Q U E S T I O N for DCC in N scale

Carl Sowell Jun 22, 2019

  1. Carl Sowell

    Carl Sowell TrainBoard Supporter

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    Most of you know that I have self installed many sound, non-sound, function only decoders in steamers as well as diesels. However I have never made my own keep alive. I have read threads regarding where to make connections on an ESU drop in decoder But I have a project I am planning and need help.

    You may or may not have seen the Lionel boxcars with the UP version of the stars and stripes that illuminate the stars. That is what I am going to attempt.

    Here I am going to use a 1 function Digitrax TL1 that I will attach 2 LEDs, one per flag on each side of car. I have never used the small surface mount resistors but I have some 100uF, 20v units. I think one would be sufficient ? ? Should I install in the red and black power feeds for the decoder ?

    Any help is appreciated,
    Carl
     
  2. chadbag

    chadbag TrainBoard Member

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    What size resistor are you using? The "time" you get from a "keep alive" is based on both capacitance and resistance. You can search on "Rc time constant discharge" or similar to get info on how to compute how long you will get a reasonable voltage etc.
     
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  3. tjdreams

    tjdreams TrainBoard Member

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    Take a look at the NCE "Light-It" Decoder. If I'm understanding what you are wanting to do correctly, The "Light-it" decoder was designed to do exactly what you are wanting to do and it has dedicated solder pads for adding your own keep-alive capacitors
    Take a minute and read through all the instructions as it has several different modes that will provide you with many different lighting options.

    David
     
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  4. vasilis

    vasilis TrainBoard Member

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    the resistors measurement unit is ohm and the capacitance farad. The smd capacitors are tantalum caps. Be Careful with tantalum caps, they failure violently. The 100uf,25v cap max stored energy, supposed your track voltage is 10v, is (100e-6*10^2)/2=0.005joule=0.005watt/sec, that is too low. Let's say your total consumption is 50mA you need 0.5W/s and with en effectivity 70% KA the stored energy for 1sec KA has to be .5/.7=0.71w. That means you need 0.71*100/0.005=14200uf capacitance. It seems more as anti-flickering (output) than keeping alive solution. You have to estimate or measure your power consumption and i think you have the place in the car for a biger electrolytic cap for KA.
     
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  5. chadbag

    chadbag TrainBoard Member

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    You won't need anything like 14200uF in capacitance to light 2 LEDs for a few seconds. How long do you want to keep them going after power goes off?
     
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  6. vasilis

    vasilis TrainBoard Member

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    decoder eats power too and needs a min voltage to work and this is a KA solution at the input side. If he want an antiflickering solution at the output side a quite smaller is enough. I'm curious too how much power is needed. I supposed 50mA current. He can measure the current with the lights on and we will know. Any way this is for a second. If the consumption is the half and the time .5 sec then the capacitance goes to 1/4.
     
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  7. chadbag

    chadbag TrainBoard Member

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    Don't most modern KA today use stacked super caps? Probably get more capacitance in a smaller volume (?)

    ETA: you can also get 100uF ceramic SMD caps if you look hard enough at 25V rating. 16V rating is a lot easier to find but not rated high enough without a more complicated circuit. Ceramic cap failure causes less excitement I believe and are easier to work with as they are not polarized.
     
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  8. chadbag

    chadbag TrainBoard Member

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    Also, do you really need a KA on a 1 function decoder? Anti-flicker type circuit on the output of the decoder will basically achieve the same affect unless you really really need to address the decoder during that 1 second.
     
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  9. vasilis

    vasilis TrainBoard Member

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    So what resitance has to use and what minimum voltage? How big capacitor do you suggest?
    So no a KA at the input of the decoder, but an antiflicker at the output of the decoder. What do you suggest for each case?
     
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  10. vasilis

    vasilis TrainBoard Member

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    And something more, there is the charging resistor and the consumer equivalent resistor. The first is chosen, the second has to be find.
     
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  11. chadbag

    chadbag TrainBoard Member

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    In my N-scale lightboard I use a voltage regulator to bring the voltage down to either 3.3V or 5V (either work -- just changes your resistors). The capacitors are outside the voltage regulator so they charge to the full track or decoder output voltage (say nominal 12V). As they discharge, you get a full 3.3V or 5V out of the voltage regulator until the voltage has gone down to about 4V or 5.7V and then it starts to taper off. The 3.3V or 5V is used to power the LEDs with a resistor. I use 600UF -- 900 uF for about 8 or 9 -- 14 LEDs depending on how big of a wagon I am lighting. But I don't run the LEDs at full capacity current as they would be too bright.

    2 LEDs at full brightness would probably need 400 -- 600uF ?? Just a guess.

    Disclaimer: I am in no way an electronics engineer.
     
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  12. chadbag

    chadbag TrainBoard Member

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    I developed some N-scale light boards to add internal lighting to passenger wagons, EMU/DMU/HST, etc. I developed three using the same basic circuit. In the photo below, the one on the top, the long one, has integral LEDs, the middle one is to be used with those cheap commercial LED strips you can buy, and the bottom (black PCB) was designed to be used with factory light boards like KATO or Tomix or whatever -- to provide some anti-flicker. The board could also drive two individual LEDs on wire leads.

     
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  13. Carl Sowell

    Carl Sowell TrainBoard Supporter

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    Many thanks to you guys but my question has not been answered BECAUSE I did not explain well enough. I said I had some resistors that are 100uF 20v and I meant to say capacitors.

    I have already made my own "board" with the TL1 attached along with a 1K resistor for the LEDs. I mis-stated that I wanted a KA but meant an anti flicker which would not require much capacity. From comments above, I think I can place it between the decoder and the LEDs for anti flicker only. Right ? ?

    The LEDs are 3v and I am looking for 5-10ma out of them. The 1K should be close assuming approx 14v decoder output. I do not want the lighting to over power the unit. BTW, I used this decoder because I had it on hand.

    I apologize for causing you extra thought and work. Must have been the Crown Royal I was sipping ! !


    carl
     
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  14. vasilis

    vasilis TrainBoard Member

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    @chadbag, nice job! I wouldn't solder even one led of these. XD
    The dissipated power for 100 leds at 2v and 0.005A at least 1W and you have to add the dissipated power on the dropdown resistor from 10v(or whatever) to 2v. Anyway as I said before the antiflicker demands smaller cap. With the 600uf, as you can see in the graph, the power consumed (given) in the led is about 60% after 100ms of the power shutdown. If this is not noticeable then all is ok.
    @carl,
    No problem, it is a discussion. The following circuit is close to your demands with an 600uF cap. The file is attached (you have to change the extension to .asc) and ltspice is free.

    upload_2019-6-23_12-57-34.png
    upload_2019-6-23_12-59-26.png
     

    Attached Files:

  15. vasilis

    vasilis TrainBoard Member

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    Carl, you can also experimenting without the decoder connected. Put the circuit on a coach and power it from the rails through a rectifier bridge. Pull the car arround with a loco and find if the result is good for you.
     
  16. chadbag

    chadbag TrainBoard Member

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    @vasilis With the solder paste in a syringe and a hot plate, poor man's DIY SMD is not hard :) A full finished board is under $2 -- closer to a buck in bulk. Better than the $6-$20 per commercial board.

    @Carl Sowell Post some pics when you are done. I am interested to see how it works out.
     
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  17. vasilis

    vasilis TrainBoard Member

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    Interesting:eek:
     
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  18. chadbag

    chadbag TrainBoard Member

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    Better late than never. Here is one of my boards populated and done using the poor man's hot plate SMD method. And an earlier rev of the board (in blue) installed in a Japanese Shinkansen cab car (passenger section). The earlier version is basically the same but with the capacitors in a different position in the circuit that is not as efficient. Imagine the white board in its place.

    IMG_3859.jpeg IMG_3860.jpeg IMG_3857.jpeg
     
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  19. Keith Ledbetter

    Keith Ledbetter TrainBoard Member

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    @chadbag. Nice job and 2 questions. Would you mind sharing the Gerber files?

    And also could you possible link to something that shows the SMD hotplate and paste method you use.

    Thanks!
     
  20. chadbag

    chadbag TrainBoard Member

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    @Keith Ledbetter

    I can share the gerber files. I need to find them on the computer. If I don't get it done in the next day or two remind me.

    I just Googled "hot plate SMD" and found some tutorials and videos on YouTube. Here is one I just found:

    It is not the one I originally used to learn about this.

    I basically just put small dabs of solder on each pad, place the components, get hot plate (the kind you cook on) hot (I just bought a cheap single plate table top one from Amazon) and when the paste melts it is obvious (turns bright silver). Take it off and let it cool. Then I fix anything that messed up (components that slide, or bridges, etc). I found that if I don't use to much solder I get hardly any movement or bridging etc. I have a hot air reflow station (cheap one from Amazon) that I sue to fix things with.

    Here is a pic from another forum I posted on showing one of my older V1 boards on the hot plate (Note that this is an earlier attempt and in general I use less paster on each pad now than is shown)

    https://jnsforum.com/community/topi...ject-pcb-based/?do=findComment&comment=193253
     
    Last edited: Oct 25, 2020
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