DCC Block Detection

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Summary: Block Detection is a method for detecting rolling stock within a given section of track. It is also known as Occupancy Detection.

Current Transformer for mounting on a PCB, similar to that used for detection.

Occupancy Detection

  • Have you ever thrown a hidden turnout with a car over it?
  • Would you like to know when your train is at the end of your hidden staging track?
  • Have you ever hit the back wall of the engine house?
  • Which of your roundhouse stalls is occupied?

Knowing where trains are on your layout can be used for many purposes. It can be used for position indicators, operational signaling, automated control, or animation. To perform these tasks, a certain amount of blocking needs to be done for train detection. Even though this does take some work, it's easier to accomplish this with DCC than with block control. While it's not realistic, we'll say that the signal blocks in our example are less than 12 feet long.

Occupancy Detectors

Current Transformer Symbol. Current Transformers (CT) are often used to detect the presence of a train by sensing the current flowing to the track.

To get detection working, you will need occupancy detectors. These are devices that use electronic methods to sense the presence of a locomotive (or appropriately-fitted rolling stock) on a section of track.

There are two basic forms of block detectors. First, and usually the preferred form, is a block detector that detects trains and reports the status to your control system. These detectors are usually more expensive.

The second type of detector simply detects the trains, but requires additional circuitry to send the status to your command station. However, they are still useful without this, as they can operate a crossing gate without the command station even knowing.

Commercial Occupancy Detectors

A number of DCC manufacturers supply detection devices.

Lenz has the LB101. NCE has their BD120.

Digitrax offers the BDL168 and BD4, as well as the more sophisticated BXP88 and BXPA1.

See Occupancy Detectors for more information.

Current Transformer used to measure high currents. The occupancy detector would replace the ammeter in this schematic.

Preparing the Track

Here are the steps to prepare your track for detection:

  1. In order to get detection working, you first need to select a section of track you wish to detect if a train is on that track.
  2. Isolate this section of track by gapping one of the rails at the start and the end of the block.
  3. Wire the DCC feeds to this section of track via the detector

The Track/Power Bus

A sub-bus is often employed to power the section of track where detection is desired. In this instance the CT will be inserted near the beginning of the sub-bus to detect all the current in the sub-bus.

It is also possible to use an entire power district for detection purposes.

  • Do not twist the wires for a foot or more prior to and after the CT
  • This reduces the parasitic currents which may cause false triggers
    • Twisting two wires together increases the capacitance between them

Inserting a CT

Insertion of a CT is easiest when the wiring is being installed. Test the CT to determine if enough current flows for detection by placing a locomotive on the track, or shorting across the rails with a 10kΩ resistor at the point furthest from the CT. If necessary, add additional turns to the CT to increase the current it sees,

In the simplest installation, one wire (or turn) will pass through the window of the CT. To maximize flux concentration, the other wire should be held against the CT.

If multiple turns are required, pass the wire through the window two or three times (more if needed) until the detector works with the minimum load required. When using multiple turns, do not concentrate the wire in one spot. Space each turn out around the circumference of the CT. This is much more effective. Again, keep the return wire against the CT as well.

  • Doing this spreads the flux out instead of concentrating it in one location, avoiding saturation of one segment of the coil.
  • The number of turns is counted from the exit side of the CT.

See Also

External Links