Best power source for LED array? - Page 4 - Overclock.net

**Boris4ka** · 05-27-09

Quote:

Originally Posted by EntropyTTU

Yes, the circuit in the top picture, is what I proposing to begin with, and it will defintely work. And with the diode equation, I see your point, and you are exactly right, there may be a difference in lifespan of the leds, but I wonder how much difference it will make?

Boris:

Either way, the original picture I posted with each led getting its own resistor is guaranteed to work, if you want to be safe.

But, I think a shot at the 50 in parallel, or 25x2, is worth it too, if that is what you want to try.

Best of luck to you.

I think I'll go for the 25x2. Would each resistor have to be 22 Ohms (according to this http://led.linear1.org/led.wiz )? Can I run a 20 Ohm one since I'm sure they don't make 22 Ohm resistors?

As for the issue with each LED being slightly different, I can't run a higher voltage anyways. Because of the amps and watts I'll have to use the battery pack, which means I can't add any cells to make more volts to run them in series.

EntropyTTU · 05-27-09

Quote:

Originally Posted by ionimplant

To Entropy's question about multiple loads...multiple LINEAR loads are fine.
Don't ignore the diode equation...it tells all. Look at the TI document.

Diodes are not linear, good point.

Ok, I have to agree, you are not going to be able to count on the circuit working that long, with the single resistor.

Here is something you can try in the link below, it will calculate, and even provide, a circuit, but this goes back to what we were saying earlier, that you were trying to avoid; but I personally don't see any other way.

http://led.linear1.org/led.wiz

Edit: Ok, I just tested running two leds in parallel, with one resistor as a current limiter, and measured the current, and it was roughly 35mA total, with one LED getting about 15mA, and the other getting about 20mA. I only ran it for a few minutes, but you never, know it might actually run for a while.

Best of luck.

**Boris4ka** · 05-27-09

Quote:

Originally Posted by Ionimplant

There are multiple questions here...

"Then how come all LED's aren't wired in series?"

Well...they ARE!
[]http://i40.tinypic.com/x1eh3s.jpg[/IMG]

You can use four 1W resistors in parallel (to make the 'bad' circuit)

"I've never heard of this before." If you are an EE in the circuits area, you should have heard it before.

To Entropy's question about multiple loads...multiple LINEAR loads are fine.
Don't ignore the diode equation...it tells all. Look at the TI document.

Well I'm not an EE

What you're saying makes sense, but there is no practical way for me to fix it. I can't add more voltage to the circuit without spending more money than I'd like. I already have the 7.2v battery. How much do you think it will affect the LED's though? Are we talking life spans of 500 hours, 10000, 10?

Quote:

Originally Posted by Reista

Im not going to post anything about the actual array, the guys seem to have it covered from what i read but you might like to look into a one-way motor that will generate a current when you pedal your bike to recharge your battery (if your using a rechargeable.)

I have thought about that but this is a mountain bike that sees 20ft jumps, water, mud, dirt, etc. so it wouldn't be a practical solution. It'd break quickly.

**Boris4ka** · 05-27-09

Quote:

Originally Posted by EntropyTTU

Diodes are not linear, good point.

Ok, I have to agree, you are not going to be able to count on the circuit working that long, with the single resistor.

Here is something you can try in the link below, it will calculate, and even provide, a circuit, but this goes back to what we were saying earlier, that you were trying to avoid; but I personally don't see any other way.

http://led.linear1.org/led.wiz

I posted a link to that in a previous post. It says to use 22 Ohm resistors for each 2 LED series. Again, how long of a life span will the LED's lose?

Ionimplant · 05-27-09

Quote:

Originally Posted by Boris4ka

I think I'll go for the 25x2. Would each resistor have to be 22 Ohms (according to this http://led.linear1.org/led.wiz )? Can I run a 20 Ohm one since I'm sure they don't make 22 Ohm resistors?

As for the issue with each LED being slightly different, I can't run a higher voltage anyways. Because of the amps and watts I'll have to use the battery pack, which means I can't add any cells to make more volts to run them in series.

If you are going for 25x2 and one resistor, it needs to be a 1.6 ohm resistor and it needs to be 1/2 Watt.

Good luck...let us know how it works. You might get lucky and the worst thing is that some of the leds are dimmer. Take a picture and post it!!

**Boris4ka** · 05-27-09

Quote:

Originally Posted by Ionimplant

If you are going for 25x2 and one resistor, it needs to be a 1.6 ohm resistor and it needs to be 1/2 Watt.

Good luck...let us know how it works. You might get lucky and the worst thing is that some of the leds are dimmer. Take a picture and post it!!

I thought I needed a 4w resistor?

Why is this so confusing

Ionimplant · 05-27-09

Quote:

Originally Posted by Boris4ka

I posted a link to that in a previous post. It says to use 22 Ohm resistors for each 2 LED series. Again, how long of a life span will the LED's lose?

That's cute!

If you use 25 resistors with your 25 diode pairs, they will last FOREVER--they will be glowing when you are dead or at least until you are so rich that you will buy the solution off the shelf!!

I thought you were gonna live on the wild side and put all those babies in parallel with one resistor!

To your lifetime question?? That would be very complicated to determine. You would have to model all of the actual diode parameters, calculate temperature increase in the diode hogging the current, get a number for activation energy and see what it tells you. ???

**Boris4ka** · 05-27-09

Quote:

Originally Posted by Ionimplant

That's cute!

If you use 25 resistors with your 25 diode pairs, they will last FOREVER--they will be glowing when you are dead or at least until you are so rich that you will buy the solution off the shelf!!

I thought you were gonna live on the wild side and put all those babies in parallel with one resistor!

To your lifetime question?? That would be very complicated to determine. You would have to model all of the actual diode parameters, calculate temperature increase in the diode hogging the current, get a number for activation energy and see what it tells you. ???

I did want to use them all in parallel with one resistor. But since you said it's a bad design and won't last, I will do it with 25 pairs. This will be ok? Can someone confirm that I need a 22 Ohm resistor for each pair?