DBD22 - DCC Block
Detector Application Information
Block
Detector:
The DBD22 can be used as a block occupancy detector for
several of Team Digital products, but specially for the
SIC24 (SIC24 refers to SIC24AD also). The DBD22 can
operate from 5 volts which the SIC24 provides on it's
input connectors. The DBD22 output voltage levels are
then compatible with the SIC24 inputs. See SIC24
application information for a diagram of two DBD22s
connected to a SIC24.
The DBD22 is designed to
sense high frequency DCC AC current and
will not work with DC current or 60 cycle AC current.
Sensitivity Issues:
There are several reasons why the DBD22 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 DBD22 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 DBD22 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 DBD22. A resistor connected between
the holes near each of the current sensors will decrease
the sensitivity. Start with a 10K resistor. The smaller
the resistor value, the less sensitive. The resistor can
be mounted on either side of the circuit board. Keep the
resistor leads short.
You can use the DBD22 for over current warning by
connecting a buzzer to one of its outputs.
Experimentation will give you the exact trigger point
desired.
In some cases the sensitivity
of the DBD22 may be to low. It is rated to trigger at a
track current of 3 ma. To get 3 ma of track current a
wheel set of ~ 4.7K ohms is required with a track voltage
of 14 volts or ~ 3.9K ohms with a track voltage of 12
volts. Since N scale track voltage is near 12 volts a
lower (than HO) value of resistor will be required.
Remote LED Indication:
The DBD22 can be used to drive a remote LED to show block
occupancy status. The first diagram shows the DBD22
driving a remote LED. When the block becomes occupied the
DBD22 turns on the LED.
The next diagram shows the DBD22 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.
Notice that to drive a bi-color LED the auxiliary outputs
are required. The normal outputs are an open collector
type. This type output allows several DBD22 outputs to be
ORed together. The auxiliary outputs are push pull type.
In each diagram only output 1 of the DBD22 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.
Relay drive:
The DBD22 can drive a relay with a low current coil. The
relay shown has a coil current of 30 ma and a contact
rating of 10 amps (Digikey PB380-ND). A clamping diode is
required when driving a relay to suppress the voltage
spike.
