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Newbie with LED question.

Discussion in 'LEDs and Optoelectronics' started by passingthru, Aug 4, 2012.

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  1. passingthru

    passingthru

    3
    0
    May 2, 2010
    I've been a member of this forum for quite some time, but as I don't play with electronics much my contributions have probly been nil, so although it may seem like I'm just picking brains here I remain a member for an occasion when I may need assistance. I have read through about 14 pages but failed to find suitable information relevant to my issue.

    I seem to have an issue that I have not encountered in the past. I make small projects which run on a transformer reducing AC mains power to 24vdc. Included in the circuit is one LED and one resistor. The circuit is completed when two electrodes are submerged in liquid, one electrode is positive and the other is negative.

    After completing one of these projects recently I found I was unable to attain my 24vdc across the electrodes. There is a massive drop in voltage, down to around 3v. I have tested the positive and negative on the power adaptor socket with a multimeter and have the correct voltage coming in so the socket is OK, I have tested the circuit between the positive on the socket to the positive side of the LED {which is soldered to the negative on the socket} and that is OK. I have tested the resistor and that is OK. I even unplugged the transformer and put a battery across the terminals of the LED and it lit up fine.

    I have bench tested the LED with a 9v battery and it lights up fine {signifying that the LED is functioning as it should, from what I've read anyway} but I can't figure out why it won't allow the 24v to continue past the LED through to the resistor and hence on through to the electrodes? If in fact it is the LED that's causing my problem, I've tested everything else but the failure appears to be at the LED.

    The LED is 3mm and I have not struck this problem in the past, I've made numerous projects such as this in the past with no problem. I have purchased these LED's recently. If I've explained my circumstance clear enough in the above, I would be grateful for any assistance which may be forthcoming. I can't fathom what is going on? I've changed LED's and still no resolution. Have I purchased LED's with some form of voltage limit - if that is at all possible?
     
  2. KJ6EAD

    KJ6EAD

    1,114
    159
    Aug 13, 2011
    Can you try another power supply?
     
  3. passingthru

    passingthru

    3
    0
    May 2, 2010
    That's one thing I failed to mention. Yes, I have tried another power supply and get the same result, 3v at the electrodes, massive voltage drop. I even switched machines and power supplies to double check and all power supplies work fine. I've worked through everything I can think of and cannot fathom what is going on?

    I've been making these projects for a few years and have never experienced this before. Because I have checked everything I can think of, and what I'm about to say next may sound stupid, but I am left wondering if the LED's I purchased recently are somehow voltage rated or limited? The issue seems to be at the LED. I have 24v going into the LED, but only 3 volt coming out? I can't understand how that can be possible? Is it possible that I may have purchased from a faulty manufactured batch of LED's? Although they light up on a 9 volt battery they don't seem to take 24 volt? I really don't know what to think?

    I'm no electronics expert, but this is about as simple as it gets with what I'm doing. 24vdc coming in and wired from positive terminal on power socket to the positive electrode, the negative from the power socket is wired to the positive side of the LED and the negative from LED is wired to a resistor which in turn is wired to the negative electrode, the connection to complete the circuit only occurs when both electrodes are submerged in a liquid medium. It's very basic stuff.
     
  4. KJ6EAD

    KJ6EAD

    1,114
    159
    Aug 13, 2011
    You've just described connecting the LED backwards. Any reverse voltage above 5V or so at the LED should destroy it but maybe the resistance in the circuit is sufficient to prevent that.
     
  5. gorgon

    gorgon

    603
    24
    Jun 6, 2011
    If you connect 24V to a 3V LED you also need to add a resistor to drop the 21V not needed by the LED. To compute the value of the resistor you need to set the current through the LED. For 21mA you can use a 1k resistor, for 10mA use a 2k2 resistor.

    TOK ;)
     
  6. passingthru

    passingthru

    3
    0
    May 2, 2010
    Thank You both for your input, I really appreciate it.

    I have so much egg on my face you wouldn't believe. I re-checked my wiring from the socket and discovered I *HAVE* wired it up the wrong way. Beejeebers I can't believe I did that, I guess I should be checking my drawing every time to avoid such stupid mistakes, or praps it's time I got stronger glasses <g>.

    I'm certainly not the sharpest knife in the draw when it comes to electricity, and I'm still smacking my head here for being so stupid, I thought I had checked everything in my process of elimination, obviously not.

    Thank You very much, I am relieved in finding resolution to such a simple problem.
     
  7. KJ6EAD

    KJ6EAD

    1,114
    159
    Aug 13, 2011
    You're welcome. :D
     
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