Installing axle resistors for block detection

Model Railroader Electronics and DCC
1

Just wanted to pass this little tid-bit along for others who may be planning to do this.

I have finished setting up my railroad for block detection so I can see where my trains are. The last thing I need to do is install the small resistors on my freight car axles so the system can see them.

I researched the task and found I need to glue 10K ohm resistors, 1/8 watt, to the axle so it creates a very small current path across the wheel’s insulator. This small amount of current allows my BD20 to detect that a car is on the block.

I ordered 200 resistors on eBay, and was surprised at what I received. I needed a magnifying glass to SEE the resistors, and it was impossible to install them on the axles with the conductive paint.

It turns out, that one of the long list of codes in the part number was the number 0201. Not knowing what that meant, I did not pay that much attention to it, as I thought all the chip resistors were similar except for resistance and wattage.

Well, the 0201 was a size code, which, had I looked into it earlier, meant this resistor was .024" in length, .012" in width and .01" in height !!! In metric that is 0.6 mm in length, 0.3 mm in width and 0.25 mm in height.

What I should have ordered (and did so this evening) was resistor size code 1206, which is the exact same resistor only about 5 times that size.

The below link is to a sizing table showing resistor sizes. If you are contemplating doing this, I suggest you refer to the table on this web site before ordering. You may wish to order sizes

1206, or the even larger 2010.

Hope this helps someone!

http://www.resistorguide.com/resistor-sizes-and-packages/

2

I user 1206 size - they’re big enough to see even with my pooor eyesight, big enough to handle with tweezers, big enough to get a dot of CA on the axle that isn’t so big it squished up around the end caps of the resistor, insulating it from the conductive paint.

I used actual silver bearing paint, it’s more coonductive than the graphite types. Yes, it is extremely expensive, but a tiny bit lasts a long time - it’s very important to cap it up properly and NOT spill it, because while that tiny 1/4 ounce of it may cost you $50, it’s also enough to do probably 1000 wheelsets. Om eBay is it most seen listed as for use with electron microscopes. I’ve never used or even gotten to see an electron microscope up close, so I am not sure where the silver paint comes in with the process. But, it does make nice reliable paths for resistor wheelsets. I also then spray over the entire assembly with flat black, to keep the silver from flaking off and also to hide the shiny line.

The biggest thing to lear is - be patient. Most conductive paints don;t actually conduct until they are dry, or almost dry. So you finish painting one, measure across the wheels, and still get infinite resistence. No, you haven’t messed it up - give it some time to set up. Any that didn’t work, I could usually find a break in the conductive paint. Part of that was because the P2K wheelsets I use have slippery engineering plastic axles, and even scuffing a path across the axle with a Dremel didn’t always allow the paint to bite. A little touch up over top usually fixed it - I had very few outright rejects.

–Randy

3

Thanks for the input guys, I have to go see what size I am using. There were Atlas HO cabooses back some time ago that had resistors in the hollow axels which I stumbled across this when I was installing Atlas’s HO three color signal heads and those particular cabooses triped the lights (I wish Atlas would get on the ball and continue producing these signal heads). The funny thing was, at least two phone calls put in to Atlas technicians way back when from my LHS about the wheel sets with the resistors came back the answer of, “There are no resistors in the wheel set’s axels.” I just pulled one off the layout, looks like the ‘‘NE6 North Eastern’’ type dated 2004 Atlas item #6300 series and the resistor looks to be a 1/8 watt, brown, black, orange, gold = 10k-5%.

4

Most any decent detector, especially ones with the sensor coil like a BD20, work well with 10K resistors. I put 2 resistor axles on each car - it’s unlikely both would be dirty at the same time, and makes it possible that if the train was overhanging the previous block by half a car, it would still show occupied.

The overall load is neglible. Say you had 100 cars, each with a 10k on 2 axles, so 200 x 10K resistors. Total resistence would be 10K/200 or 0 ohms, at a DCC track voltage of 14 volts this would be 280 ma.

Even 1000 cars would be 2.8 amps - but if you had a layout big enough to handle 1000 cars on the tracks at the same time, you probably would have a layout with multiple boosters anyway.

–Randy

5

you may be better off ordering larger resistor that can easily reach between the wheel and axle as shown below. maybe randy can comment if the 1206 size he uses would be appropriate.

6

We’ll see when I get some Intermountain wheels that are insulated what way. I’ll make the 1206’s work somehow, since I have some 900 of them left. Outside of cars that already had metal wheels, all mine are P2K, which have metals wheels with all plastic axles. I think the Kadee wheels are hub insulated, but it’s not a protruding collar, it’s flush with the inner wheel surface if they are. I’d have to dig those cars out and take a look, I know I have two that I added resistors to, they are in my ‘club train’ carrying cases.

2010 and 2512 are usually only for high power resistors, and are starting to get quite large. Maybe for O scale 2 rail use - a 1/4" long resistor, you might as well just use 1/8w leaded resistors A 2512 is as wide as a 1206 is long. That’s why I found the 1206 to be a decent compromise between being able to handle the thing versus it standing out and/or getting in the way - even so, on open hoppers I’ve had to but the resistor wheelset on the outer axle rather than the inner on most other cars because even the 1206 interferes with the center sill, at least on Accurail, Athearn, and Bowser models. And that’s with the 1206 offset as far to one side as possible.

There are a couple of “how good are you REALLY at soldering” kits available that challenge you to solder all sizes of SMD from 2512 right down to the 0201 (for the ‘expert’ kit) and the maker will even send you a certificate if you submit video proof of the 0201 working. No thanks. I tried to fix a loose capacitor on an Arduino Nano, it was an 0402 size, and only made it worse. The 1206 size I am pretty sure I could solder to a PCB, but any smaller - nope. I know I could make my circuit designs more compact using SMD instead of through hole, but putting the parts in, soldering the leads, and then trimming them is almost cathertic. Peering at my screen while manipulating tiny componends under the USB microscope and trying to solder with e

7

I have made quite a few of those resistor wheel sets, however father time has caused my hands to get shaky. It got to be too much trouble to struggle with the shakes, so I bought some premade from an e-bay vendor. Fortunately I only ordered the minimum 4. Two of them were electrically open and had to be repaired. I guess I will continue to struggle.

8

i’m not sure any soldering is required. The articles talks about using conductive glue to attach the resistors to the wheels.

9

Attach with a very small amount of CA (Crazy Glue) being careful not to coat the ends. Then use the conductive paint to attach the end plates to the wheel and axel.

No solder is required, but be sure you are on the insulated end of the axel and that the axel, in fact, is conductive and connected electrically to the other wheel.

10

What I found works well is a thick CA from Locktite, that came in a really strange looking bottle that had sort of ‘wings’ along the side of the sount that you squeeze to dispense a drop. I got this I think at Lowes, not a hobby shop. It dispenses a very nice small drop without fiddling with special applicators or needles, and being the thick slow set kind of thing, you don’t have to rush to get the resistor in place before it sets up.

–Randy

11

Start small rant.

I just want to be clear, the image above was not created by Larry Puckett. I created this image. I originally posted this on April 23, 2012 in the MR forums. Here is the link: http://cs.trains.com/mrr/f/744/t/205307.aspx. I have contacted Mr. Puckett about this and he has been in touch with me about this and has now included credit on his site.

I am very much about fair use and believe in spreading knowledge with the caveat that credit be given where credit is due.

Just a gentle reminder to everyone that if you use an image that is not your own site where you got or include a link to the original source.

I apologize for the rant but I take this issue very seriously when I spent a great amount time coming up with images like this to help people here on the forums.

End rant.

Finally, back on topic, I use 0603 10K ohm resistors. I have two resistor wheel sets on each car, usually the inside axle. I paint all my wheelsets that I have replaced with a rust color paint when done to let me know I have converted the car.

Like Mel, i use silver bearing paint ($$$). its was about $45 for a 1 oz jar but I still haven’t run out and I have done about 400 axles so far. the tricky part is not letting it dry out. It is toluene based and it evporates quickly.

The image above utilzes a 0603 Resistor, and an intermountain wheetset. I have both 33" and 36" wheelset. one thing i found necessary was to do was t

12

I’ve used the wire glue mentioned . The $5.95 bottle can do about 500 wheels. if it dries out just add some distilled water mix and it’s good to go. Make sure you stir it every so often as the graphite will slowly settle out to the bottom. On metal axles I do the angle mount spanning the insulating bushing. On plastic axles I acc the resistor dead center on the axle and run two beads of wire glue from wheel back to each side of the resistor. Let them dry overnight then test with a meter to make sure they are working. ---- Ken

13

Well, in addition to the 600 0603 resistors, I now also have 700 1206 size 10k resistors, and still not correct!![:(]

The latest batch came in and were the correct size, but would not work on the axles when I installed them. I found that I had one more criteria to look at when selecting the resistors, whether the conducting edges were on the long ends or the sides. The ones I ordered conducted from side to side, not end to end.

Once I rotated the resistor 90 degrees, I was able to get it to work, but they do not sit well on the axles in that orientation, so I ordered a third batch, only in the correct orientation.

Mouser.com was the only place that had them (unless I wanted to order from China and wait a month to get them). It would have cost me $250.00 to order 700 of them from Mouser or only $50.00 to order 5,000 of them. Guess they REALLY don’t want to split a reel. So, I ordered 5,000, they should be here this week and I can always sell the remaining 4,400 of them in strips of 400 on eBay.

All this so a signal can see where a train is! [*-)]

14

I’m lucky enough to be using a signal system that doesn’t need resistor wheelsets. But they don’t hurt. I’m gonna save this discussion, and maybe someday put resistors on my cabeese.

Very interesting, I do say!!

Ed

15

That’s odd, I got mine from Mouser, 1000 pieces, part reel, for $15. I just checked, same part number is not 16 cents each for a part reel, so it would be $18. Mouser part number 263-10K-RC

–Randy

16

I did make a type, it was not $250, should be $150.00.

Definately not $15.00.

I tried to post a photo of the pricing, but cannot put a photo on the site. Here is a link to their site showing their current pricing.

https://www.mouser.com/ProductDetail/Xicon/263-10K-RC?qs=%2Fha2pyFaduhBdHOkDdZjvKl12x96Fotv2IQ53RVpDYA%3D

800 x .18 = $144.00 + $7.00 (reel charge) = $151.00 plus shipping.

17

The reel charge for less than full reel quantities is optional. Unless you’re going to load them on a pick and palce machine, you don’t need a reel. Mine came in a small box, a rolled up section of tape from a reel, but no reel.

–Randy

18

$7.00 or no $7.00, they still wanted $144.00 for 800 lousy resisters when 5000 was only $50.00. Nowhere can I find them for $15.00. Maybe a few years ago or so, but not now.

19

I noticed Arrow has some options that do not incur the “Mouser Tax”, I can get 1000 pieces for $28.40 shipped. I have yet to order anythign from there, but if Mouser’s pricing has changed so drastically on other components as well it is not likely I will be ordering from there any longer. I don’t need those mass qunatities to get the good price breaks for most things. Shame, they were always my go-to, I have several BOMs with Mouser parts for various projects. They didn’t use to surcharge on smaller orders like Digi-Key.

Edit: I just checked DigiKey - same as Mouser. 10-30 cents a piece in single unit quantities, buy a whole reel and they are less than 1 cent each. Over $80 for 800, but $50 for 5000! So much for the age of the maker, you better be making a zillion of whatever it is you design, or you’ll end up with more leftover parts than you know what to do with.

–Randy

20

Yes, I agree. Unfortunately, in my case, I got fed up with ordering the wrong resistors from not knowing what every little detail meant, so I just bit the bullet and ordered from them with the part number stated as the correct resistors.

I went to their site and, for the heck of it, entered 4999 resistors and the price that came up was $899.82 plus shi. WOW. I added 1 resistor to that and the price dropped to $50.00.

I do think I can eBay the remaining 4200 resistors and get most if not all of my money back, just crap to have to go through that.