Cree P4 under-cabinet lights with dc power supply

[Gryloc]

yes im using a big laptop cpu heatsink, and no it is not sufficient. the light does warm up considerably. after about 15-20 minutes it approaches "too hot to touch".

You said that the light got hot to the touch. Did you mean the entire heatsink, or did you put your fingers on the LED emitter itself? If the heatsink is getting warm, then how big is this CPU heat sink? I know some laptop heat sinks are tough, but if you get one from an older, less powerful laptop, it may not be big enough. How much space do you have for a heatsink? How about a desktop CPU heat sink?

If the LED is getting very warm, but not the heat sink, then how well did you attach the LED? (Thermal paste, screws, epoxy, thermal tape?). As long as you got good thermal contact, then you are just limited to heatsink size and the amount of current you use. LEDs are pretty rugged, they aren't living, so a little extra heat will only reduce the brightness (because of the phosphors) and shorten its life fractionally (I bet you will replace this with brighter and greater LEDs in the near future, anyway he he he). I should take that back. Maybe they are living since I care about LEDs so much . Luckily they are getting cheaper and it won't be the end of the world if they lose 10% of their brightness over a year or two. I don't know. Oh, I am sure a nice sized aluminum or copper plate will work, too. Its low profile and has decent surface area.

Do you have the resistor close or in contact with the heat sink? That may hurt performance. If you worry about the resistor, just support the resistor(s) up by the leads so there is air underneath it and it is isolated from the wooden cabinets. I have purposely used a 10 Ohm 10W ceramic resistor (with the exposed coils on the one side) to put a heavy load on a small lead acid battery. The coils got red hot, but the ceramic outside was fine as it was held in the air. It never did ignite (even after 10 min of this burn). I am so cruel. It never hurts to put a small 1A low voltage fuse in line with the output of the transformer. Keep heat managed, use heavy enough wire, and maybe use a fuse, and it should be worry free. Does this sound right?

Anyways. Crees are nice choices for interior lighting. I use three in a small desk lamp and is bright white and wonderful. The narrower beam pattern is very nice, too. Well, good luck and have fun with them.


-Tony

 

[2xTrinity]

I acutally use battery cell-phone chargers -- they are switching power supplies, and are current regulated as opposed to voltage regulated. For example, I have a 400mA 5V charger from a Nokia cell phone that I am driving a Cree with -- it supplies exactly 400mA at whatever voltage the LED happens to want, provided it's less than 5V. I'm not using any additional resistance. IMHO this is the best way to power LEDs for this sort of application -- electronic switching power supplies (not magnetic wall-warts). They're designed to charge batteries, so they must be well regulated, and prevent things like power surges etc. from propagating -- as the same kinds of power supply fluctuations that might kill an LED would also kill a cell phone.

 

[walkabout]

I acutally use battery cell-phone chargers -- they are switching power supplies, and are current regulated as opposed to voltage regulated. For example, I have a 400mA 5V charger from a Nokia cell phone that I am driving a Cree with -- it supplies exactly 400mA at whatever voltage the LED happens to want, provided it's less than 5V. I'm not using any additional resistance. IMHO this is the best way to power LEDs for this sort of application -- electronic switching power supplies (not magnetic wall-warts). They're designed to charge batteries, so they must be well regulated, and prevent things like power surges etc. from propagating -- as the same kinds of power supply fluctuations that might kill an LED would also kill a cell phone.

Now that is totally cool. The things you learn around here! Thanks for the info.

(I'm modding with Luxeon I LEDs, though -- 350 mA normal, 399 max. Pretty sure I still need a current limiting resistor.)

 

[2xTrinity]

Now that is totally cool. The things you learn around here! Thanks for the info.

(I'm modding with Luxeon I LEDs, though -- 350 mA normal, 399 max. Pretty sure I still need a current limiting resistor.)

Well, problem there is that if you add a resistor, the power supply simply supplies a higher voltage to try to bring the current back up to 400mA. You'd be better off finding a lower-current driver, or running two LEDs in parallel with a low-value resistor on each -- each would be underdriven, which is good if you're running these for fixed lighting (potentially hours straight usage) to prevent as much heat buildup. Two LEDs at 200mA will put out a lot more light, and run cooler (heat is more spread out), than one LED at 400mA.

 

[walkabout]

Ah, good point. I'll try this with a reading lamp I'm modding.

BTW: is there any way to tell if a supply is current regulated just by looking at it? You can often buy boxes of them at yard sales (almost free).

 

[evan9162]

Well, problem there is that if you add a resistor, the power supply simply supplies a higher voltage to try to bring the current back up to 400mA.

But if the supply truly has an upper output voltage that it will regulate at, you can lower the current by putting a large enough resistor in there.

 

[AndyTiedye]

BTW: There are some great threads on LED-based home lighting in "Homemade and Modified Lights." Take some time and browse, it's definitely worth it.

They can also be found in the LED Fixed Lighting Index, to which this thread is being added.

 

[LEDite]

I have used and modified a lot of small switching power supplies.

None of them were current regulated.

They will try to deliver the rated voltage and increase current to at least 125% of the rated nameplate value.

Larry Cobb

 

[moon lander]

You said that the light got hot to the touch. Did you mean the entire heatsink, or did you put your fingers on the LED emitter itself? If the heatsink is getting warm, then how big is this CPU heat sink? I know some laptop heat sinks are tough, but if you get one from an older, less powerful laptop, it may not be big enough. How much space do you have for a heatsink? How about a desktop CPU heat sink?

If the LED is getting very warm, but not the heat sink, then how well did you attach the LED? (Thermal paste, screws, epoxy, thermal tape?). As long as you got good thermal contact, then you are just limited to heatsink size and the amount of current you use. LEDs are pretty rugged, they aren't living, so a little extra heat will only reduce the brightness (because of the phosphors) and shorten its life fractionally (I bet you will replace this with brighter and greater LEDs in the near future, anyway he he he). I should take that back. Maybe they are living since I care about LEDs so much . Luckily they are getting cheaper and it won't be the end of the world if they lose 10% of their brightness over a year or two. I don't know. Oh, I am sure a nice sized aluminum or copper plate will work, too. Its low profile and has decent surface area.

Do you have the resistor close or in contact with the heat sink? That may hurt performance. If you worry about the resistor, just support the resistor(s) up by the leads so there is air underneath it and it is isolated from the wooden cabinets. I have purposely used a 10 Ohm 10W ceramic resistor (with the exposed coils on the one side) to put a heavy load on a small lead acid battery. The coils got red hot, but the ceramic outside was fine as it was held in the air. It never did ignite (even after 10 min of this burn). I am so cruel. It never hurts to put a small 1A low voltage fuse in line with the output of the transformer. Keep heat managed, use heavy enough wire, and maybe use a fuse, and it should be worry free. Does this sound right?

Anyways. Crees are nice choices for interior lighting. I use three in a small desk lamp and is bright white and wonderful. The narrower beam pattern is very nice, too. Well, good luck and have fun with them.


-Tony

yep, the whole heatsink heats up, now im up to 1.5ohms, 700ma and its still heating up too much. your right i think, its now a balance between current and heatsink size. i have a larger heatsink but it is too big fort under the cabinets. ill try to find a sheet of copper or aluminum and thermal epoxy some thin heatsinks to that. or better yet, use 2 smaller heatsinks with 2 crees spaced out about 3 feet at around 500ma each.

any idea where to find a nice thick(ish) sheet of copper or aluminum? home depot had nothing thick when i was there.

funny, i did exactly as you suggest with the resistors right before you suggested it. definately a good idea.

i agree, the crees make wonderful light, i wish my whole house was cree'd up. one day...

thanks for the info and suggestions!

 

[mjzraz2]

. and let me tell you my kitchen counter lighting is now beautiful (and efficient too)!

Any pictures of your setup or before and after with the lights?