From: ce11son@yahoo.ca   
      
   On Fri, 20 Jun 2014 17:35:31 -0500, Larry Sheldon   
    wrote:   
      
   >On 6/20/2014 3:45 PM, mroberds@att.net wrote:   
   >   
   >>> Side note about capicitor size:  Landline telephone sets have always   
   >>> had a capcitor to separate the 20 Hz ringing current from the 48V DC   
   >>> signal and talk power.   
   >>   
   Not "always", just generally with manual or automatic systems using a   
   central battery for powering the telephones.   
      
   >> Well, 48 V at the exchange.  Maybe 5 V by the time it gets all the way   
   >> out to your off-hook telephone.  :)   
   >   
   >Not sure if there was a point here or not--I didn't find it.   
   >   
   >But in the interest of historical accuracy, the capacitor in question is   
   >important when the telephone is ON-hook and the DC voltage is pretty   
   >close to 48 volts, but more interesting is that's job is to pass the   
   >105V 20 Hz (most common) ringer current.   
   >   
   Historically, the capacitor had to be a particular value (thus   
   affecting its size) to match the inductive properties of the   
   mechanical bells so that the circuit did not stray too far from   
   resonance with whatever was the standard permitted number of bells in   
   any particular field of use. Even more historically there wasn't   
   necessarily a capacitor in the bell circuit as calling and clearing   
   signals didn't all require the provision of DC blocking. As for the   
   line voltage, most modern telephones require a minimum current for   
   proper operation of the electronic circuitry and the use of what   
   generally amounts to constant-current feeds makes the terminal voltage   
   merely an observeable feature rather than a meaningful measurement.   
   With electronic telephones there is no general requirement to involve   
   the capacitor in the speech circuit so (at least with UK/EU   
   telephones) there no longer needs to be one in each telephone and it   
   now goes only in the main jack; the diagrams that I can Google up so   
   far don't indicate this is US practice. Even without mechanical bells,   
   the capacitor has to be of a set value (+/- x%) if it is part of   
   determining whether or not there is a telephone at the end of the line   
   (or in the UK, whether or not any telephones are actually plugged in)   
   so we're still not at the stage of shrinking them to invisibility or   
   omitting them (unless e.g. the AC is detected via resistors in excess   
   of the required line insulation value).   
      
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