Sensitivity Issue with BOD-8
The following was compiled from an exchange on the LCC groups.io list in December 2021.
I am attempting to use a BOD8 to monitor track occupancy in a 5 track O scale staging yard. The yard is in the form of dog bone reversing loops with each staging track approximately 50′ long. Each track has its own PSX-AR enabling trains to come and go independently via the approach track ladders at the neck of the dog bone. The feeds to the track busses (1 per staging track) pass through a CT close within inches of the PSX-AR output. To accommodate track drops the busses follow the tracks under the table – and are also about 50’ long. No DCC Snubbers installed on the bus.
Ed
Issue: Four of the 5 staging tracks are being successfully monitored by the BOD-8. The innermost loop is not. The dilemma: with less sensitivity I get no detection – more sensitivity I get a false positive. If the sensitivity is turned up on the BOD8 pot it will detect a decoder equipped locomotive as it enters the staging track – but when the locomotive is slowed to a stop the BOD detection goes off. If I blow the horn the BOD-8 detects occupancy momentarily. When I slightly increase the sensitivity of the BOD8 to detect the idle locomotive it will not turn off when the loco leaves the track section. With less sensitivity I get no detection. I’m apparently on the hairy edge of the BOD-8 adjustment range. Prior to wiring the blocks for LCC I was able to monitor staging tracks with NCE BD20’s. However, to get the B20’s to work I installed 100pF capacitors shunting on the track busses as they passed through the BD20 coils. (One leg of the cap wire passed through the coil along with the bus wire).
Q: would it be safe to try the same 100pF cap wiring with the BOD-8 CT?
Ed, Just put the cap into the same terminals as the pair from the CT coil. The added cap will indeed make things work better. It snubs out more of the spikes caused by the track and feeder capacitance.
Dick 🙂
Success! Using an on hand .1 micro farad capacitor installed as Dick suggested did the job. Reliable detection is now in the range of adjustment of the on-board BOD-8 potentiometer.
Ed
The ability to extend the tuning sensitivity range of the RR-CirKits Block Occupancy Detector is useful information for the LCC tool kit. It would be especially applicable in instances of troublesome or less than optimal wiring. In a December 1 post, Dick mentioned using a .033uf capacitor whereas you used a .1uf cap. Further, the RR-CirKits BOD-4 and BOD-8 documentation contains the following: “Sensitivity may be decreased with the on-board pots for situations requiring less sensitivity. To further reduce sensitivity place resistors across the coils.” My added underline.
Steve
For us less informed types, it would be good to know how using a resistor differs from using a capacitor? Also, what are the useful ranges of either and are there any limits and precautions involved?
There are two different reasons that you might want less sensitivity. One could be that you have large scale and used lower resistance resistors for your rolling stock to better deal with ballast resistance out doors. In that case you need to lower the current sensitivity with a parallel resistance. In Ed’s situation the problem was too much capacitive coupling. It is dealt with by adding parallel capacitance at the input as he just did.
Dick 🙂
Both are cases of can’t turn down the sensitivity enough. In the case of too much stray capacitance what usually happens is that the adjustment window gets so narrow that there is no setting that works.
Thank you for the clarification on use of resistor vs. capacitor. Good to know. I take it that capacitor values used will depend on existing conditions and desired amount of adjustment. Starting value is not critical? Like Ed, I also have one block that is flakey and will begin with a .033uf cap. Perhaps one of those multi packs listed on Amazon could come in handy when starting out using existing wiring. Again, thank you and best wishes for the holidays
Steve
The issue with some capacitor kits is that they are SMD. Be sure to select leaded CAPs because they are the easiest to play with. You simply stick them in parallel with the twisted pairs right at the terminal strips. A quick look on Amazon shows mostly leaded parts, so I guess they are catering to the DIY crowd. I have never actually tested for what maximum values will still allow normal detection. I imagine that too high a value could also reduce sensitivity. For those that want to play, it shouldn’t matter how large a cap you try, it will not damage anything.
Dick:)