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I'm putting together a custom aquarium lamp as a hobby project and I'm not sure of the best way to configure the LEDs.

All the LEDs are Cree and I am looking to push ~800ma through them.

5x whites with a forward voltage of 3.4v (XTEAWT-00-0000-000000H51)

2x blues with a forward voltage of 3.4v (XTEARY-02-0000-000000Q04)

3x reds with a forward voltage of 2.1v (XPERED-L1-0000-00801)

I am not sure whether to go with a dedicated LED driver or a standard 12v strip supply. I would prefer a 12v supply as they are available locally.

I figured to run with 12v:


3.4v - 3.4v - 3.4v - 2.20hm 5W resistor = 818ma, 1.5w in the resistor.

parallel

3.4v - 3.4v - 3.4v - 2.20hm 5W resistor = 818ma, 1.5w in the resistor.

parallel

2.1 - 2.1 - 2.1 - 3.4 - 2.7ohm 5W resistor = 851ma, 1.8w in the resistor.


I am concerned about the power dissipation in the resistor being high and adding to the thermal load.

Is there a better way to go about this? I looked for 'dual channel' LED drivers so I could hook up two strings of 3.4v and 2.1v but they don't seem to exist.

I considered using two 800ma drivers into one series line of 3.4v and one series line of 2.1v but that seems ugly.

I looked into some preassembled aquarium lamps and they seem to use SMD resistors only, so there must be a trick I am missing.

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  • \$\begingroup\$ will they be always on and with 800mA? You could put them all in series with a single constant current driver. Also note that you will need to dissipate quite a lot of heat (20~25W) \$\endgroup\$ – Wesley Lee Apr 13 '16 at 6:42
  • \$\begingroup\$ Are the prebuilt ones using 3W leds? Probably not. \$\endgroup\$ – Passerby Apr 13 '16 at 7:17
  • \$\begingroup\$ @WesleyLee Timed, on for about 8 hours a day. I want them bright so yes 800ma... Does a single constant current driver work though with different forward voltage LEDs in series? Won't the voltage drop be the same across all the LEDs resulting in some being overvoltaged or some undervoltaged? \$\endgroup\$ – Lascaille Apr 13 '16 at 9:21
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    \$\begingroup\$ Since the LEDs will be in series, what will be in common amongst them is the current, not the voltage. So If all of them are rated and properly cooled to handle 800mA, it will be ok. \$\endgroup\$ – Wesley Lee Apr 13 '16 at 9:30
  • \$\begingroup\$ @WesleyLee so a LED constant current driver for 800ma and with a voltage range of 25-45v should be adequate with all the diodes hooked up in series and no resistors in the circuit, I guess? \$\endgroup\$ – Lascaille Apr 13 '16 at 9:35
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I would seriously consider using a switched mode driving scheme to prevent wasting a ton of power in resistors and/or linear drivers, superheating your poor fish, upping your electric bill, and lining the pockets of greedy electricity companies. There are a ton of application notes about designing them using DC DC converter IC's. Depending on your experience, skill, and desire to make a project out of this, you could design your own, or buy one which already does this.

http://www.digikey.com/en/articles/techzone/2011/jun/buck-regulators-make-driving-high-brightness-leds-easy

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