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Thread: Laptop power supply for driving LEDs

  1. #1
    Flashaholic anuragwap's Avatar
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    Default Laptop power supply for driving LEDs

    Update: I've tested my laptop brick with DMM, it outputs 0.5V higher than specified 19.2V. This overvoltage may damage the LEDs. Or maybe, voltage sags a bit under full load? Anyways, I'll go with dedicated solutions like Meanwell LPC/CLG series.
    Last edited by anuragwap; 01-25-2013 at 12:26 AM.

  2. #2
    Flashaholic* idleprocess's Avatar
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    Default Re: Laptop power supply for driving LEDs

    Generally, DC power supplies are constant-voltage since that's both more useful and considerably cheaper than constant-current. A laptop power supply will generally be regulated at its rated voltage and be fairly consistent through its current range; the current rating will be a maximum rating.

    Standalone power supplies capable of current regulation are most often found in labs and in factory QC stations where they're used for testing purposes (these devices tend to offer variable voltage and current, with the peak voltage determining if the device can supply the specified current). They're appreciably more expensive than constant-voltage power supplies and of limited use in "production" where current demand tends to be variable. Inside devices, there are often constant-current sources of course, but those are typically performing DC-DC conversion.
    Last edited by idleprocess; 06-08-2012 at 10:35 PM.
    I apologize that this letter is so long; I did not have time to write a short letter

  3. #3
    Unenlightened M79's Avatar
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    Default Re: Laptop power supply for driving LEDs

    I`m using several DC constant voltage PSU`s, including laptop PSU`s - because they are very cheap and often just lay arround. They are all constant voltage. I am in design phase of my project(s) and still use the mentioned PSUs, because I spent all the cash for LEDs, can`t afford nice PSU right now . I did a little research online and measured a bit too.

    First thought is that you will rather want to use some driver (between leds and PSU) to get constant current, because your leds will be fed with as much current as they only take, and will probably fry (at 4 ampere or sooner), or will live very, very short. This is rather important...

    2nd: the driver (step-down/buck)will take about ~0,7-0,75V for his own usage. So you will have ~18,25V left for your disposal. I would definetely go for a "I" regulated (potentiometer or pins) driver, it is much more flexible and it will allow you to regulate the current to the level, where your LEDs eat less voltage than it is disposable.

    I use some 31V HP printer PSUs, which are pretty much over of needed voltage of my setup (24V). It looks like that the difference of voltage is toasted (6V) somewhere, I see that the unit draws so much power, as there would be 30V worth LEDs applied. So the more gap beetween voltage of the PSU and voltage drop of your leds, the more power is lost. I don`t think that this is in the driver (alone!), probably the laptop PSU are not designed for less voltage, and allways eat at their V specs.

    4th: efficiency of your laptop PSU will be best above 50% of its max current design, preferabely above 70%. So if you will constat drive (with a buck driver) your leds at 1A, then it will be around 23% of max output of your PSU. In this area your PSU will be as low as (very roughly) 40-60 % efficient. Modern laptop PSU`s are about 80-90% efficient at about 90% of their max current output. So for better efficiency I would take a smaller PSU or I would attach more LEDs
    Last edited by M79; 06-22-2012 at 10:44 AM.

  4. #4

    Default Re: Laptop power supply for driving LEDs

    I use constant voltage sources all the time for driving LED's. Provided the supply is stable and forward voltage aligns right they work fine. Not the most efficient power supplies on the black, but typically stable and throw a lot of current. I typically use HP Bricks which are 18.5 volts, and drive 5 older series LEDs like Asian's or older XP-E's. This gets me around 700-900mA, and have run for years at 24/7.

    A 19volt laptop brick will drive six Rebels at 3.17 volts, and over time forward voltage will drop a bit. Looking at the Rebel ES current rating that's around 1.2amps. The regular blue and green have higher forward voltages. Provided you have good heat -sinking, and I mean GOOD heat sinking there should be no problem with this. You can also drop another series of six in parallel if you want on the same supply without any issues. That's a big advantage with fixed voltage sources - no drama with parallel runs.

  5. #5

    Default Re: Laptop power supply for driving LEDs

    because your leds will be fed with as much current as they only take
    No. LED's will push as much current as their forward voltage allows. If you have 6 LEDs on a 19volt DC circuit each LED will be getting ~3.17 volts. You then use the chart associated with each specific LED and this works out to around 1.2amps for a royal blue. Been running LED's off laptop bricks and other fixed voltage supplies for years and it works out surprisingly well. Forward voltage will drop a tad as the LED's burn in, but nothing you can't guess for. I use current regulated supplies when I want an exact current amount or absolute efficiency.

  6. #6
    Unenlightened M79's Avatar
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    Default Re: Laptop power supply for driving LEDs

    Thanks Blasterman for correcting me, I didn`t think about this scenario. I admit that such solution is very tempting.

    My thinking is right only in case of connecting less than 5 leds in Anuragwap`s setup...

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