LED power questions - i know bog all about electronics...

Discussion in 'General Electronics Chat' started by i8cookie, Nov 12, 2009.

  1. i8cookie

    Thread Starter Member

    Nov 12, 2009
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    Hi guys,
    I would be very grateful for some help if you have the patience with a electronics numbnut.
    I just bought one of these:
    http://www.virtualvillage.co.uk/20w-white-bright-1000lm-power-led-prolight-light-lamp.html

    It says it uses forward voltage 13.5 - 15v DC, and forward current 1400ma - 1800ma

    Can I use a 15v dc one of these to power it even though it says 500ma?:
    http://www.maplin.co.uk/module.aspx?moduleno=13453

    I never really understood the difference between volts and amps... I just need this LED to be as bright as possible, how do I do this?

    thanks in advance
    8
     
  2. i8cookie

    Thread Starter Member

    Nov 12, 2009
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    also, how do i find out the polarity of this led?
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers

    It will go through the basics.

    An LED will light given current and enough voltage to start conducting. You need a resistor as well as a power supply. The power supply shown is totally inadequate though, you need something about 5 V greater than dropping voltage of the LED, and at least the current (more is a lot better). This is just the minimum specs.


    Plan on either a largish resistor whose exact value will be determined after we see the power supply, or a simple regulator integrated circuit (they come in all flavors, and really are easy to use).

    Another way to go about it is to buy a PuckBuck (google it for your area, since you are in the UK). The power supply is still a must though.

    As for the specs, there should be a datasheet somewhere for it, that is where the answer is. You probably need to contact the vendor to get one, but any decent retailer should have it on their web site.

    You can get this to light with a lot less current, but it would be a waste of the money you're spending, most of the capacity would never be used.

    Another thing, lamps like this get hot. Plan on mounting it to a chunk of metal to soak the heat away. Compared to a light bulb or whatever they are still much more efficient, but that is a lot of current and voltage, there will be some heat on the side.
     
    Last edited: Nov 12, 2009
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Voltage = electrical pressure; analogous to water pressure
    Amperes = flow of electrons; analogous to water flowing in a pipe or a river.

    The LED you have purchased is rated for 20 Watts.
    A very rough calculation shows that 20 Watts will be at about 13.8V, 1450mA (1.45A). The individual LED that you receive may have a somewhat higher or lower Vf (forward voltage) for the current rating.

    You will need some type of heat sink to mount the LED to. Don't try to power it up without being mounted on a heat sink, or you will very quickly overheat and destroy it.

    LEDs must have their power source regulated by current. With small, low-power LEDs, it is typical to use low-cost resistors to limit the current. However, with high-power LEDs like the one you bought, you really need to use a "buck" or "buck-boost" type regulator.

    In the States, a couple of popular models go by the trade names "BuckPuck" and "PowerPuck". You would need one rated for around 1400mA to 1500mA, preferably adjustable.

    These regulated current supplies are not 100% efficient, and they require "headroom" of at least a couple of volts above the LED's operating voltage.
    A scientifically calculated wild guess suggests that for a BuckPuck with a 1.5A output, you will need a power supply that will output at least 17 Volts DC at 1.7 Amperes; this is based on an LED Vf of 14v at 1.5A current, 88% efficiency. An 18V 2A supply would work smashingly.

    Up to a certain point, the more current you put through an LED, the brighter it is. However, operating an LED at maximum or over maximum current will shorten it's life considerably.

    I must caution you that the LED you ordered will be extremely bright; so bright that it will quickly cause permanent eye damage if you stare directly at it. Use some type of light diffuser between your eyes and the LED; even a plain sheet of typewriting paper about a foot away from the LED should be enough diffusion to protect your eyesight.

    The polarity of the LED should be indicated in the datasheet, or at least indicated by the vendor.
    If that information is not provided, it will be easy to find out; as if it is connected with the wrong polarity, no light will be emitted from the LED.

    I recommend that you experiment with inexpensive, low-powered LEDs prior to experimenting with this high-powered LED. Even the low-powered super-bright LEDs can cause permanent eye damage if you stare at them - you have been duly cautioned. Bill_Marsden's blog has plenty of such experiments.
     
  5. i8cookie

    Thread Starter Member

    Nov 12, 2009
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    thanks for the advice!

    I'm actually going to use the LED to replace a conventional projector bulb in my old Super 8 Projector. I was sick of having to pay £15 or more to replace a rare old-fashioned bulb that burns out quicker than grandma on a hike.
    So I thought I'd power the LED separately and it'll last much longer than the bulb.

    Here's the projector:
    http://i760.photobucket.com/albums/xx243/i84cookie/projector.jpg
    And where the LED will be mounted:
    http://i760.photobucket.com/albums/xx243/i84cookie/insideProjector.jpg

    How long do you think this LED will last, if it is possible to guess at this kind of thing based on your previous experience?

    Will an old computer CPU heatsink be suitable for cooling?

    one last question before I head out to the electronics shop,
    this is going to be expensive isn't it...?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Hilarious! :D
    Sounds like a logical conclusion. The LED will also be much more rugged than an incandescent bulb, after it's suitably anchored down.

    A typical LED can have a very long life, 100,000 hours, if operated at less than it's rated power. As LEDs age during operation, they become less bright. When they reach about 1/2 of the original intensity, they need to be replaced. If you operate the LED at maximum brightness initially, it will rapidly become dim - and there is no good way to reverse this dimming.

    That is a good bet. The best heat sinks are made of copper; copper conducts heat nearly twice as efficiently as aluminum/aluminium. You will need to use a little bit of heat sink compound between the base of the LED and the heat sink. This heat sink compound stuff usually looks like a white paste. You will need to drill holes through the heat sink to attach the LED firmly with very small screws or bolts. Make certain that the heat sink is positioned so that the projector's cooling fan will blow a good stream of air on the heat sinks' fins.

    Well, that depends.

    You may be better off transferring all of your movies to video.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    It probably obvious we'll help you with the electronics, you may have to do a bit of machining though. The electronics behind LEDs is actually pretty simple.
     
  8. i8cookie

    Thread Starter Member

    Nov 12, 2009
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    ok, the main electronics shop in the uk is Maplins. The people in there know absolutely nothing about anything except how to sell me the latest dvd/blueray player...

    I've been looking around this morning on the net. I've found this power regulator:
    http://www.reuk.co.uk/buy-LM317T.htm

    And this power supply:
    http://www.sunpower-uk.com/product/40W-18V-2-22A-Desktop-Power-Supply/P40A-5P2J/default.htm

    The power supply has higher amps than I need but will it be ok? I'm having trouble finding a psu with a close enough spec.


    Also I found some electronics lying around in my house that I could practice on. This breadboard and a microprocessor that I got during my degree but never used and failed to give back. But it's got an LED on there that I can try to light:
    http://i760.photobucket.com/albums/xx243/i84cookie/breadboard.jpg

    Plus some extra bits and bobs. A power regulator that's too weak for my big LED. Excuse my ignorance, are these little blue things resistors? I've lost all the documentation that came with this stuff:
    http://i760.photobucket.com/albums/xx243/i84cookie/bitsandbobs.jpg
     
  9. i8cookie

    Thread Starter Member

    Nov 12, 2009
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    BTW just got that LED on the breadboard going with the ts7805 power regulator and a 9v battery. I'm 99% of the way to becoming a fully fledged mad scientist!!
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    Nope, that requires a Tesla coil and/or Jacobs Ladder. They give your hair that special I don't care anymore look.
     
  11. i8cookie

    Thread Starter Member

    Nov 12, 2009
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    thanks Bill. But some advice on those components that I'm gonna buy would be great...!
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    The no-name-brand expensive LED does not have a datasheet.
    Buy a name-brand less expensive one that has a datasheet.

    Don't overload a 500mA adapter with more than 3 times its max allowed current.
     
  13. i8cookie

    Thread Starter Member

    Nov 12, 2009
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    Thanks for the advice. However, it's not much use telling me things I have done wrong without explaining anything. I have no experience with electronics at all.

    I don't know any brands, much less where to buy from. I don't know what a datasheet is! What will it tell me more than I already know about how to power this thing? What more do I need to know other than that it needs 13.5 - 15v and 1.4 - 1.8A?

    Where are you getting 500ma from? The specifications of the adapter says 2.22A, how will I overload it?
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Sorry about that, i8cookie. It's hard to know what experience level someone might have.

    Where are you located? I'm assuming the UK, because websites are located there. It helps a great deal if you put your general location information in your profile. Country and time zone are usually sufficient.

    In the UK, Farnell, RS Components and Maplin seem to be popular. There may be other shops, I quite frankly do not know.

    Cree, Lumiled, OSRAM; there are many others. Look at the above vendors.
    A "datasheet" is supplied by a manufacturer of a device.
    It contains very detailed information about the device; electrical as well as physical.

    That is a pretty general range. A datasheet would likely have graphs/plots showing the information.

    He apparently just looked at your 1st post. The 2.2A 18v switching regulated supply would be a very good choice for this application.
     
  15. i8cookie

    Thread Starter Member

    Nov 12, 2009
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    ahh ok thanks sarge. I'll give that power adapter a go. Is this voltage regulator ok for this circuit acting as a 'PowerBuck' or 'puck' that you guys were talking about before?
    http://www.reuk.co.uk/buy-LM317T.htm
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    I think that power supply will work fine. How much current are you planning for your LED? If I read the specs correctly the power supply will shut down if it gets hot, so don't enclose it in a tight space where it will be an oven.

    The LM317 will work, but it isn't very efficient. Efficient in this case translates as heat, the regulator will get pretty hot if it is used at an amp.

    Yes, that is a resistor. Here is how you read them. There is also a little neumonic we teach new techs, "Bad Boys Ravaged Our Young Girls, But Violet Gives Willingy", which translates to 0=Black, 1=Brown, 2=Red, 3=Orange, 4=Yellow, 5=Green, 6=Blue, 7=Violet, 8=Grey, and 9=White.

    A PuckBuck doesn't get very hot as I understand it, it is a lot more efficient. Wookie and I have some differences on how they work, but I think we both agree they are better than a LM317 for this application.
     
  17. i8cookie

    Thread Starter Member

    Nov 12, 2009
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    ok cool thanks Bill. I was planning about 15-17 volts for my LED.

    I'm having trouble finding a puckbuck, they dont seem to sell them at any of the uk electronics shops. All i can find under 'buck' are these liner voltage regulators eg:
    http://uk.farnell.com/jsp/search/br...pliedparametrics=true&locale=en_UK&catalogId=

    Are they similar? None seem suitable anyway they all go from max output 12v straight up to 20 something.
    Also one of these has a minus voltage and amps spec, -15v -1.5a, what does that mean? Would be perfect apart from the minus rating.
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    Google puckbuck, you'll get better results. PuckBuck is a proprietary name, although I've found what I think are two different companies making it. I'm trying to come up with some alternates, and have actually found some schematics with a lot of help from my friends, but a simple module is the way to go.

    You might think of using the LM317 on a temporary basis, till better parts come in. I suspect you have some machining to do and parts to build. Again, how much current are you currently planning to use?
     
  19. SgtWookie

    Expert

    Jul 17, 2007
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    An LM317 is a linear regulator. As Bill said, it will be very inefficient, and will dissipate a lot of heat.

    "BuckPucks" are switching current regulators. They are very efficient, very small, and do not dissipate power as heat like the LM317's do. They are certainly more expensive than LM317 regulators are, but energy is VERY expensive nowadays; the increased efficiency of the "BuckPuck" will soon pay for itself in energy savings.

    Unless you need another room heater, that is.
     
  20. i8cookie

    Thread Starter Member

    Nov 12, 2009
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    I was thinking of a current of 1500ma.
    (I googled 'puckbuck' before when you first mentioned it, you get a lot of strange things.)

    A quick look at the 'BuckPucks', there doesn't seem to be any that go higher than 1000ma, same with the 'powerpucks', which I assume are pretty much the same. Such as:
    http://www.ultraleds.co.uk/buckpuck-03023de1000p-1000ma-1w3w-luxeon-driver-p-1374.html

    Will 1000ma be alright with my LED? Surely it would make it significantly dimmer or fail to light it at all. Here's the spec again from the website:

    • Forward voltage [VF]: 13.5 - 15V DC
    • Forward current [IF]: 1400mA - 1800mA
     
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