NerdKits - electronics education for a digital generation

You are not logged in. [log in]

NEW: Learning electronics? Ask your questions on the new Electronics Questions & Answers site hosted by CircuitLab.

Basic Electronics » Power Supply for LED's

October 31, 2009
by soundman003
soundman003's Avatar

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

October 31, 2009
by mrobbins
(NerdKits Staff)

mrobbins's Avatar

Hi soundman003,

I found this datasheet but I can't be 100% sure that it refers to exactly what you're talking about...

In any case, on page 4 they say that typical forward voltage for red is 2.4V, and for blue and green is 3.4V, all at 350mA of current.

With a 24V supply, for blue and green you might be talking about perhaps 6 LEDs in series, for 3.4 * 6 = 20.4 volts. Three parallel sets of 6-series chains for blue, and three parallel sets of 6-series chains for green, is 6 total current paths @ 350mA each so far. You could put red on the same 24V supply and just have two parallel sets of 9-series chains, with 2.4 * 9 = 21.6 volts. Then, if all your LEDs are on at full brightness, you would have 3+3+2 = 8 total parallel paths with 350mA each, for 2.8 amps. So somewhere in the 3-4 amp range for estimating how much current your transformer needs to handle sounds right.

An example transformer might be like this although you don't need the center tap. But this assumes that you want to build your own power supply: line connection, fuse, transformer, bridge rectifier, filter capacitors, etc, and make sure it's all safe. I'd really rather you consider an off-the-shelf power supply like this one, or even an old ATX power supply from a computer (just switch to using 12V instead of 24V). Just trying to save you some time and be more safety-conscious... I've found that if there's a way to avoid working with 120VAC that doesn't actually cost much more (and probably saves time), it's best to do it.


February 13, 2011
by sask55
sask55's Avatar

Hello I have bean trying to build a refrigerator control to replace the mechanical control that is not working on an old frig. The project works well on my bench. I wanted to experiment with what I could be done. I have incorporated a number of timers and provisions to record information (highest temp lowest temp, duty cycle ext.) as well as a simple four button control panel to allow a user set various factor. The control buttons, programming and information reporting seams to be working wellon my bench. The control circuit will reliably open and close the relay that is intended to control the frig compressor, as long as it is not actually near the refrigerator. The issue that I cannot seam to solve is that it seams that my project is very sensitive to any electrical interference. I am suspecting that I am getting some kind of voltage spikes or RF pulse. I have tried a number of different relays the results are always the same. I have verified that there is no measurable electrical connection within the relays, i.e. between the activation coil and the switching contacts. The nerd kit micro reboots almost ever time the compressor on the frig start or stops. That is to say that the lcd screen goes blank for a fraction of a second, all timers and data variables are reset. The strange thing is that this will occasionally occur even when the relay is not connected to anything at all if the project is near the frig. It has happened even when I am running the project on a battery or an ac adapter plug in a completely different circuit. I have tried temporarily disabling the temperature sensor in the code just prior to the relay changing states that does not help. I have noticed that it will sometimes restart when some other electrical appliance in the same area starts up. i.e. when the deep freeze or the spin cycle on the washing machine start in the same room as my project is running. These appliances are not connected to the project in any way so it must be some kind of week EMP that the project cannot deal with. I have tried shielding the circuit board by wrapping it in aluminium foil connected to ground, that does not seam to help. I have also determined that the project will restart just about every time I turn on the heavy duty 12V power supply on my bench, even if there is nothing connected to that supply. I am suspicious that the problem is related to the fact that I have a number of longer cables connecting various components of the project. There is a six for long shieled cable between the LM34 and the circuit board. I did incorporate an R-C damper at the temperature sensor end of this cable as per the data sheet. The frig control relay is also separated from the circuit board by a six for cable with three conductors a 12v, a ground and a control wire to the gate of a 2n7000 at the relay. Just to clarify things the relay does work to control the frig compressor if I manually make the gate voltage go high on the relay control, and the control pin on the MCU works well on the bench to open and close the relay circuit , it will control a VOM meter connected across the relay output terminals just fine. The problem is once the MCU closes the relay when the frig is connected, the compressor begins to start up and 80% of the time the MCU restarts. I assume (now) that all this wiring is acting like antennas and delivering pulses to the circuit board or maybe I am missing something else. So, to some things up I do not have a clue why this project is behaving the way it is. If anyone has any suggestions or ideas that I could try, or could point out some error that I am making, I would greatly appreciate any help I can get. I would really like to get this project to work with the frig compressor as well as it seams to work sitting on my bench with no larger electrical loads switching near it.

February 13, 2011
by sask55
sask55's Avatar

sorry I posted my last post to the wrong location I wanted to be in basic electronic. I am not sure what I should do now. Repost or just hope this gets some helpfull comments?

Post a Reply

Please log in to post a reply.

Did you know that our customers love us? Learn more...