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Impedence Bond
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   Author  Topic: Impedence Bond  (Read 232 times)
Virginian2004
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Impedence Bond
 
« on: Jul 2nd, 2004, 7:32am »
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Can anyone explain how the impedence bond works to separate AC and DC power and why it was used on electrified territory?

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Pennsy
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Re: Impedence Bond
 
« Reply #1 on: Jul 2nd, 2004, 10:46am »
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Hi,
 
Separating AC from DC is relatively easy.
 
First way would be to use capacitors, condensers. A capacitor will pass AC and block DC. It also has the ability, based on how many microfarads it is, to filter frequencies, pass high frequencies, and present a high impedance to low frequencies. This is what a capacitor does in your Stereo system to protect the Tweeters.
 
Second would be Diodes, semiconductors. They will rectify the incoming signal and pass only DC. They are current sensitive, and so have to have a current rating large enough to handle the circuit you are presenting to it.  
 
Hope that helps.


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Dyed in the wool PRR fan.
k5sss
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Re: Impedence Bond
 
« Reply #2 on: Jul 2nd, 2004, 2:18pm »
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on Jul 2nd, 2004, 7:32am, Virginian2004 wrote:       (Click here for original message)
Can anyone explain how the impedence bond works to separate AC and DC power and why it was used on electrified territory?

I don't know the exact details, but IIRC the need for them is related to cab signalling (AC) and train detectors (DC).  Impedance bonds allow one or the other to pass through, letting you build distinct but overlapping blocks for the two systems.


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firstbelt
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Re: Impedence Bond
  5216a_weeZ_01-23-1979.jpg - 126734 Bytes
« Reply #3 on: Nov 28th, 2012, 9:56pm »
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Here's a photo of an impedance bond on Atlanta's Metro, taken in 1979 before the system went into operation.
My 2 cents:
Trolleys and subways with overhead wire or third rail supplying power use running rails as the return path.  Impedance bonds equalize current between the two rails.  You can see the large cables connected to J-shaped bars that pass through coils inside.
Impedance bonds like the one pictured define the ends of a track circuit on either side of it.  This one has a receiver and transmitter tuned to two different frequencies transmitting a signal which the train shorts out to enable train detection and signal circuits.
They also transmit a coded speed command to the trains when operating in automatic mode.  Trains on Atlanta and Washington DC systems can run the entire length of a line on these speed commands, stopping and starting at stations along the way.  Most transit agencies prefer to have train operators run in manual mode, as a normal practice.


http://Forums.Railfan.net/Images/Infrastructure/5216a_weeZ_01-23-1979.jpg
Click Image to Resize

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