SYSMAX Ind./Clike to find more about Nitecore flashlights!
Results 1 to 30 of 30

Thread: Cree P4 under-cabinet lights with dc power supply

  1. #1
    Flashaholic
    Join Date
    Feb 2007
    Location
    boston
    Posts
    287

    Question Cree P4 under-cabinet lights with dc power supply

    Im creating under-cabinet lighting, and my plan is to run a single Cree P4 on a dc power supply. heres the setup: radioshack dc power supply, set to 4.5 volts, 1 amp rated maximum, run thru 1 ohm 10 watt resistor and the led. im actually using 2 .5ohm 5 watt resistors in series. the led is mounted with thermal paste on a big heatsink.

    i couldnt find a power supply with a voltage closer to 3.7v, so i had to work with what i had. Obviously 4.5v is too much for the led, without the resistor it drew 1.5 amps (too much for the led or the power supply). i added a .5ohm resistor and it went down to 1.2 amps. added another resistor (total of 1 ohm) and got 800ma to the led. perfect. at that power, it shouldnt get too too hot (especially with the heatsink) and the power supply is happy. and let me tell you my kitchen counter lighting is now beautiful (and efficient too)!

    the QUESTION is: is this setup safe? according to my knowledge it is, tho my knowledge about these things is extremely limited (i read electronics for dummies). the power supple should have no trouble because it is rated for 1 amp (1000ma) and the led is drawing 800ma (measured with my multimeter in series). the led itself cant start a fire, can it? i cant think of any reason (after some testing) not to leave it on unsupervised. can you?

    my kitchen counter has never looked better. the light is perfect for about 4 square feet but i think it would be better with 2 leds spaced out about 3.5 feet. i think to do this, i could wire 2 leds in series and use something like a 7.4v power supply (give or take a little and/or add a little resistance) that should give me around 800ma to each led (total of 1600ma so the power supply would have to have a high amp rating as well). makes sense right? any thoughts?

  2. #2

    Default Re: Cree P4 under-cabinet lights with dc power supply

    If the dc power supply was set to 4.5 volts @ 1A, how did you draw 1.5 amps to start off with? I think you meant 1A. Correct me if im wrong.

  3. #3

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by moon lander
    the QUESTION is: is this setup safe? according to my knowledge it is, tho my knowledge about these things is extremely limited (i read electronics for dummies). the power supple should have no trouble because it is rated for 1 amp (1000ma) and the led is drawing 800ma (measured with my multimeter in series). the led itself cant start a fire, can it? i cant think of any reason (after some testing) not to leave it on unsupervised. can you?
    If you're using a UL-listed power supply, there shouldn't be reason to worry.

    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.
    Last edited by walkabout; 04-04-2007 at 06:38 PM.

  4. #4
    Flashaholic
    Join Date
    Feb 2007
    Location
    boston
    Posts
    287

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by bascerballer4
    If the dc power supply was set to 4.5 volts @ 1A, how did you draw 1.5 amps to start off with? I think you meant 1A. Correct me if im wrong.
    actually it did draw 1.5amps. the maximum rating for the power supply is 1 amp but i exceeded that for a very brief moment (luckily nothing happened). the 1 amp rating is the maximum recommended current before it starts to melt. apparently its flexible for very brief power.

  5. #5

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Many supplies will exceed their current ratings as they approach a shorted condition, but their voltage will sag at the same time.

    BTW, when you say, "Radio Shack power supply set to 4.5VDC," are you talking about the wall warts (wall transformers)?
    "You reached for the secret too soon, you cried for the moon.
    Threatened by shadows at night, and exposed in the light." -- Roger Waters, Shine On You Crazy Diamond

  6. #6
    Flashaholic Biker Bear's Avatar
    Join Date
    Sep 2006
    Location
    The Greater Los Angeles Metropolitan Sprawl
    Posts
    279

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by moon lander
    my kitchen counter has never looked better. the light is perfect for about 4 square feet but i think it would be better with 2 leds spaced out about 3.5 feet. i think to do this, i could wire 2 leds in series and use something like a 7.4v power supply (give or take a little and/or add a little resistance) that should give me around 800ma to each led (total of 1600ma so the power supply would have to have a high amp rating as well). makes sense right? any thoughts?
    Your calculation of the needed current is wrong.

    When you run LEDs in series, the needed voltage goes up, but the amperage is unchanged. When you run LEDs in parallel, the needed voltage remains the same but the required amperage goes up.

    You could look at LEDs as water valves that only open (emit light) when the water in the pipe is of a certain pressure (voltage). Thus, if you have two in the same pipe (in series), your input pressure/voltage will have to be higher than with just one - but the two of them will pass the same amount of water (current/amperage) because it's only one pipe.

    If you have two pipes side by side - one valve in each - that are fed from a bigger pipe, you're going to need just enough pressure for one valve - but you're going to need to deliver twice as much water.

    Granted, these analogies can only be pushed so far - but I hope that makes what's going on a bit clearer.
    Violence is the last refuge of the incompetent. -- Salvor Hardin
    Censorship is telling a man he canít have a steak just because a baby canít chew it. -- Mark Twain

  7. #7

    Default Re: Cree P4 under-cabinet lights with dc power supply

    The problem with LEDs and water analogies is that water acts in linear fashion, whilst LEDs (being a semiconductor junction) are exponential. That's why the OP needed a resistor from the very beginning, even if he was to find a 3.7VDC supply that would "work." A change in voltage across the junction results in a very large swing in current, without a resistor.
    "You reached for the secret too soon, you cried for the moon.
    Threatened by shadows at night, and exposed in the light." -- Roger Waters, Shine On You Crazy Diamond

  8. #8
    Flashaholic
    Join Date
    Feb 2007
    Location
    boston
    Posts
    287

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by walkabout
    If you're using a UL-listed power supply, there shouldn't be reason to worry.

    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.
    im sorry, but i dont know what UL-listed means. its a radioshack wall wart that can be set to 3, 4.5, 7, 9, or 12 volts. 1 amp rated.

  9. #9
    Flashaholic
    Join Date
    Feb 2007
    Location
    boston
    Posts
    287

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by riffraff
    Many supplies will exceed their current ratings as they approach a shorted condition, but their voltage will sag at the same time.

    BTW, when you say, "Radio Shack power supply set to 4.5VDC," are you talking about the wall warts (wall transformers)?
    yes. wall wart with a switch on the front to set the voltage, rated 1 amp max.

  10. #10
    Flashaholic
    Join Date
    Feb 2007
    Location
    boston
    Posts
    287

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by Biker Bear
    Your calculation of the needed current is wrong.

    When you run LEDs in series, the needed voltage goes up, but the amperage is unchanged. When you run LEDs in parallel, the needed voltage remains the same but the required amperage goes up.

    You could look at LEDs as water valves that only open (emit light) when the water in the pipe is of a certain pressure (voltage). Thus, if you have two in the same pipe (in series), your input pressure/voltage will have to be higher than with just one - but the two of them will pass the same amount of water (current/amperage) because it's only one pipe.

    If you have two pipes side by side - one valve in each - that are fed from a bigger pipe, you're going to need just enough pressure for one valve - but you're going to need to deliver twice as much water.

    Granted, these analogies can only be pushed so far - but I hope that makes what's going on a bit clearer.
    right you are! thanks for the correction. i really like your analogy. so i can use the same supply for 2 leds, ill just set the voltage higher and check that the current doesnt exceed 1amp. thanks!

  11. #11

    Default Re: Cree P4 under-cabinet lights with dc power supply

    UL = Underwriters Laboratories. Required for just about anything that connects to house current (certainly all consumer goods).

    Most of the wall warts are not regulated, so you may not be getting a true 4.5VDC.

    You said you saw it go up to 1.5 Amps...I assume you have a multimeter? Do you have two? So you can monitor the current and the voltage at the same time?
    "You reached for the secret too soon, you cried for the moon.
    Threatened by shadows at night, and exposed in the light." -- Roger Waters, Shine On You Crazy Diamond

  12. #12
    Flashaholic
    Join Date
    Feb 2007
    Location
    boston
    Posts
    287

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by riffraff
    UL = Underwriters Laboratories. Required for just about anything that connects to house current (certainly all consumer goods).

    Most of the wall warts are not regulated, so you may not be getting a true 4.5VDC.

    You said you saw it go up to 1.5 Amps...I assume you have a multimeter? Do you have two? So you can monitor the current and the voltage at the same time?
    will it say UL on the back? im not home so i cant check until 1am.
    i checked the voltage (not under load) and it reads high. the 3v setting read 3.4 and the 4.5v setting read 4.8. i only have 1 multimeter so i cant check that. i havent checked the voltage under load. not sure how to do this. do i place the test probes in series with the led/power supply circuit? or parallel across the led? thanks again.

  13. #13

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Have a look here for a series/parallel LED calculator.

    If you have two LEDs drawing 800mA each, that's 1.6 Amps. You gonna need a bigger boat, er, supply, rather.


    "You reached for the secret too soon, you cried for the moon.
    Threatened by shadows at night, and exposed in the light." -- Roger Waters, Shine On You Crazy Diamond

  14. #14
    Flashaholic
    Join Date
    Feb 2007
    Location
    boston
    Posts
    287

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by riffraff
    Have a look here for a series/parallel LED calculator.

    If you have two LEDs drawing 800mA each, that's 1.6 Amps. You gonna need a bigger boat, er, supply, rather.



    i think in parallel i would, but in series i should be ok right?

  15. #15

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Should be okay, but there's a greater likelihood of the brightness being different (due to differences in the performance of the junctions, a difference which is minimized in a parallel circuit).

    You're looking at a 2.2 ohm resistor of about 3 watts (to be safe; don't want to set fire to the cabinets). This is assuming a true 9VDC supply, a forward voltage of 3.7VDC, and 800mA draw.
    "You reached for the secret too soon, you cried for the moon.
    Threatened by shadows at night, and exposed in the light." -- Roger Waters, Shine On You Crazy Diamond

  16. #16
    Flashaholic
    Join Date
    Dec 2002
    Location
    Dallas, TX
    Posts
    313

    Default Re: Cree P4 under-cabinet lights with dc power supply

    moon lander;

    Do you have a good heat sink on the Cree ??

    If it gets hot, the voltage accross the LED will fall and the current will go up.

    You need to test it @ warm temperatures to see if the current increases as the LED heats up.

    Run it about 10 minutes at least to check for higher current.

    My Cree heatsink is shown on this post:

    http://www.candlepowerforums.com/vb/...d.php?t=155211

    Larry Cobb
    Last edited by LEDite; 04-04-2007 at 11:08 PM.
    UV Lights, Panasonic 3400mah #18650 cells available

  17. #17
    Flashaholic
    Join Date
    Feb 2007
    Location
    boston
    Posts
    287

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by LEDite
    moon lander;

    Do you have a good heat sink on the Cree ??

    If it gets hot, the voltage accross the LED will fall and the current will go up.

    You need to test it @ warm temperatures to see if the current increases as the LED heats up.

    Run it about 10 minutes at least to check for higher current.

    My Cree heatsink is shown on this post:

    http://www.candlepowerforums.com/vb/...d.php?t=155211

    Larry Cobb
    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". i guess ill have to add more resistance. it is currently running at 870ma, i bet it will be ok around 750ma. i did not check the current when it was hot but i bet your right. also the resistors heat up a lot. i wonder if any of this could get hot enough to ignite something? Larry i love your triple cree house light!

  18. #18

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by riffraff
    Should be okay, but there's a greater likelihood of the brightness being different (due to differences in the performance of the junctions, a difference which is minimized in a parallel circuit).
    The difference in brightness will be minimized when the two LEDs are wired in series. That ensures that the same current is going through both LEDs.


    A couple of things: the power supply is rated at 1 amp, meaning it can safely supply that much current. It is not a regulated supply, meaning that if you ask it fore more than 1 amp, it might deliver it, unsafely. It's up to you to make sure that the load does not draw too much current.

    If you're going to use a meter to measure the current, don't just put the meter in series with the power supply. The voltage drop across the ammeter will reduce the total current. The proper way to measure the current is to put a low value resistor (0.1 or 0.05 Ohms) in series, and measure the voltage drop across the resistor.

  19. #19

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by riffraff
    The problem with LEDs and water analogies is that water acts in linear fashion, whilst LEDs (being a semiconductor junction) are exponential.
    Well it's not the water that's non-linear in this analogy. Think of the LED as a pressure release valve that has a non-linear spring in it.

  20. #20
    Flashaholic Biker Bear's Avatar
    Join Date
    Sep 2006
    Location
    The Greater Los Angeles Metropolitan Sprawl
    Posts
    279

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by balazer
    Well it's not the water that's non-linear in this analogy. Think of the LED as a pressure release valve that has a non-linear spring in it.
    And I did say that water analogies can only be pushed so far before they obscure more than they elucidate....
    Violence is the last refuge of the incompetent. -- Salvor Hardin
    Censorship is telling a man he canít have a steak just because a baby canít chew it. -- Mark Twain

  21. #21
    Flashaholic* Gryloc's Avatar
    Join Date
    Jan 2006
    Location
    Cincinnati, Ohio & North Lewisburg, Ohio
    Posts
    600

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by moon lander
    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

  22. #22
    Flashaholic* 2xTrinity's Avatar
    Join Date
    Dec 2006
    Location
    Arizona
    Posts
    2,383

    Default Re: Cree P4 under-cabinet lights with dc power supply

    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.

  23. #23

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by 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.
    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.)

  24. #24
    Flashaholic* 2xTrinity's Avatar
    Join Date
    Dec 2006
    Location
    Arizona
    Posts
    2,383

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by walkabout
    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.

  25. #25

    Default Re: Cree P4 under-cabinet lights with dc power supply

    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).

  26. #26
    Flashaholic*
    Join Date
    Apr 2002
    Location
    Boise, ID
    Posts
    2,639

    Default Re: Cree P4 under-cabinet lights with dc power supply

    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.

  27. #27
    Flashaholic* AndyTiedye's Avatar
    Join Date
    Oct 2006
    Location
    Santa Cruz Mountains
    Posts
    2,034

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by walkabout
    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.

  28. #28
    Flashaholic
    Join Date
    Dec 2002
    Location
    Dallas, TX
    Posts
    313

    Default Re: Cree P4 under-cabinet lights with dc power supply

    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
    UV Lights, Panasonic 3400mah #18650 cells available

  29. #29
    Flashaholic
    Join Date
    Feb 2007
    Location
    boston
    Posts
    287

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by Gryloc
    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!

  30. #30
    Unenlightened
    Join Date
    Mar 2007
    Location
    Severna Park, MD
    Posts
    10

    Default Re: Cree P4 under-cabinet lights with dc power supply

    Quote Originally Posted by moon lander
    . 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?

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •