Z scale resistor wheels, advice needed.

drken Jul 27, 2018

  1. drken

    drken TrainBoard Member

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    I'm thinking about converting my rolling stock so it'll be noticed by block detection. The most common method seems to be gluing a SMD 10K resistor to an insulated wheel and using conductive paint to connect to it across the insulating bushing. This sounds pretty doable. Has anybody here tried this? What size SMD resistor did you use? They definitely make them small enough, but I'd rather not figure out what works best by trial and error if I can avoid it.

    Also, there's a resistive paint (Aquadag) that SBS4DCC seems to like and it definitely looks easier since I won't have to glue anything to the wheelset. How well does that work?
     
  2. SJ Z-man

    SJ Z-man TrainBoard Supporter

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    The Aquadag I know was Graphite in Alcohol, so no sealing agent or ability to maintain mechanical attachment. You might try the pens for circuit board repairs.

    There is no real clearance issue so any size should work. But a 0402 is a bit small for me.
     
  3. drken

    drken TrainBoard Member

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    https://www.amazon.com/gp/offer-listing/B008OVGH9A/ref=dp_olp_new_mbc?ie=UTF8&condition=new
    I have a friend with some 0603s, so I'll see if that fits. But, what SBS4DCC uses is called Aquadag E, a water based formulation that comes in a little nail polish bottle w/brush. they say the resistance is 1K/sq (I am assuming inches). It's available on Amazon. It's not that much, so I might just pick up a bottle anyway.
     
  4. markm

    markm TrainBoard Supporter

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    I've looked at block detection several times, the most recent last year after a how-to story came out in the NMRA magazine. I've ultimately decided that it doesn't meet my needs considering my layout and operation, but as they say in the commercials: your results may vary. But I'm happy to share my work with you.

    The upper limited on resistance for reliable detection is 10Kohm (my example was the Digitrax detection unit). However if you plan to run a 100 car train, the resistance of all the cars is 1/100th that value or 100 ohms. With a 12V track voltage that's 120mA or about 4x the power draw of an AZL locomotive. And the cars will be drawing that power anytime they are on a powered track. So you need to give some thought to how you power the layout.

    As far as physical resistors either 0603s or 0402s will work, it's just as Jeff pointed out, how comfortable you are with the size. I would advise a vacuum pen for handling either. I would also suggest using a silver fill PC board repair fluid to make a conduction path.

    The SBS4DCC material is exactly as specified: 1Kohm/sq. Paint a 1 mm square and it's 1kohm, paint a 1 inch square and it's 1kohm. If you want a 5kohm resistance paint a rectangle that's 1 x 5. I know it seem counter intuitive and I can take you through the physics, but trust me on this.

    Hopes this helps,

    Mark
     
    drken likes this.
  5. drken

    drken TrainBoard Member

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    Thanks for the advice, Mark. I'm not planning to do any 100 car trains, so I don't think it'll be an issue.
     
  6. RobertAllbritton

    RobertAllbritton TrainBoard Member

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    I've used EMF shielding paint for years on hundreds of axles. The paint I use is made by GC electronics, but it is no longer available. One bottle has lasted me forever.

    I just dilute the paint about 1:1 with alcohol, and then paint across the insulator on the axle. The paint has a fair amount of graphite in it, whihc conducts a bit. After the paint dries in about 20 minutes, I check each axle with a meter. I accept anything in the 1K to 3k ohm range. Because you are making a resistor out of paint, sometimes you get too much, sometimes you get too little, so you must check your work.

    Putting an actually resistor on Z scale axles has been tried, but its very difficult, and they fall off. Like I said, with the paint method, I've got rolling stock that has been running well for over 10 years without adjustment.

    One note: since your wheels are now slightly conductive, they are going to collect dirt off the track faster than non-treated wheels, so you will need to clean them every once in a while. The only effective way I have found is to remove the wheels and hand clean them with a little bit of T-Shirt material, then re-install. Its a good thing to do while your watching TV.
     
  7. drken

    drken TrainBoard Member

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    Thanks Robert,

    I have some strong epoxy, so we'll see how much luck I have with resistors. Of course, even if they do work, paint will be much easier anyway. Is this EMF shielding paint like regular conductive paint? Because I have some MG Chemicals 838AR carbon paint along with the Aquadag. I'll have to see if that works too. All I have to do now is get up off my rear end and get to work on them.
     
  8. RobertAllbritton

    RobertAllbritton TrainBoard Member

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    The paints are probably similar. Ive also used some stuff off amazon:https://www.amazon.com/Woremor-Shie...UTF8&qid=1533397115&sr=8-6&keywords=emf+paint and it works well.

    The trick is to thin all of these a bit and experiment. But WAIT FOR THE PAINT TO DRY before you test it on a meter. Its resistance will change as it drys.

     
  9. drken

    drken TrainBoard Member

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    Well, I've started on the wheels without much luck. While a DCC equipped loco shows up, a painted wheel using either paint (see above) did not. To see what would, I put a 10K resistor across the track to test and still got nothing. Same with a 1K. Oddly, a 220 resistor didn't set off the detector despite getting very hot. A wire will still set off the circuit breaker, so something works. I'm using a Digitrax BXP88 with a LT5 tester, if that matters.

    Edited to add: I put a multimeter across the wheels and they showed up almost closed about 50-1 ohm. Do I need more paint or less?
     
    Last edited: Aug 4, 2018
  10. drken

    drken TrainBoard Member

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    Well, I shut everything off and went outside for a bit. When I came back a few hours later the BXP88 could pick up the resistors. Not sure what happened, but good to know the ol' turn it off and on again still works. I also tried using much more of the Aquadag and got some flickering detection of the wheel, so I'm putting even more on by using the supplied brush directly. Hopefully I'm on the right track. The wheel seemed to spark a bit at the beginning, but otherwise seems to function normally.
     
  11. SJ Z-man

    SJ Z-man TrainBoard Supporter

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    50-1 ohm ??? (read: 50 to 1 ohm range???) The DCC voltage would evaporate that graphite powder !

    @ 1Kohm (1K or 1000 ohm), that would be .14 Watts The wheel should be warm. @100 ohm, it would be 1.4 Watt and the wheel should a) get hot and b) spark when touched to the rail.

    LESS graphite !

    NOTE: with put sealing it, moisture, etc. are going to change that over time.
     
  12. drken

    drken TrainBoard Member

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    To much current frying the paint would explain the sparking as well as why it only happens when I first put the wheel on the track. I guess this is another case of "less is more". I'm still having issues with the BXP88 having to be reset to default to pick anything other than a decoder after a short time, if it persists I'm going to have to ask Digitrax about it.
     

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