And Run of 3 LEDS in series
This would give you 3 Groups that would run in Parallel to power.
Drop your numbers into this Calc. Select Schematic and it will Draw what you need (tells you Resistor wattage and every thing) Use Solution 1! You'll need 3 X 270 ohm 1/4W Resistors and 9 2.5v 20MA LEDS. Why 9, these are not real bright LEDs so an extra will make it brighter.

If you have to use 8 (it's a pattern or so forth)
Go with the 2 groups of 4 LEDS in series with 100 ohm 1/4W resistors.
Again Drop your numbers into this Calc. It will even give you the wiring diagram if you ask it!

By the way I assumed 12Vdc based on your numbers

Hope this Helps!

CrazyBillyBob

Thanks for the fast response

sorry i forgot to add the 12v part of my idea i was going to run it from a molex in my case maybe to a PCB and then with a switch and whatnot but wasnt sure.

yeah thats one of the sites i got a few of the ideas from.

Just wasnt sure if it was a good idea to go that way or literally just do a big parallel loop.
i'll play with the 2 x 4 loop and then see what to do with that i think. I may go for the 9th as suggested or split it down to 4 x 2 or some such crazy thing

[crazybillybob]

The issue with going with less LEDs on a loop is that you still have to do something with the power... What happens is you turn those resisters into little heaters as they convert the power to heat. What you want is to convert electricity into light. The 4x2 or 3x3 are more efficient setups, and more heat is not what you want in your case.... That GPU your running is heating things up enough

[JimChud]

thanks for the heads up, i did some testing last night with a string and had a few problems, but have started to get round it seems the numbers on the data sheet for the LEDs were wrong so its going to be abit of trial and error to work out what works and what doesnt.

[crazybillybob]

I was wondering about that, as most Red LEDs are 1.7 to 2 V with varying mA based on regular (20-30ma) or Ultra bright (20-60mA)... But each company is a little different. Blue and Green LEDs are normally 2.5-3V mA pretty much the same as red.

A note on LEDs in series. They must be the same type and same color. This is due to the different operating voltages and current drops. Normally if you mix them up you'll either see nothing, or the red LED will burn up in seconds (yes I've done this )
Some people will say they even need to be the same brand...Different brands will normally work but my have a shorter life time (10,000 hours versus the 100,000 hours there rated for) but that's still 1.14 Years of light

If you've got any more issues or just want to bounce any ideas around let me know I'll help as much as I can.

CrazyBillyBob

[JimChud]

thanks for your help really appreciated.

I bought 8 Red LEDs and 2 Ultra Bright Blue LEDs.
The thing i read in the specs were that the 8 reds are 2.5v and max current of 30mA with an optimal running of 20.
I tried running one series with my old test shuttle PSU. I got a circuit when testing the cables but not when putting the resistor and LEDs in the circuit.

I ended up getting abit strung out after an hour of testing and getting nowhere. I ended up searching through and tagging on a few resistors to get it to be able to run one LED yet it wouldnt light up so i tried another and another and alas i thought GAH! BALLS etc and just started dropping the resistance down and suddenly it lit with a 200R 1/2W.

I've got some time this weekend and tonight so i'm going to try and rework the maths and see what i can do then

oh just saw your briefcase mod and jesus is that cool! got any further with it yet? i know the pain of travelling alot with a big ol case i have to do it weekly when i stay with the mrs... pain in the ass

[crazybillybob]

as for the math What your going to use is Ohms Law E=IxR (voltage = Current x Resistance) here's a good web site for all the forms of Ohms law
For your use, you know E =12V and I =20mA so you want to find R so R=E/I or R=12/.02 R=600 ohms This is not a common resistor so 580ohms would be a good choice. Now when you add LEDs in series you need to add the LED's voltages together (ie. 2.5+2.5) to get the total voltage used by the LEDS
So If you have 4 LEDs that are rated at 2.5V you would have 2.5x4=10, Now subtract that voltage from the supply voltage in your case 12-10 =2 That is the E in your ohms law formula now so E=2, I=20mA R=? or 2/.02=R R=100.

The webpage calc makes the math easier, but you can double check it.

Now I have some ultra bright LEDs that I run 2 in series with a 150 ohm resistor.... Man Are they bright (there lifetime is way shorter then the 100,000 hours rated). But I don't recommend trying this with "standard" LEDs they will not last long.... most are like a flash bulb but If you don't mind killing a few to learn experimenting is the best way to learn the limits of LEDs.
If your going to mod or play with LEDs get a bunch of them, their cheap and small One place to get them is Ebay, I just got 100 Ultra brights for about $10 shipped. The other place that's been recommended here at TBCS (I've never used them) is LEDShoppe.com
Prices don't look bad. The bonus of ebay is that some auctions have a mixed bag of colors like 25 each of 4 colors or the like... makes it easy to get a wide variety for little cost. Just watch the Ultra bright should be at least 2000mcd, the 200mcd are just a little brighter then "standard" LEDs.



And yes I've got more done on the Pack-n-Play, Hopefully I'll get another update up tonight... but I'm a little busy at work now for that.

CrazyBillyBob

[JimChud]

i won the war, though i had to change my plans very slightly.
I worked out that i still couldnt get them running with a 100R or a 68R so alas i got annoyed then realised something really stupid that i had overlooked.
Right next to my 12V line is a 5V line and they are infact 2.5V LEDs.
I put them in pairs and put a 1 ohm resistor on the lines and BOOM up they all came

definately going to change these for superbright ones comparing these to my blue ones is crazy really.

think i may well do what you advised and grab a few from the ebay and have a play with them. i wanted to light my case without using the cool cathodes. i used to use. plus wanted in some areas a more direct light.
thanks for your advice

look forward to reading the worklog as thats looking fairly awesome hehe

[crazybillybob]

Getting the right voltage always helps
The Red and black wires are 5VDC and Ground the Yellow and black wires are 12VDC and Ground. I thought about asking if you had it hooked up to the right ones but said nope I'm sure it's right... Next time I'll ask

Good Luck (LEDs are addictive, you can never use just 1 )

CrazyBillyBob