Adding LED's to your locomotive! WARNING!

Discussion in 'FAQs' started by kf4jqd, Feb 15, 2004.

  1. cidchase

    cidchase Active Member

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    I'm really still trying to understand what causes
    the original problem in this thread. :)

    I'm guessing the flyback theory is a good possibility.
    Fred, do you know if that low of a PIV rating is
    common to a lot of LED's??

    Has anyone noticed a protection device in a loco
    factory-equipped with an LED?

    Goin' back to sleep now:sleeping: :D
  2. ezdays

    ezdays Out AZ way

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    Yes, OK, OK, no, maybe.

    An LED is a diode and adding a series diode only serves to give about a .7 volt drop in the forward direction. The problem is that either the LED or a diode only work to block the AC in one direction resulting in half-wave rectification. Half-wave rectification is the top half of the AC sign wave. The voltage goes from zero to full and back, then stays at zero for an equal time and then repeats the cycle. This results in bumps on the DC called ripple. Some devices that run on DC don't mind, other are quite sensitive to this and hence the use of capacitors to filter out the ripple.

    Oh yeah, an LED works fine on AC. It basically is flickering on and off, but at a rate that is hard to detect. I have used LED on 110 VAC, but with a large series resistor, and only inside an enclosure that is not easilly accessed.
  3. Fred_M

    Fred_M Guest

    I agree that leds can be ran on 110 AC with just a high value resistor, but most show in their specs that the have a max reverse voltage of 4 to 5 volts. Most leds ran this way on AC are reported to have a shorter than normal life due to rush and reverse voltage damage. A Led used this way may last forever or burn out instantly. It depends on how much better it is than min. spec. and how close to the edge it is being ran. It's no big deal to throw in a cap, diode, and resistor anyway. We are really discussing a "so what" thing. FRED
  4. jon-monon

    jon-monon Active Member

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    That's right, dash, he's already solved his problem, so we are just learning and trying to understand what's going on. Best to understand it than jus tmake it work, IMHO.

    I think a diode will only protect agaisnt flyback in a unidirectional application. And it works by conducting the flyback, not blocking it, as I recall. I'de have to go look at a schematic, which is against my religeon while on vacation :D
  5. Fred_M

    Fred_M Guest

    My way of seeing it is the LED has a PIV of 5 volts. So any source of voltage greater than -5 v could "jump" the semiconductor and cause burnout. If you put in a 1n4002 in series the PIV of it's 100v (from memory) so it would take a -100v reverse to "jump" it and then reach the LED. In the forward direction it would be a "resistor" that would cause a aprox .7 v drop in voltage. So your real resistor value would be reduced depending on the amp draw of the LED. IMHO the real culprit was probally rush which the cap absorbes thereby fixing the problem. And what happens when the loco is reversed? Then the LED sees -12v and goes out and has a 12v PIV load? If it were mine, I would use a small full wave, a resistor, and a 10 mfd 25v cap. This would fix the PIV problem, fix the ripple problem, and fix any voltage rush. And Jon,
    :) that the difference between science and technology. Science wants to know why and technologist don't care why, only how to make it work.:) FRED
  6. krokodil

    krokodil Member

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    The problem with upper audio range freqs is that many motors have huge impedance on this frequency (exception are the modern coreless motors), so you will have extreme losses (power reduction) because of inductivity of the coils in the motor).
    The freq is the main issue on any controller (on my own I can adjust up to 5 kHz to control almost all available motors).
    Above this freq I must be careful to avoid wireless transmission to my neighbores (the layout is probably the best antenna, unfortunately not invented yet ;) ).
  7. krokodil

    krokodil Member

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    I would never use a diode in series with the LED. The LEDs cannot resist to high voltages in reverse, so if you add a diode in series, it still allows high reverse voltages on the LEDs, and usually you will burn them. The best way is to add the diode in antiparallel, i.e. solder the Anode of the LED to the cathode of the diode and vice verse. This allows max. 0.7 V reverse voltages on the LED.
    If you want to reverse the loco lighting (same color) then you can add instead of the diode an another LED (if it is RED it will automatically switch the color forward yellow/white, reverse RED).
    Or you can add a small bridge (4 diodes in one package) to the whole circuitry, i.e. there will be always the same polarity to feed the LEDs, in such cases you do not need to block each LED with the reversing diode. Of course in this case the light will not change with the direction of the train.
  8. Fred_M

    Fred_M Guest

  9. krokodil

    krokodil Member

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    I fully agree with you


    :)
  10. rcwatkins

    rcwatkins Member

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    I know this may sound like a stupid question, but what are LED's & what do they do?
  11. cidchase

    cidchase Active Member

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    Hi rc,
    It is a Light Emitting Diode, has the same function as a light bulb,
    but it uses much less current and generates much less heat.
    It operates on DC, on one polarity, and generally requires a
    resistor in series to limit the current. :) :) :)
  12. rcwatkins

    rcwatkins Member

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    Thanks cidchase. ;)
  13. ezdays

    ezdays Out AZ way

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    Light Emitting Diodes.

    A diode is a small electronic device used to control the flow of current. An LED has the ability to emit light as well. These are the same as the ones in your alarm clock (assuming you have a digital clock) and usually light red. They also have green, yellow and amber ones a long time, and most recently, we have other colors including blue and white.

    Hope this helps.
  14. krokodil

    krokodil Member

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    LED - Light Emitting Diode is a modern source of light on models (an other applications). It does not create heat and has extremely long life and can be extremely small (e.g. SMD LEDs are only few square mm big!)

    The basic LEDs are Red, Green and Yellow, now extended to Blue and White as well.

    :)
  15. Fred_M

    Fred_M Guest

    Be aware that if you do this and try to run the loco in reverse the diode becomes a direct short. FRED
  16. cidchase

    cidchase Active Member

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    In a way, Fred, but no more than the LED is a short
    in the forward direction. The resistor limits the current,
    and the diode pair will drop 0.7 V like krokodil says in either
    direction.
  17. Fred_M

    Fred_M Guest


    But when the LED is in forward bias it is limited to current flow by its high internal resistance. An example is at 3 v a LED will flow 20ma in a forward direction and near zero (leakage allows some) in reverse. A rectifier diode has almost no resistance at 3 volts and will flow enough current to fry itself if allowed to at 3 volts (almost instantly). The limiting resistor will protect it, but say in a 12 volt application running a resistor, we use a 400 ohm resistor. In reverse it will flow .03 amps for .36 watts. My 1/4 watt resistor might get a bit hot. As to running a diode in series with an led, it's nearly the same as running a bridge, only we still have ripple. I can't see where it would cause harm? I've never had a problem using them on 3 color leds. I use a 3 position rotary switch with 1n4001 on two legs (biased each way) and the center is straight through. The power is 5VAC with a 220 ohm resistor. They work just fine that way in all 3 colors, and after all, it's just a green and red LED in one package that mixes for yellow on AC. So is this like one of them Matrix things that work because I didn't know better and will now burn out? FRED
  18. jon-monon

    jon-monon Active Member

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    LOL

    Back then we didn't know nothin' 'bout safety, so no one got hurt.

    Hey guys, why not scribble out what your're doin' on a napkin and scan it and post it for reference and clearer discussion? Some of hte worlds greatest ideas started out scribled on a cocktail napkin :D Or, I suppose, draw a little schematic with some software and post that, but it just wouldn't have the same impact :D :D :D
  19. Fred_M

    Fred_M Guest

    Well, here's how I power my searchlight signals that are red/yellow/green and use a 3 position rotary switch. I'm not 100% on the diode polarity, but if its red when it should be green you reverse the leads at the LED (signal). Works fine for many many hours 24/7/365. Mostly on green. FRED

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  20. Fred_M

    Fred_M Guest

    And here's one of the home made towers that it runs. It's on AC (yellow) during the picture. If anyone wants to see how to make them post and I'll start a new thread in HO. FRED

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