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Block Detectors

NCE offers the BD20 current sensing based block/occupancy detector.  There are two version and they are interchangeable in terms of current sensitivity.  However the newer BD20a has more features for the same price.  The term "BD20x" refers to BOTH versions.

This Section Covers:

1) BD20x Basics
2) BD20x and DC power
3) BD20 (The original version with no "a" suffix)
4) BD20a
5) Installing BD20 into your layout
6) Operation of the BD20 series of detectors
7) Downloadable Schematics


1) BD20x Basics

The BD20x detector falls into the "current sensing" class of detectors.  Current sensing detectors all monitor the current in a given isolated block of track where all current use to power the train inside the block must pass through the detector.  When the current level exceeds the detection threshold current, the detector will report the block occupied.

The BD20x detectors use a current sensing transformer as the device to sense the track current.  Transformers can only pass AC signals.  DCC is a form of AC where DC is not.  Hence the B20x is designed for and limited to DCC applications.


2) BD20x and DC Power

Some people have report the BD20x works with DC.  The problem with that statement it is conditional on what type of DC power is being used.  Many DC power packs contain some form of pulse power.  It can be rectified but unfiltered DC (Half Sine) power such as the old MRC ThrottlePack.  It can also be from a modern transistor throttle that generates pulses mixed with DC.  Regardless how the pulses are created, pulses are a form of AC power.  So the BD20x may work or not work depending on what type of DC power they are using.  Even motor noise or intermittent track power pickup can get involved.  Stated another way, these people are simply LUCKY for it is by ACCIDENT that it works.  However,  there is NO GAURANTEE it will work nor does NCE advertise it would.

Regardless if it works or not, one thing will always be true: When you turn the DC throttle to 0, then all power to the track goes to zero and no detection is possible.  That is NOT true with DCC for track power is never lost and independent of the movement of a train.


3) BD20 (The original version with no "a" suffix)

"BD20" Original Version is identified by the black boxy looking IC with 14 legs.  In the picture it is the device between the upright resistor and the tall tombstone looking current transformer.  The IC is nothing more than 4 individual NPN transistor (2N2222) type installed in a DIP (Dual Inline Package) format.  It is sold with a 2 pin connector that allows one to attach wires.  This version is discontinu.ed and replaced by the "BD20a" below.

LED Option: There are two holes that allow one to connect an external 5V to 12VDC power source that will allow on to light up a LED that is not installed.  The transistor needed to make this work is also not installed.  One should note that if you do hook up an external power source, the power supply will slightly influence detector sensitivity at the very edge of detection.

4) BD20a

"BD20a" or "A" Version is identified by the use of low profile surface mount parts.  This version has the full 4 pin connector which brings out all the feature offered by this version of the BD20.









5) Installing BD20 into your layout

The BD20 series of occupancy detectors are current based detectors. The sense the difference between a open circuit on the track (nothing conducting current) and closed circuit on the track (Something conducting current).  The advantage of current sensing detector is that the entire block of track is monitor form end to end.  It does not matter where the car or engines is on the track.  Track power consist of two wires, one for each rail.  The circuit requires one to install the detector in series with one of the rail track feeds.   We call this rail the detected rail. 

The track feed must be the main wire that all the track feeds are tied into.  Stated another way, all the track feeder wires for the rail must be reduced down to a signal wire before it passes through the BD20.

6) Operation of the BD20 series of detectors

The schematic on the left is the starting point for understanding how the BD20 works.  (You can download a higher resolution schematic at the bottom of this webpage.) On the far left is T1 which is a current sensing type transformer.  The winding on the left not connected to anything are in fact the loops of wire you run through the hole in the current transformer.  Stated another way, this transformer only comes with a secondary and you must install the primary.  The reason for the user installed primary is to allow one to set the sensitivity versus full current capacity of the detector.


See the BD20 schematic shown on the left which is also downloadble below.

Current transformers T1 works just like normal transformers in that there is a primary and secondary windings.  However the primary winding is built by the installer who must run a loop or wire through the hole.  Each loop of wire through the hole is called a turn.  The wire is wired in series with the track block to be monitored by the detector.  So any current consumed by any rolling stock or engines on the track must pass through the primary winding of the transformer.  The more turns (loops) made in the current transformers hole, the more sensitive the current transformer will become.  In other words it will detect lower levels of current.

When current is flowing through the primary, it will generate a magnetic field in the current transformer's magnetic core.  The secondary winding which is also mounted on the same magnetic core will be forced to establish a current flow in the secondary winding wire.  It is this secondary current flow that is used by subsequent circuits to implement detection.

One end of the secondary winding is connected to the detector's ground.  The other end of the secondary winding  is feed to the base (Pin 9) of a NPN transistor who's Emitter is connected to ground (Pin 10).   Resistor R1 is used to limit the base current.   This circuit creates a current path between the transformer secondary and the transistor Emitter-Base connections allowing the transistor to detect current flow.  The collector (Pin 8)  of the transistor is connected to the output terminal of the dectector.  It creates what is called a "Open Collector" interface.  When current is flowing in the Emitter-Base junction, the transistor will turn on and connect it's Collector to the Emitter.  In other words the collector becomes grounded allowing it to sink current.  Stated another way, it shorts the detector output terminal to ground.

Resistor "Rs" is used to adjust the sensitivity by routing some current directly to ground instead of through the Emitter-Base connection of the transistor.  Lower values of resistance will reduce the sensitivity while higher values will raise it.  However it is expected the user will use the number of turns on the primary as the main method of controlling the detection sensitivity.  Capacitor C1 is used to filter any noise in the DCC current so the detector output is steady and free of noise.  It also creates some delay in the clearing of detection. 

The detector output is intended to be connected to a input device which provides a pull-up current source (10K Resistor connected to some logic supply resistor.)   The resistor will charge C1 up to the logic supply voltage when there is no current detected in the track block being monitored.  As soon as sufficient current flows in the transistor, it will discharge C1 to zero volts.  Hence a logic low means the track is occupied and a logic high means the track is not occupied.

The NCE AIU board is designed to work directly with the BD20x. 

Why use a current transformer at all?  There are two major reasons:

1) The secondary is a very small fraction of the primary current making it easy to work with using small parts on the detection side.  The amount of current depends on the "turns ratio" of the current transformer.  Another way to think of this is the transformer "Steps Down" the primary current level to more usable level.

2) The secondary is fully isolated from the primary such that the DCC track power cannot show up on the detectors output.  This allows one to easily interface the BD20x to any circuit you want without concern of sneak current paths or shorts.  Keeping DCC power out of control circuits is very important for success.


7) Downloadable Schematics

Below are PDF files of the schematics for both the BD20 and BD20a.  They are provided to help you integrate the device into what ever devices you may wish to hook up to them.  There are marking that I have added to the schematics the provide additional information to help you get an idea of what going on.  

The BD20 schematic, I show the maximum current the detector supports to the right of the sensitivity setting and added a 4 turn information.  I also show what is not installed and the transistor part number if you so desire to install one.

The BD20a schematic does not show the diode front end but only the detection part.  I corrected some resistor values.
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Mark Gurries,
Aug 21, 2015, 7:37 AM
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Mark Gurries,
Aug 21, 2015, 7:38 AM
Ċ
Mark Gurries,
Aug 21, 2015, 7:38 AM
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