In the May 2008 issue of Modeel Railroader,"Easy inteior lighting with DCC power", by Wayne McNab, claims that LED's require DC voltage. I have LED's in all of my Kato passenger cars with only the LED and a drop resister and they light up in either direction the car is facing on DCC. I would like to add a capacitor to them so they don't flicker. Why is a brigde rectifier need if the lights work on DCC without it? Also, sence the capacitor is polarized, does it matter which direction the car is facing on DCC or does that only apply to DC? I sometimes switch back and forth depending on what I feel like running at the club or at home that day. Hope someone can shed light on this. Thanks.
Steve---shaggzworld@verizon.net
LED stands for Light Emitting Diode. Like all diodes, it passes current when the voltage is one polarity, and blocks it when the voltage is the other polarity. A standard AC signal flips polarity on every cycle, so half the time the LED will pass current, and therefore light up. So, the light is actually ON half the time. You can't see any flicker, because the polarity changes so quickly. A DCC signal is a "modulated square wave," which looks enough like AC that the LED will behave the same way.
A capacitor, whether polarized or not, accepts current and stores it as charge. But, if you flip the polarity of the input signal, the capacitor will discharge. If you put a capacitor on an AC circuit, it will repeatedly charge and discharge, for no net storage and therefore no value.
The bridge rectifier circuit, which consists of 4 diodes, will pass the "positive" side of the AC cycle untouched, and invert the "negative" side, so you end up with a waveform which goes from zero to some positive voltage, but never below zero. This allows you to light the LED on both sides of the cycle, and charge the capacitor as well.
As a side note, I disagree with the idea of drawing power for structure lighting from your DCC track bus. Even the small requirements of individual LEDs add up after a while. The addition of multiple LEDs reduces the net capacity of your DCC system, for no particular net gain. Also, it adds another chunk of electrical stuff that you have to throw into the mix when you have a problem and have to troubleshoot it.
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First of all to the forums. Second the bridge rectifier filters the AC and makes it DC which the LED likes. While a LED will work on AC it does not like AC. As the above post explained on an AC voltage the LED is only Lighting half the time but you can not tell this, to most people it would look as if the led is on all the time. That is way the bridge rectifier is there. The capacitor is used it the "Car" lighting circuit so the LED does not lose power while moving over a gap or dirty track while it travels around the layout. If not the LED would flicker out every time it lost power. If you look closely at each circuits, the Capacitor is the only thing that is different in the two. The structure lighting gets full voltage all the time, while the car lighting circuit can lose power for short moments of time.
You say you have LEDS in your "Kato passenger cars", did you install them? Were they installed at the "Kato" factory? Did you install a lighting "KIT"? While LEDS will work on AC they do not like AC.
Johnnny_reb Once a word is spoken it can not be unspoken!
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Thank you all for the help.
For my N-scale Kato passenger cars I used Kato lighting kits and removed the incondesent bulbs. I then opened up the holes in the fixture to accept the LED posts,(the anode and cathode), I then added the drop resistor to the back side of the fixture. I used Miniatronics Corp., Yeloglo White LED's- ultra bright 3mm, model # 12-310-05. They come with the drop resistors. It was a little cheaper this way than to use the LED light kits that Kato offers. I then tested the lights to make sure I got the polarity right and installed the lights per the instructions. After testing again before putting the shell on I soldered the conections at the fixture while in the car and put it all back together.
Shaggy---Hagerstown,MD
Also, my reason for using LED's initially was to prevent melting my passenger cars due to the heat created by the incondesent bulbs. It didn't happen to me but I have been told by fellow railroaders at shows it happened to them.
Shaggy----Hagerstown,MD
shaggy wrote: Also, my reason for using LED's initially was to prevent melting my passenger cars due to the heat created by the incondesent bulbs. It didn't happen to me but I have been told by fellow railroaders at shows it happened to them. Shaggy----Hagerstown,MD