Lighting Project Pictures

[SQUEAK]

Hello all,

I have been working on a design project. A few of you may vaguely remember me making a thread talking a bit about LED lighting in the planted tank. I've progressed further with it, and made a prototype model. The dimensions were exaggerated to avoid thermal problems - the actual board you would use could (and probably would) be significantly smaller than the one I have, as mine is a whopping 13" by 7" by .125"! The basic design is a CCM-mode buck regulator at top, driving a series-chain LED string using a hysteretic feedback (internal comparator between .2V to .3V). This board uses that measurement to regulate the current to a 'fixed' value (sort of).

Using a CCM-mode regulator provides two large benefits we care about for now:

1. Very low resistance - the only resistor in the current path is 0.43 ohms! Whenever you have resistance, you have power loss. This uses a more advanced technique to deliver the power, instead of limiting your current through just a resistor. For 20mA this is fine, for 350-450mA the power loss adds up quickly.

2. Current limiting. As LED's heat up (and believe me, they do!!!), and just as they are used, their properties change - a passive circuit will not scale back to protect the LED's from burning out, this one should hold to a steady current (dictated by the hysteretic feedback).

The part we actually care about is that there are 10 ultrabright LED's being driven at 450mA each, with a total voltage drop of about 31V. Meaning, that we can approximate this to mean that there is about 15W of electrical power being dissipated across the LED chain. I am still working on extracting the optical performance, but absolute measurement is not simple. You cannot directly relate the electrical power to optical power, unfortunately (that I know of).

Additionally, these LED's can also be dimmed at the users choice, using a simple DC voltage. What actually starts happening is the LED's 'flicker' on and off faster than we can see - resulting in an optical dimming which to us looks solid, however does not change the light frequency. Using a small DC voltage, you can control how long the LED's remain on per cycle (say, 80% per cycle... or 100%, or 50%, etc). For normal planted tank I can't think why this would be anything but a fun effect, but for some marine applications there may be some reason to use it.

Below I have attached a few photograhps I took a few days ago. I cannot actually photograph this circuit in operation, as it is dangerous to look directly into the LED's while operating, and the intensity would saturate my CCD (and can cause permanent damage to my camera). I assure you though, it is very bright, if you cannot even look at it. It is an interesting project, even if it is a complete pain in my rear.

OSRAM slightly conflicts on their LED specification. Their website states a nominal 55lm/watt, while their datasheet lists 40lm/watt. I do not know offhand which one is correct, however we can approximate the theoretical:

15W * 55lm/watt = 825lm

15W * 40lm/watt = 600lm

Remember that this is not directly correlatable to say, a fluorescent light or incandescent. While the total intensity matters, you also need consideration of directionality. These LED's are Lambertian emitters, and for all intents and purposes we can deal with them being largely on a half-circle radius (180 degrees). Fluorescents and similar are the full 360 degrees - and must use reflectors to reclaim part of that light. Part of the light is still lost.

Incidentally, thank you to National Semiconductor for sponsoring this project.






*note: the white stuff is thermal grease, I swear.

[ctlam]

Hi,

Can you share the schematic to us.
It looks like you are using high DC voltage to supply to your board. Is that correct?
Are you using PWM for the control circuit?

Thanks
Regards
Lam

[SQUEAK]

Hi,

Can you share the schematic to us.
It looks like you are using high DC voltage to supply to your board. Is that correct?
Are you using PWM for the control circuit?

Thanks
Regards
Lam

Hi Lam,

The schematic still has some errors in it, I need to redo a few parts of the schematic when I have time - but I am struggling to graduate from university so I am going to wait on that.

Yes, it is a relatively high DC voltage. I use a 48V DC voltage in, but it can reasonably be between about 45-52V DC for correct operation. This schematic generates its own PWM. If you see the 3 chips on the top right of the board, those are operational amplifiers (op-amps) that I use to generate the PWM. That will dim the lights by turning them on and off about 200 times per second, and vary how long per time they are on (if that makes sense).