The following items were discussed at the Signal Class on March 26, 2003:
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| Track Circuits: |
DC - |
Battery and Relay and opposite ends of Block
Trains "Shunts" track circuit and drops out relay
Polarity reverses at every block change
<0.06 ohms Shunt will drop relay (equiv to 14.7 ft of #14 wire)
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AC - |
Transformer feeds track through 1 ohm at usually 60 hz
Phase sensitive relay monitors track at other end of block
This relay compares AC line to received AC on track
Match causes motor to move contacts
Train in block cause mismatch and gravity pulls down contacts
Freq is either 60hz or 25 hz (opposite of propulsion if AC propulsion)
OERM is DC propulsion so any freq is OK for us
Phase reverses at every block change
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Coded Track Circuit - |
Voltage source is pulses at 50, 75, 120, 180 or 270 per second
Relay at other end of block is sensitive to pulse rate
Still must have insulated joints
Usually good for "one-direction" only
Example: Yel=50, AppYel=75, Clear=120, Red=Absence
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Audio - |
Audio Tone transformer coupled to rails
Tone at other end of block pulls in relay circuit
Adjust audio level until relay barely picks up
Don't need insulating joints except at switches
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Electricode (pulse pairs) - |
Expensive and Newer so not at OERM
Spacing of 110 millisec pulses change based on track condition
Provides bi-directional signaling info
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| Impedance Bonds: |
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Need DC Return for Traction
AC/DC Propulsion still flows but AC audio circuits stop
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| Relays: |
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Track Relay - 4 ohms 1-3 volts
Line Relay - 500-670 ohms 10 volts
(can be 1000-1500 ohms if needed for multiple relay circuits)
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| MISC: |
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Track Circuit patented by Dr. Robinson
Typical Modern Railroad:
Main Signals = Electricode
Crossings = Audio
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