"Keep Alive" nonsense

Erik84750 Feb 27, 2023

  1. jbonkowski

    jbonkowski TrainBoard Member

    40
    27
    16
    The product pages for the DigitraX keep alives (they call them Power Xtenders) list the total capacitance value.

    Jim
     
  2. BigJake

    BigJake TrainBoard Member

    3,259
    6,172
    70
    A capacitor's failure rate rises abruptly when operated near the rated operating voltage. Choosing a rated voltage of double the actual operating voltage is a good investment in reliability.

    Physically larger capacitors, even when rated to the same operating voltage, are more reliable than smaller ones (of the same electrolytic.) The military and many commercial standards recommends doubling the rated voltage above the operating voltage, and avoiding the largest capacitance in given case sizes. And the military prohibits aluminum electrolytic capacitors in almost all applications.

    Of course, the consequences of their equipment malfunctioning are considerably higher than those of our consumer electronics.
     
    Sumner and badlandnp like this.
  3. DCESharkman

    DCESharkman TrainBoard Member

    4,396
    3,025
    87
    One of the things to be aware of, is that simple calculations based on the magnitude of the DCC signal is close but not 100% accurate. The calculations do not properly include the discrete nature of the the composite signal. But as the joke goes, it is close enough for government work.

    It is in the multiple levels to the packets sent by the controller to the track, and then to the locomotive and it's capacitive capabilities with the devices installed.

    Sorry my first degree was in Electrical Engineering as was my Masters degree. You never forget your dance with the devil of frequency calculations and their Convolution Integrals, and the Fourier Transforms!
     
    BigJake, Sumner, MK and 1 other person like this.
  4. MK

    MK TrainBoard Member

    3,494
    4,790
    82
    Uh, FT and FFT. I remember those. Still have nightmares! :eek:
     
  5. wvgca

    wvgca TrainBoard Member

    499
    305
    21
    they have all been for HO scale, and usually used five or six of the 1 farad 3.0 volt caps ..
     
    Sumner likes this.
  6. BigJake

    BigJake TrainBoard Member

    3,259
    6,172
    70
    True dat!

    Funny how engineers learn the theory in school, then learn the working shortcuts, rules of thumb, etc. on the job. My father (a Chemical Engineer) told me (as his father told him) my Electrical Engineering degree was a mostly-empty toolbox, with just enough basic tools to get by, and lots of room for the tools I would acquire on the job. Technology and its applications change, especially the growth of computers during my electrical engineering career, and never ceased requiring, and supplying, newly improved tools to my toolbox.

    But discard the theory at your peril!

    It all boils down to physics, but then it was physicists who screwed up the direction of current flow (flow of electrons vs flow of charge.)
     
    badlandnp likes this.
  7. DCESharkman

    DCESharkman TrainBoard Member

    4,396
    3,025
    87
    I would have to disagree about the "Empty Tool Box". It is a base from which an Engineer can spread out as needed. For example, my program, had a class all EE's had to take. It was nicknamed "Bridge Building 101" because it was all about shear and tension and compression of concrete columns, rebar designs etc. Well I still understand all of that and pointed out the errors in the concrete pour of the pool we had installed, and made the company rip it all out and start over on their dime. So no education is really wasted, even Bridge Building 101 prepared me to spot the issues in the concrete.

    I loved my careers, first in the Navy and then as an Engineer, and after my doctoral degrees, all the theoretical research I have done since.

    I actually calculated why the max length of a DCC signal is about 300 feet, it has to the signal harmonic frequencies combining and because of Entropy, an extinction function is created by those frequency mismatches that completely degrades the signal.

    This where in the DCC world, you have power districts to have very large layouts.
     
    badlandnp likes this.
  8. Dave1905

    Dave1905 TrainBoard Member

    265
    284
    22
    I don't know about the electronics, but if you compare the operation of an engine with the keep alive and without the keep alive, the one with won't stall on dirty track and will keep going when it runs into dead track. You can argue that it doesn't work all you want but in actual operation there is a distinct difference between an engine with a keep alive and those without.
     
    badlandnp and Sumner like this.
  9. Erik84750

    Erik84750 TrainBoard Member

    334
    134
    12
    Thanks Andy, I stand ashamed.
     
  10. BigJake

    BigJake TrainBoard Member

    3,259
    6,172
    70
    Ignorance is never shameful!
     
    badlandnp likes this.
  11. Erik84750

    Erik84750 TrainBoard Member

    334
    134
    12
    Hi Andy, it was not ignorance but negligence.
     
    BigJake likes this.
  12. 308GTSi

    308GTSi TrainBoard Member

    432
    957
    16
    It's all good.

    I learned something through reading this thread.
    Let me check , , If fitting a keep alive I need to know the highest voltage DCC track I'm likely to run on ??? But not go crazy with a silly high voltage ?
     
    badlandnp likes this.
  13. Sumner

    Sumner TrainBoard Member

    2,798
    5,837
    63
    I assume you are saying that in regards to picking the capacitors.

    My reading says that the more expensive Tantalum-Polymer Capacitors are safer than cheaper capacitors in that it they fail it probably isn't going to be with a big boom that can destroy things. I'm using 12 volts so I'm willing to try the 16v Tant caps but only because I'm going to build the voltage limiting circuitry that Iowa Scale Engineering is using. From what I can see the zener diode in their circuit (similar to other's circuits also) will limit voltage to 13.25 volts. Without that circuitry I'd go to the 20v caps.

    Sumner
     
  14. Todd Hackett

    Todd Hackett TrainBoard Member

    93
    201
    8
    All: Electrolytic or Tant's will rapidly dis-assemble on an over voltage condition; the more the voltage is over spec, the bigger the bang and blast. Electro's are a more 'directed' blast as the casing is kinda like a rifle barrel and the force goes more or less in one direction; top or bottom. Tant's on the other hand do not have a directed blast per-say; the force goes out at the weakest spot in the casing which could be anywhere on/around the device. Both make a bang with flame and lost smoke; electro's rain metal; usually thin Aluminum pieces, paper and electrolytic, tant's rain casing and electrolytic. Also, both have a built-in safety margin. For instance, if the working voltage is 20Vdc, the safety margin could be 5% or 21V which in the grand scheme of things is not diddly-squat.

    Later
     
    badlandnp likes this.
  15. wvgca

    wvgca TrainBoard Member

    499
    305
    21
    i thought the safe margin was around 40-50 percent .. safe momentary maximum, not the working voltage
     
  16. 308GTSi

    308GTSi TrainBoard Member

    432
    957
    16
    So I can't jumpstart the layout with a fully charged Tesla ................. :(
     
  17. BigJake

    BigJake TrainBoard Member

    3,259
    6,172
    70
    It'd sure 'nuff 'jump!'
     
    308GTSi likes this.
  18. Todd Hackett

    Todd Hackett TrainBoard Member

    93
    201
    8
    Ya you are right, and that margin depends on the peak voltage value and the duration. Not something that you would want to plan on though.
     

Share This Page