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Team Digital, LLC
DBD2 - DCC Block Detector |
| The DBD2 is a dual block occupancy detector. It
provides complete isolation from the track by using small transformer
type sensors. This also means there is no drop in track voltage as
there is with sensing diodes. The DBD2 is designed to work well with
the SIC24 signal and indicator
controller. In applications where relays are required it has enough
output current capability to drive them directly. MSRP - 19.95 USD Available at these Dealers Manual in PDF format - 40K Board size: 1.5" X 1.8" Application Information Support Tools |
| Operation: The DBD2 is capable of sensing current in two isolated sections of track called blocks. This provides for detecting the presence of locomotives or other rolling stock that draw current from the track. The track feeder wire for one rail of the block is passed through the hole in the sensor (see diagram below) before connecting the feeder to the track. When the current in the feeder going though the sensor reaches the trigger level, the output is turned on and the LED lights showing the block is occupied. Passing the feeder though the sensor more than one time will increased the sensitivity. However, this is NOT recommended because the maximum sensor current specification will be greatly reduced. Locomotives and lighted cars draw enough current to trigger the DBD2. In order for other rolling stock to draw current, resistive wheel sets will have to be added. For HO scale, one 4.7K ohm resistive wheel set will work per car. If you want to use 10K resistive wheel sets you will need two wheel sets per car. Duty track can cause intermittent current flow. A built in filter keeps the DBD2 from rapidly turning off and on if the current flow is interrupted. Additional, there is a small amount of hystersis so that small changes in track current near the trigger point do not cause the DBD2 to turn off and on. The filter keeps the output on after a train leaves the block for a short period of time. This time is typically about 1 to 2 seconds. The amount of track current has an effect on this time. Since the DBD2 does not use diodes to sense track current there is no drop in track voltage. This allows you to “double sense” a block. For example, suppose you want to add a grade crossing with gates and flashers to a section of track that already has a diode sensing block detector. The block could be cut into three smaller blocks to detect traffic on each approach and island blocks. DBD2s can be used for the three new blocks with out interfering with the operation of the original detector or causing an additional track voltage drop. See the SIC24 signal and indicator controller for information on implementing a grade crossing. |
| Detector Output: The DBD2 output configuration is a push-pull and can sink and source current. The output is typically connected to a signal system input. However, the DBD2 has enough current capability to drive a relay. There is clamping diode build in so no external diode in required when driving a relay. Detector Label: Gerry Albers provided identifier template labels for the DBD2. They have a place to label the output and power connector terminals. Version 2 has a box to write the Power District identifier and the rail (Rail A or Rail B) being detected. These templates are set up to print on business card stock, Avery #8376. Only the bottom needs to be trimmed after printing. They are intended to be placed under the DBD2 where it is mounted. det label.doc det label2.doc |
| Specifications: Operating voltage: 5 to 15 volts DC Operating current: 6 ma @ 5 volts, 10 ma @ 12 volts Maximum output load: 400 ma Typical trigger current: 3 ma @ 5 volts, 6 ma @ 12 volts Maximum sensor current: 5 amps w/one pass of feeder wire |
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| Applications |
| Detector for SIC24: The DBD2 can be used as a block occupancy detector for the SIC24. The DBD2 can operate from 5 volts which the SIC24 provides on it's input connectors. The DBD2 output voltage levels are then compatible with the SIC24 inputs. See SIC24 application information for a diagram of two DBD2s connected to a SIC24. Sensitivity Issues: There are several reasons why the DBD2 may indicate a block is occupied when in fact there is no engine or rolling stock in the block. If there is a LED or some kind of indicator that is powered from the track, this could cause the DBD2 to show occupied. High humidity conditions can cause a problem. Perhaps the ballast is damp for some reason. Water can act like a resistor between the rails. There can be enough distributed capacitance in the DCC wires to cause a small amount of current to flow and thus causing a false indication. To keep the capacitance to a minimum, place the DBD2 close to the block it is detecting. Keep the the block feed wire or any of the block track feeder wires from running right next to other wires for long distances. Do not twist the block feed wire with another wire. In some cases it may be desirable to decrease the sensitivity of the DBD2. This picture shows where to solder resistors to decrease the sensitivity. A 2.2K resistor will give about a 20 ma trigger current. A 1K resistor will give about a 75 ma trigger current. If possible use a 1/4 watt resistor. Also, keep the resistor leads short. You can use the DBD2 for over current warning by connecting a buzzer to one of its outputs. Using a resistor in the 22 to 27 ohm range will cause the DBD2 to trigger in the 2 to 3 amp area. Experimentation will give you the exact trigger point desired. |
| Remote LED Indication: The DBD2 can be used to drive a remote LED to show block occupancy status. The first diagram shows the DBD2 driving a remote LED. When the block becomes occupied the DBD2 turns on the LED. The next diagram shows the DBD2 controlling a two lead bi-color LED. When the block is unoccupied the LED is green. When the block is occupied the LED is red. In each diagram only output 1 of the DBD2 is shown connected. Output 2 could be connected in any of the ways as output 1. When driving a LED a resistor is required to limit the current in the LED. |
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| Train stopping block: Warning: This is an experimental circuit and has had limited testing. It intended to give ideas to experimenters. Many decoders support the stopping of trains when DC is applied to the track, rather than DCC. The DBD2 along with a couple of relays and a diode bridge can be used to construct a simple circuit that will stop a train in a block if the block ahead is occupied. When the activating block is occupied relay 1 will be energized allowing relay 2 to be energized when the stopping distance section of the stopping block becomes occupied. When relay 2 is energized DC is put on the track which causes the train to stop or slow to a stop if deceleration is active. The DBD2 does not sense DC current, however, in this case there is enough DCC "noise" in the DC to keep the DBD2 triggered. When the activating block becomes unoccupied relay 1 is de-energized causing relay 2 to be de-energized putting DCC back on the stopping block track. This causes the train to resume normal travel. The diode bridge rectifies the DCC signal providing power to the DBD2 and DC for the track. A suppression capacitor of .1uF is used across the coil of relay 2. |
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| Bi-Directional Train stopping block: Warning: This is an experimental circuit and has had limited testing. It intended to give ideas to experimenters. The above circuit only stops trains traveling in one direction. It may be desirable to stop trains going in either direction. Relay 2 now needs to be at least a 3PDT (a more common 4PDT relay is shown). When either activating block is occupied relay 1 will be energized allowing relay 2 to be energized when the trigger section of the stopping block becomes occupied. Notice the DBD2 is sensing both activation blocks. When relay 2 is energized DC is put on the track which causes the train to stop or slow to a stop if deceleration is active. Since the trigger section may become unoccupied as the train slows, relay 2 must stay energized so it is "latched on" with a set of it's contacts. When the activating block becomes unoccupied relay 1 is de-energized causing relay 2 to be de-energized putting DCC back on the stopping block track. This causes the train to resume normal travel. The diode bridge rectifies the DCC signal providing power to the DBD2 and DC for the track. Diode D1 1N4001 is required because when output 2 of the DBD2 is off, relay 2 may still need to be energized. A suppression capacitor of .1uF is used across the coil of relay 2 |
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