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Power Supply Amperage Question

  1. Apr 6, 2012 #1
    Hello. I am trying to wire 40 LED bulbs to a power supply i bought with them. Each bulb has a forward current of 100 mA. So, I calculated I would need 4 amps of current to power the lights if I wired them all in parallel.

    However, the power supply I purchased Says 12V DC 12.5 Amp. Is that ok? Do I need a 4 amp supply? Or is that just the maximum rating?

    Also, I understand how to hook up the 110 AC to the back of the power supply but I don't know how to hook the positive and negative of the LEDs to it. Besides the 110 AC connections it has four terminals side by side. Two that say "COM" and two terminals that say "+V". Do I connect the positive line of the LEDs to "+V" and the negative line to "COM"? Does it matter which one?

    Please help!
  2. jcsd
  3. Apr 6, 2012 #2


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    Do you know what voltage each LED requires? It might be written on each one somewhere and will be a number like 3 Volts, 3.5 volts etc.

    Do they already have series resistors with them? These limit the current, although high powered LEDS often have current limiting regulators instead.

    The power supply is OK. That is just a maximum rating.

    The current of 100 mA seems a bit high. It it a cluster of LEDs in each package?
  4. Apr 6, 2012 #3
    They are 10mm 40 degree 12v dc white led. Vf 6.0-15.0 Iv 220,000 wired with a resistor in heat shrink and 6 inches of wire red and black.

    They come in ten packs but oddly, I had a confusion with my order and they accidentally sent me a twenty pack directly from their supplier in china. It had that information I wrote on the top line printed on it.

    THIS is what their website advertises.

    Specifications below for WHITE

    * Source Material:InGaN !
    * Emitting Colour: 12VDC 10mm 40° 5-Chips 0.5 Watt WHITE LED
    * LENS Type:Water clear
    * Color Temperature: 6000-8000K White or 3000K Warm White
    * Luminous Intensity-MCD: Min: 260,000mcd Max: 280,000 mcd
    * Reverse Voltage:5.0 V
    * DC Forward Voltage: Typical: 12 V Max: 15V
    * DC Forward Current:100mA
    * Viewing Angle: 40 degree
    * Lead Soldering Temp:260°C for 5 seconds
    * Intensely Bright

    Here is what their website says about the power supply; I think it partly answers my question.

    12V 150W Power Supply 12.5A

    Perfect power supply for small to medium sized LED projects. Low heat, high effiecent at 80%. Easy to hook up, just wire it up to any cord. Input and output are labeled.


    * 150W Max Load at 12V
    * Low Noise & Ripple
    * Output over Voltage Protection
    * Short Circuit Protection on all outputs
    * Complies with FCC part 15J Class B 115VAC operation and CISPR 22 230VAC operation
    * Approved by UL.CUL.CE & CB
    * 100% Burn-in under high ambient temperature(50C)
    * 100% Hi-pot & Function tested
    * Dimensions: 6" L x 4" W x 1.5" H
    * INPUT AC PLUG NOT INCLUDED - Can use any wire you have laying around or any computer cord can be cutted and wire to its terminals.
    * INPUT: 110V/240V Switch controller - Terminals: L and N
    * OUTPUT: DC 10.6-13.6V Adjustable via knob. V+ / COM are used to connect your device such as any LED or 12V device.

    So it says the V+ and com are used to wire the leds so I'm guessing V+ is positive(black wire) and COM is neutral(red wire)

    But which V+ and COM terminal do I use? There is two of each.

    Here is a link to a picture of the power supply so you can see the terminals

  5. Apr 6, 2012 #4


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    You could try connecting just one of the devices across the power supply. Be careful with polarity as the reverse voltage rating is only 5 volts, so if you get it wrong, the LED may blow up.

    If you are happy with it, and if the resistor doesn't get too hot, you could just wire up the other LEDs in parallel with the power supply output.

    40 of these will use 4 amps and that is 48 watts at 12 volts.

    I don't know if it would be worth the trouble, but it would be possible to reduce this power consumption by putting two LEDs in series with one smaller resistor (of about 50 ohms) so that both LEDs still get 100 mA but the resistor uses less power.
    This way you would have only 20 strings drawing a total of 2 amps. This would be 24 watts instead of 48 watts at 12 volts.

    If you can, measure the output voltage of the power supply before you connect anything to it. Some power supplies only give the rated voltage when they are loaded to maximum current.
  6. Apr 6, 2012 #5
    Thank you, that is very helpful

    Did you check that picture in the link at the bottom of my post?

    Which terminals should I use to do them all in parallel?

    There is two COM terminals and two V+ terminals. does it matter?
  7. Apr 7, 2012 #6
    The two COM terminals should be common ground so they should be the same. I would assume the two V+ terminals are the same as well, as the power supply only lists one output voltage.

    Just to clear something up you mentioned earlier, with DC stuff:

    Red is usually positive
    Black is usually negative(or ground)

    So red wire on LED to V+, and black wire on LED to COM should work.
  8. Apr 7, 2012 #7


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  9. Apr 7, 2012 #8


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    Yes, they are. They should at least be covered with a solid shield after the connection is made.
    I would open up the box and disconnect these terminals before it was used.

    Not only 110 volts either. This power supply works on 240 volts as well.
  10. Apr 7, 2012 #9


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    Maybe they are the AC line inputhttps://www.physicsforums.com/images/icons/icon5.gif [Broken]
    Last edited by a moderator: May 5, 2017
  11. Apr 7, 2012 #10


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    Yes they are. We would call them active and neutral, but calling active "line" is fairly common.

    This looks like a good case for mounting an IEC socket on the chassis somewhere and disconnecting those metal terminals just waiting to kill someone one day.

    Last edited by a moderator: May 5, 2017
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