I'm working on a project that will end up being 5 groups of RGB LEDs. Each group will always be the same color, but I need probably about 9 in each group to get the brightness I want. My plan was to have 3 series of 3 in each group and use 12v to drive them. That would be 180mA per group for all three colors. So .9amps for the whole thing. But then I realized my LEDs are common anode and I cannot put them in series like that. That puts me at 2.7amps if they are all in parallel. Is there anything I can do to take advantage of my available 12v to reduce the current needed for this with the LEDs I already have?
To make things a bit harder the LED groups will not be near the controlling device, I was planning on using 4 conductor telephone cable to connect to them so it would be especially helpful if it only required 4 wires to solve this problem, BUT I am still interested in anything that could solve this problem even if I need to throw out this part and use something else.
Thanks.
But then I realized my LEDs are common anode and I cannot put them in series like that.
CA or CC, why does it matter?
Rob
It doesn't matter whether it's CA or CC but it does matter that it is either. How could I put two or more of them in series? As far as I can imagine I cannot. So I need an alternative way to keep my current lower.
I may be missing a subtlety here, but series circuits have the anode of one thing connected to the cathode of another, whose anode is then connected to the cathode of the next element, etc. What difference does it make whether the LED is CA or CC in this?
They are RGB LEDs. Each package has one anode and three cathodes. The only way to put all of the reds in series, all of the blues and all of the greens would be to connect all three cathodes to the next anode. Then when I pull, say for instance, the red cathode low, all three cathode of the next two LEDs are all pulled low. Not just the reds.
They are RGB LEDs.
Vital piece of information that has so far been missing.
You are right there is no way to put these in series. So you have to drive them individually leading to a larger current.
The very first sentence of the first post. I apologize for not mentioning it sooner.
So there is certainly no sorcery to take advantage of my 12v to lower the current with them?
What would be my best alternative? Would it be worth getting new LEDs or is it not completely absurd to try to use almost 3 amps.
Would any switching regulator be able to make full use of a 12v 1A power supply and be efficient enough to source that kind of current or would I definetely need a higher rated power supply?
Actually I should end up with a 2amp power supply soon. I've got a DC-DC switching power regulator rated for 1.5amps. If I used two of them, 1 for 3 of the groups and 1 for the 2 others and my arduino would that possibly keep me under 2 amps from the powersupply? (12v -> 5v)
I know it depends on the regulator, I don't have the info for it on hand, but does that seem like it may be a possibility?
The very first sentence of the first post.
Doh I missed it, yes sorry.
While you can use a 3A power supply all that extra power has got to go somewhere and you end up burning it off as heat.
I would suggest you get RGB LEDs that have both anode and cathode separate for the three colours, you can get them.
I've actually searched for them but couldn't find much... Could you suggest some keywords? I'd love to be able to find them on eBay or something and keep them under 20 cents each, if they're much more I may not be able to afford it.
I thought switching regulators did not burn off most of the energy and actually took more advantage of total power available.
I'd love to be able to find them on eBay or something and keep them under 20 cents each,
Well I would be surprised if you could, they are going to cost in the region of $6 each
http://uk.farnell.com/osram/lrtbc9tp/multiled-ceramos-rgb/dp/1827016
I thought switching regulators did not burn off most of the energy and actually took more advantage of total power available.
Yes they do, are you thinking of switching down to 5V or less. I use this sort:-
Yes at 6 dollars I'd rather just glue red green and blue LEDs together. The only reason I didn't in the first place was because my RGB LEDs were only about 16 cents each.
With that power regulator, it says 88% efficiency at the lowest. So 5.5v * 3A = 16.5 watts * .88 = 14.52 watts / 12v = 1.21A from the 12v power supply. Is this an accurate calculation? That might be the road I take since I'm already expecting a 2A power supply.
Haha nope, wrong number and wrong calculation. Let me think about this for a moment more. It does still seem like it would be under 2A though, am I wrong?
Ok do 80% efficiency means 20% inefficiency. So 5.5 * 3 / 12 * 1.2 ??? 1.2 being the power+ inefficiency? So 1.65A?
Ok do 80% efficiency means 20% inefficiency.
Yes. A 100% efficient circuit would take in 12V @ 1A and delver 6V @ 2A.
With 80% efficiency to get 6V at 2A you would need 20% more input current, in this example that would be 1.2A
I hate to have to power it so inefficiently but it might be most economical for me. I may look into using individually colored LEDs if I can find a good deal on some that match.
Thanks for your help.