Hello
I was wondering if anyone could help me with a question. I am wanting to make a power supply that will handle 18 3w RGB LED's. Basically the LED 's are 3 1w LED's Like the Luxeon ones all on one base. I was thinking about connecting them in series parallel with the Red LED's at 18v. The Green LED's at 24v and the blue LED's at 24v. Each color will have its own constant current driver so I can control them separately. I am a little confused on the current they will pull if they are all on at the same time. What I am trying to find is a transformer for the power supply so I can drop the voltage from wall (115v) down to 24v then from there I can use a regulator for the other 18v. I am trying to find a transformer that is PCB mountable and is not huge. Does the transformer need to handle 3 to 4 amp loads or am I going about this wrong?
Thank you
Each color will have its own constant current driver so I can control them separately.
Then there is no need to feed each LED with a different voltage. These voltages you quote seem way too high, where did you get them from.
power supply that will handle 18 3w
So you need at least 60W of power. At 24V this is 2.25A. In practice it will be more because you constant current supply will be burning some off.
To find out how much current you need simply add up the currents taken for all three LED strings.
This is a big power supply and using a transformer is not the best way. The best way is to but a switching power supply. These are more efficient and much smaller.
As was mentioned you really want a switching power supply (possibly two).
IIRC 3W LEDs are 750mA. Three independent 750mA loops totals 2.25A.
For one supply I would get a 24V switching supply and then use a switching regulator to get the 18V. You could use an LM317 to get the 18V but you will need
a decent heatsink since it is going to dissipate 4.5W (6V * 0.75V).
(* jcl *)
Hello
Thank you Mike and jluciani for the reply's. I was playing around with the LED calculator at this site http://led.linear1.org/led.wiz and 18v for the red LED's seemed to be best because the resistor wattage was lower same with the 24v for the other two colors. The specs for the LED' are Red: 2.5V ~ 3.0V, 350mA, Green: 3.2V ~ 3.8V, 350mA and Blue: 3.2V ~ 3.8V, 350mA. What I seem to be confused about is does the transformer need to be 4amps because I haven't found one that is very small. If you guys can point me in the right direction that would be great since I am not sure on how to power this thing.
Thank you for your time and answers.
I think we were pointing you away from the transformer to a switching power supply 
A 4A transformer that outputs 24VAC is going to be big.
(* jcl *)
I guess I need to do some research on switching power supply's as I don't know how they handle current and such. I was just thinking that the transformer since it was having to handle the current load would have to be that big. Do you guys know of any links where I can read on switching power supply's?
Try doing a search in Digikey or Mouser for DC/DC converters and AC/DC converters.
That will you give you an idea about what is available, costs and sizes.
If I were doing this I would buy an AC/DC power supply, 24VDC@2.5A out and
then buy a 24V to 18V DC/DC converter with an output current of 1A. If you want to
build the 18V converter check out National Semiconductor switching regulators.
(* jcl *)
Would the AC / DC power supply at 24v 2.5A also have to handle the 1A current from the 18V regulator? if so then the main power supply would have to handle 3.5A. This is where I am confused.
Thank you again
IIRC you needed 750mA for each LED color. That would be 2.25A.
All of the current would come from the 24V supply so you would need
at least 2.25A. I was thinking 2.5A should cover it.
If you think you may want to add more LEDs you could go higher.
The switching converters are the most efficient when running between 75%-100%
load so I would not get much more than you need in your application.
(* jcl *)
does the transformer need to be 4amps because I haven't found one that is very small.
You won't, it's physics. That much current equals big size. With switching regulators you trade voltage and current and so can reduce the transformer size.