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LED help again!

  1. Apr 6, 2012 #1
    Hi, newby here, hope I can get some help on this question.
    I have a project that requires me to use a standard 9 LED torch head, normally running on 4.5 volts (three AA batteries) My project needs me to use 12 volt batteries to supply a control box and finally the 'torch heads' a distance away (miniature railway signalling). To provide the voltage needed at the final point I intended running the 12 volts through a Voltage Regulator almost at the final stage of wiring, that delivered 5 volts from the voltage which starts out at 12 volts, and obviously drops a little due to the length of wiring. If I leave the voltage at 5 volts it damages the 'torch heads LED's' so what do I need to do the drop the other half a volt to provide the final 4.5 volts. Not sure though what the impedance of the torch heads are.
  2. jcsd
  3. Apr 6, 2012 #2


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    1) are you sure that the LEDs cannot handle 5 V without being damaged?

    2) generally you can just put a resistor in series with each LED. If you know how much forward current "I" that it can handle, and if you know the forward voltage drop of the diode when it's on (call this V_diode), then you just pick a resistance value R such that

    (5 V - V_diode)/R = I
  4. Apr 6, 2012 #3


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    Welcome to the PF.

    Use a variable output voltage regulator IC, like the LM317.
  5. Apr 6, 2012 #4
    Thanks for those quick answers, unfortunately I have just been called away on an urgent task away from home. I will try to sort out following your ideas and get back once done. Thanks again.
  6. Apr 7, 2012 #5
    Okay, back again. Tried a cluster of LED's straight out of a torch, fed them with the 5 volts from the Voltage Regulator. ran all morning with no problems, so thought right I will continue. Then I had a thought and looked closely at the circuit board, there are three resistors built into the circuit. I then took another, more plentiful supply, cluster that does not have the three resistors and wired that up as the previous. Shortly after turning it on one LED stopped, then another. Obviously they need a resistor of some value. I checked the amperage draw and it was running around the 135/150 milliamps. As a complete and utter novice I am at a loss from hereon.
    So, L7805 Voltage regulator, 12 volt supply from battery dropping down to 5 volts. 9 LED torch head (no resistors built in) Current draw around the 140 milliamps. What now?

    Hope I can get this simple problem sorted as it is doing nothing for my patience or pocket.
  7. Apr 7, 2012 #6
    On the datasheet for an LED I can see that peak current, for the specific model I am looking at, is 100mA pulsed at 0.1ms. So 135mA would definitely damage the LED. Looking at this datasheet, I would say you should be shooting for about 20 to 25mA to start.

    Put a resistor in series between the 5 volt supply and the LEDs. If you have a bunch of resistors, or a potentiometer, you can start out at a relatively high resistance and work your way down, measuring the current along the way. Make sure that you don't drastically drop the resistance value you are using.

    Let me know what happens.
  8. Apr 7, 2012 #7


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    Can you not simply incorporate the circuit board that comes built-in with the "torch head?" This seems like the simplest solution, since the system has already been designed to safely operate at 5 VDC. If not, then do the following. In the circuit board version:

    1) Measure the voltage across the leads of an LED when it is turned on. This is the forward voltage drop of the LED, which is the thing I called V_diode.

    2). Measure the resistance value of the resistors on the board. Either learn to read resistor colour codes, or simply use a multimeter if you have one.

    3). The current being driven across the LEDs in this "safe" system is just equal to the voltage across the resistor divided by the resistance value. So what's the voltage across the resistor? It's the 5 V of the source, minus the voltage drop across the diode. In other words:

    I = (5 V - V_diode)/ R

    as I stated before. Now you'll know what the safe and typical operating current is for the LEDs, and you can act accordingly.

    A simpler (and equivalent) thing to do is to do ONLY step 2, and then just go out and buy resistors of that value, and put them in series with the LEDs in your own wiring. You know that this will work. I just suggested doing all 3 steps in order to fully understand the system and what's going on.
  9. Apr 7, 2012 #8
    Okay, thanks for those words of wisdom. Two thirds the way through a bottle of red at moment, so best leave the electrics until after tomorrow. Running the railway tomorrow so no time. Thanks again will keep you all up to speed on progress or otherwise
  10. Apr 20, 2012 #9
    Where can i run wires for a Led tube supplier on my sub/amp?
    I am wanting to put lights in my sub box. LED Tube They will be visible via plex iglass insert. outdoor led strip Where can I find a 12v source on my sound system that hits with the bass?
  11. Apr 21, 2012 #10


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    Hi Arnetta, welcome to Physics Forums. You should start a new thread for your question as it is different to the topic of this thread. And if people start answering your query here, those answers will get mixed up with discussion on the original topic and everyone will get thoroughly confused.

    I think you should send a message to a monitor asking for your post (and this one of mine) to be deleted, and you should post afresh with a subject header of your own choosing. That way, people will see your posting and its subject, and may be able to help. Good luck!
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