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I have been adding lights to all of my Cabooses. Running DCC, installed two axles for pick up, but still get a lot of flicker. I did three interior incandescent in series to get a softer glow, and Aristo lanterns that must have LED's. Does any one have a diagram and component ratings to add a capacitor for some power storage, I also understand a resistor to limit a surge when powering up. Thanks for some help.
 

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For capacitor storage, you need a diode bridge, current limiting resistor for large capacitors, and bulbs/leds tied in series with a possible resistor to 'tune' the voltage.

So, we need to know your track voltage, and ratings of your bulbs and Led's to give a proper/accurate answer. (Voltage and current ratings and how many of what in series).

Light bulbs are easiest as 2 12 volt bulbs in series on 22-24 volt supplies will be OK. Leds at 3 volts each can have as many as 7 in series with a small current limiting resistor. Leds in series should all be the same type/mfg for all to be the same brightness, same for light bulbs.
 

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Mike - you may want to try the method that I used with my Mr. Rogers Trolley. The circuit uses "super caps" and will not only keep them from flickering, but can keep LEDs going for 5-15 minutes after the track power is removed. Here is the schematic:


and here is a link to the article that gives much more information on how it works and how to wire it up.

Article on Lighting

dave
 

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Interesting Dave, no surge resistor to limit current to the super caps? Am I right in supposing that the overcurrent circuitry in the regulator is enough?

Regards, Greg
 

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Greg - the circuit works like a charm with as shown - I have a small aluminum heat sink on the 7805 as it does get warm when first charging the caps as they draw a few amps for a very short time. The diode (D7) is important as the voltage from the regulator is just a bit too high for the caps. The diode drops it to a safe level.

George - regular caps do work very well - the cool thing about the super caps is that they will keep a few white LEDs going for dozens of minutes before they are exhausted. This is particularly nice if you have a point-to-point like the one we are putting in at Children's Hospital.

dave
 

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I would caution users to not use 2.3 volt super caps with only 2 in series for 5 volts.
I am a firm believer in capacitors should have a 10 percent minimum margin of safety on voltage and 20 percent is much better, therefore I would use 2 of the 2.7 volt supercaps, or 3 of the 2.3 volt units.
I know the diode can drop .3 or .7 volts depending on the type, but there is no guarantee that the end users here will know which diode does what.
Also, a note for Greg, the LM7805 does have a current limit/clamp of 1 amp so the super cap charging will be regulated as far as a surge is concerned. I do not know the response time of the current limit of the LM7805.
Also, the resistor seems large to me, 470 ohms at 10 ma gives a 4.7 volt drop, and the bright white leds would be on the dim side with this value.

I am not picking on Dave, just trying to make sure if someone uses his circuit that they understand some variables that were not mentioned here and going to Dave's site would be better than trying to make the above circuit meet their expectations if the correct parts were not used. The site does need a note on the proper diode selection as Dave assumes .7 volts for the diode voltage drop and no mention of a particular type/part number to guarantee this.
 

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Dan - I agree that one would normally use a capacitor that has a voltage rating that is 20% to 50% above the expected voltage. I am not sure that this holds true for super caps as the circuits and documentation that I have seen indicates that no such precaution is in order. They note that the life of the caps will be reduced if the voltage is exceeded but nothing more.


The diode that I am using to drop the voltage is a 1N5408 which is rated for 3 amps. It does a reliable job of dropping the voltage by approximately 0.7 volts - I failed to put that number in the article and have added it.


Thanks for your help!

dave
 

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I (pontentially) disagree that there is "no harm no foul". Not by sience but out of experience:

I accidently had several times in my electronic work used a too low voltage capacitor that just blew up in my face (boooom) after a few minutes. Now it could have been just coincidence that the low voltage rating had nothing to do with it. But I don't want to see that happening again.

Also as another precaution, I reccently experimented with the new transformer less 110 to 24 Volt power converters as used in halogen lighting, and after a few moments a capacitor in the rectifing electronic that I had connected to the 24V side of the power converter blew up. This begs the question are the capacitors also freqency rated? And if so what impact that it have when we use DCC output with frequency regulated voltage?
 

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Discussion Starter #12
I now have more complete information on my project. I have 22.5 volts DCC to the rail. I have 3-12V Grain of Wheat bulbs in series, plus 2-LED Aristo Lanterns part #ART29500 in series with a 1K resistor. I don't need to keep anything going with track power off. Just want to reduce or eliminate flicker, while rolling. I do have 2 LGB electrical pick up axles. Track may not be real clean but Locos are runng well. I could go to 4 axle pick up, but hate to use more of these axles I have been holding on to.
 

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Mike,

I would go with George's responce.. Put in a bridge rectifier ( 50 volt ) & a capacitor ( 100 to 1000 uf @ 50 volts ).. This should stop the flicker..

BulletBob
 

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if the goal is to stop flickering, the standard electrolytic capacitor is the most cost effective and easiest approach. The supercap treatment is good for long unpowered sections.

The 7805 will go into current limiting in a few 10's of microseconds which prevents a spike from being long enough for anything else to notice.

If a capacitor of any kind is used as a filter for a high frequency PWM converter, then it has to be able to handle rapid charge/discharge cycles. The ESR, Equivalent Series Resistance of that capacitor will determine how well it filters and how much heat it generates from repeated current surges. Lower ESR capacitors will filter better and heat less BUT they also can charge to higher spike currents and put extra stresses on other components from those current spikes. You pays your money and takes your chances....
 

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You need the rectifier since a standard electrolytic *must* be connected with the proper polarity. Our fixture assemblers occasionally reverse one (I am assuming that it is not done on purpose!) and the capacitors *explode* within a few seconds of reverse voltage application.

The 7805 is the LM7805, a three terminal voltage regulator - most popular in the TO-220 plastic package. Put 20 V on the input and 5 volts come out the output with (as stated) current limiting and over temperature protection. Pretty close to bullet proof.
 

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you need the bridge rectifier to make constant polarity DC from whatever is on your track.

It is also easier to dispense with the regulator entirely and just run your LEDs with current limiting resistors. You can run 3 or 4 on the same 20 mA.

Figure each LED wants 3 volts. Multiply 3V by the number you are stacking in series to get the stack voltage, for example 12 volts for a stack of 4. Subtract that from your track voltage. Subtract 1.5 volts more for the drop of the rectifier. Then divide the result by 0.02 to get the proper resistor.

The upside of stacking is that you draw less current. The downside is that it takes more track voltage to get a stack to light up. If you are using DCC or PWC, then this is not a problem. If you are using DC, then you don't want to stack them so that they will light at the lowest voltage.
 

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I have 6 bachmann coaches running with LEDs, and in each coach I have a bridge rectifier and then a large value capacitor--the largest radial lead capacitor Radio shack stocks, I forget the exact values. Im pretty sure it's 50 volts and 100 uf

Becaue I run at a constant 20 volts, I just followed Dave Bodnar's site--just run track power to the rectifier, rectifier + and - to the cap, then a resistor in front of the leds, the value depending on the number of LEDS

I usually use this site to help with resistor values:


LED resistor calculator


I bought a string of warm white christmas lights and have been cutting it up and using it in various places. I'm assuming that a warm white 5mm LED wants to see 3 volts and 25 ma. It's been working so far. The car below has four leds. I've been painting the christmas-string leds with polyscale reading yellow





 

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Then in my case I see no need for the 7805. I just need to add a rectifer and a capicitor. Does that seem correct.


As long as the voltage rating on the capacitor is greater than your track voltage, sure.
 
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