LED power supply

Discussion in 'General Electronics Chat' started by saspa, Dec 16, 2009.

  1. saspa

    Thread Starter New Member

    Nov 16, 2009
    If somebody could please help me, i want to wire up a room with LED ceiling lights and i need to know what kind of power supply to use, the configuration will be, 4 ceiling lights, 12vdc each, 700mA each, 9 watts each, each ceiling light has a built in constant current driver. i was told i need to used a constant current power supply but i need to know what kind of power supply to use in.
    the wiring configuration will be parallel so i can keep the voltage down. what wattage and current do i use???
    36watts 700mA, 36watts 2800mA??? or something else??
  2. Duane P Wetick

    Active Member

    Apr 23, 2009
    A constant current power supply varies the voltage to keep the current constant with changing load conditions and I don't think that is what you want. A regulated 12 Vdc @ 3 amp. output power supply is what you need. This supply will provide 12 vdc at any current up to 3 amperes. I would buy one with more capacity than you need in the event that you want to add more lights.

    Cheers, DPW [ Spent years making heaters out of op-amps.]
  3. electronicsunzz

    New Member

    Dec 2, 2009
    1. you can use an adapter of 12v;0.7amp as out put, so the total out put power would be 12 times 0.7 = 8.4. And all the led's must be connected in parallel connection to make all of them to glow though maximum light emission is not seen in any of led but we can achieve around 85% of its maximum light intensity.
    2.To get maximum efficiency(i.e 100%) you have to use an appropriate IC which gives 4 out put pins each of 12v,0.7amps. to which the 4 led s are connected. in this process each led posses maximum power of 8.4watts which results in high light intensity as out put.
    this is my idea THANK YOU......
  4. Wendy


    Mar 24, 2008
    Let's assume you have a 12VDC 0.7A constant current power supply. You can not connect the LEDs in parallel, because each one need 0.7A. If you connect in parallel the current will be split between them.

    You can connect them in series however, if the Vf (dropping voltage of the LED is 3.6) you can have 3 of them in series, for a total LED voltage drop of 3 X 3.6V, or 10.8V. This would work. If you have a 24VDC power supply you can have up to 6 LEDs, for a dropping voltage of 6 X 3.6V, or 21.6V.

    Talking about hypothetical ICs isn't very helpful, you need to get into specifics with something like this. The IC likely doesn't exist, unless you have a candidate in mind.

    I have seen 24VDC 6.5A power supplies for $15, you could use on of these to drive around 27 or so LEDs with current regulators using something like the LM317 per 6 LEDs.
    Last edited: Dec 16, 2009
  5. Audioguru

    New Member

    Dec 20, 2007
    You said that each 12V/700mA light has a constant current driver but forgot to say its output current and its minimum input voltage. It is 700mA? What is its minimum input voltage?

    Then connect the four lights in parallel and power them all from a 2.8A power supply.
    The voltage from the power supply must be at least the minimum input voltage of the drivers.
    A power supply with more current will allow for future additional lights.
  6. Wendy


    Mar 24, 2008
    For a current regulator you could use the LM317 I mentioned earlier...


    R1 would be 1.786Ω @ 1W, or using 7 10Ω ¼W resistors wired like this...


    This works out to 1.818Ω, which in turn is .688A from the regulator. The resistors must have spaces between them for cooling.

    If the LM317 had to drop much voltage it will get hot. A heat sink is mandatory, a fan might be needed also, depending how you use it.

    The kind of LEDs you are talking about also require cooling, a mandatory heat sink and possibly another fan.

    If you aren't very familiar with LEDs I recommend reading Chapters 1 and 2 of this...

    LEDs, 555s, Flashers, and Light Chasers
    Last edited: Dec 16, 2009