3w LEDs run @ 500ma vs 700ma

[jonboy79]

Q1- So i have a selection of different coloured LEDs all in the 3w flavour. Mainly rated for around the 750ma max, i'm thinking about what driver to buy, shall i get the 700ma or the 500ma. This is for a aquarium within a small space. LEDs will be stuck onto 22mm copper end caps, these will be painted with high temperature paint to eliminate any electrical shortages. They are emitter only and do not have the PCB heat spreader and will be attached to there own end cap via thermal epoxy. These will have air flowing over but more of a gentle breeze from left to right so i would be inclined to call them passive. Do you guys think i'm pushing it with 700ma with temp/lifespan?

Q2- My calculations show at 500ma i will be using around 21w, i feel the 25w is to close for comfort so was going to get the meanwell apc 35-500 or 35-700. So i'm going to put these in series, please double check on this as my brain is fried at the moment, and as the different colour LEDs have different forward voltages connect the ones that draw the most first, closer towards the power supply?

Q3- One last question do i still need a resistor for this set up as i thought the resistor was only for running in parallel or where a constant current is not supplied??

Regards

 

[DIWdiver]

I'd go with 500 mA, for a number of reasons:
1. I'm guessing this is something that you'd like to last a good many hours (10,000+), so you really want to keep the dies cool.
2. I wouldn't think that one endcap would be enough for 2W of heat (the other watt goes out as light), especially if there are a group of them together.
3. 20W of LEDs is a lot of light for an aquarium.

If they are all in series, it does not matter what order they are in.

No, you don't need a resistor.

 

[jonboy79]

I'd go with 500 mA, for a number of reasons:
1. I'm guessing this is something that you'd like to last a good many hours (10,000+), so you really want to keep the dies cool.
2. I wouldn't think that one endcap would be enough for 2W of heat (the other watt goes out as light), especially if there are a group of them together.
3. 20W of LEDs is a lot of light for an aquarium.

If they are all in series, it does not matter what order they are in.

No, you don't need a resistor.

Cheers mate you've saved my bacon again, yeah i might half that amount then. I've also brought a sheet of aluminium, 3mm thick that i will drill holes in and set the end caps in that at a space of 10mm apart. Hopefully solder them in to help with heat spread.

 

[DIWdiver]

If you find a way to solder aluminum, let me know. I've brazed it with a propane torch and a stick I got at the home center, but I don't know if that would stick to copper. If so, it would be an awesome way to heatsink the LEDs, but it would electrically short them all. Don't know if that's an issue on your LEDs. It is on some, not on others.

 

[jonboy79]

If you find a way to solder aluminum, let me know. I've brazed it with a propane torch and a stick I got at the home center, but I don't know if that would stick to copper. If so, it would be an awesome way to heatsink the LEDs, but it would electrically short them all. Don't know if that's an issue on your LEDs. It is on some, not on others.

Hi, thanks for your reply, i was leaning towards silver solder, but i havent looked to see if this is possible. I will step drill the hole, hoping that it is a nice snug fit, make a jig to hold the aluminium slightly of the floor the sliver solder to take up any micro gaps and to fill any voids. A dash of epoxy in three places to add bonding to the two metals. Then to spray paint in high temp paint, then thermally mount the emitters to the copper with thermal epoxy. Does that sound optional?
Regards