Wiring LEDs using 12 volt source

Discussion in 'General Electronics Chat' started by romeroom, Aug 18, 2016.

  1. romeroom

    Thread Starter New Member

    Apr 17, 2014
    I am a novice at this stuff, so I would appreciate a bit of kindness and restraint in judgment.

    I have a bunch of LED flashlights from Harbor Freight and would like to connect them using a computer (ATX) power supply. (These flashlights require 3 AAA batteries. I am fairly certain I can take these flashlights apart and find the two connecting points needed for a circuit.

    I suspect there are options in terms of connecting them in series or parallel. Regardless, I would really appreciate "How to" direction.

    (I didn't realize how versatile these power supplies were.)

    Last edited: Aug 18, 2016
  2. Morvan

    New Member

    Jun 24, 2014
    Firstly, you need realize two things: computer PSU are overkill for this project, even they could be utilized; second, LED´s are current sensible, and hence, to be affected by the same amount of this, must be in series (a matrix of trhee leds could shine via 12Vcc sector, red wires, cause leds will receive same current and divide amongst VCC, by 12 (trhee times 3.2, quiescent voltage, time 3. It receives a limitator resistor of 100 Ω to sake of security).
    This page shows something very useful for you (and anyone) about LED´s Torchs and Circuitry, from Colin Mitchell´s Site.
    Last edited: Aug 18, 2016
    romeroom likes this.
  3. romeroom

    Thread Starter New Member

    Apr 17, 2014
    Thank you for such a prompt response. It sounds like connecting 3 flashlights in series to a 12v source may work.
  4. hp1729

    Well-Known Member

    Nov 23, 2015
    Would it work? Yes. How many Amps can the 12 V side deliver? Figure 0.03 amps per string of flashlights. That will tell you how many strings you can drive. 8 Amps could drive 266 strings. :)
  5. avayan


    Oct 30, 2015
    Power the three flash lights and measure the voltage across the LED's when they are lighted up (this is known as the LED's forward biased voltage - I will call it Vfdx). Each flashlight will have their own Vfd, albeit they should be kind of similar.

    Then use the following equation to determine what series resistance to use:

    RSeries = PSVoltage - (Vfd1 + Vfd2 + Vfd3) / Idiode


    RSeries is the serial resistor you need to use to limit the LED current.
    PSVoltage is the power supply voltage (I am assuming 12V)
    Vfd1 is the flashlight 1 forward voltage (same applies to the 2 and the 3)
    IDiode is the current you want the diodes to see (20 mA???)

    Now, a flashlight LED may want more than 20 mA, so you may need to experiment with this. Do note that if you put too little current, then the flashlight will be dim and the flashlight will last a very long time. If you put too much current, however, then the LED will be bright but will not last long at all. It is all about heat. If you somehow manage to keep the LED's cool you can get away with more current and more brightness. Up to a certain point, of course. Not to get into semiconductor physics but an LED cannot sustain infinite current regardless of how much you cool it.
  6. avayan


    Oct 30, 2015
    Oops! The equation should be:

    RSeries = (PSVoltage - (Vfd1 + Vfd2 + Vfd3)) / Idiode

    In other words compute the total voltage before dividing by the current.
  7. ElectronicMotor


    May 1, 2016
    I have never been able to pull the LED sub-circuit out of one of those LED torches because they have a certain type of retaining nut with slots at the top rather than a hex nut, and I have never gotten around to buying the special tool

    But if I could get one out, then I would hook it straight up to a power supply

    The first thing I would do is find which of the two main power terminals on the sub-circuit is +

    I would get my +12 V (yellow wire) and my GROUND (black wire) and very quickly touch them to the terminals (polarity doesn't matter) .

    If I get a VERY bright, Very quick flash of light, then I know I have the right polarity, and haven't burnt my LED's

    If I don't get a very quick flash of light, I know I have the polarity reversed

    Then, I would connect it up through a resistor that gave me the brightness I wanted

    If the circuit is faulty, it wont get past that stage

    Now I would 'run tests'. I would check it with all the tools I have at my disposal

    That is, multi-meters, oscilloscopes, special tools (?)
    Last edited: Aug 19, 2016
  8. ian field

    AAC Fanatic!

    Oct 27, 2012
    Most PC PSUs won't deliver on the 12V rails unless the 3.3 & 5V rails are loaded. Usually they have bleed resistors on the outputs - if they draw enough current to keep the 3.3 and 5V regulation circuits in range; the 12V rail may come up good enough for a few LEDs - it won't supply the ticketed current under those conditions.

    If you run the LEDs side by side - each one needs its own current limiting resistor. With 12V to play with - that's not many Vf at about 3.4V a pop if you go for series chains.

    If your supply voltage is all over the place, the best plan is single LEDs each with their own resistor. If the volt drop on the resistor is more than Vf; the tolerance to varying voltage is greater - but wasted power is greater.

    If your 12V is rock steady, you can just about scrape by with 3 series LEDs and a small resistance - but 2 LEDs in series and a bigger resistance is better.