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G8MNY  > TECH     22.07.17 08:32l 121 Lines 5272 Bytes #999 (0) @ WW
BID : 50550_GB7CIP
Read: GUEST
Subj: 88mH Coils, Explained!
Path: IW8PGT<IR2UBX<SR1BSZ<LU4ECL<I0OJJ<GB7CIP
Sent: 170722/0618Z @:GB7CIP.#32.GBR.EURO #:50550 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                    (Update Dec 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
After seeing several buls on 88mH coils, I thought I would explain what they
were used for by the millions in the UK telecomms industry.

LINE THEORY
The basic 600ê 2 wire telephone line system is quite lossy. This is due to too
high a capacity between the wires & the copper resistance, this causes high
loss & also higher loss at higher frequency than ideal.

         Simple Line                      A Loaded Line section
    ÄÄÄÂÄÄRÄÄÂÄÄRÄÄÂÄÄRÄÄÂÄÄÄ                ÄÄÄÂÄÄ((()ÄRÄÄÂÄÄÄ
600ê  ===   ===   ===   ===  600ê       1200ê  === ::::   ===  1200ê
    ÄÄÄÁÄÄRÄÄÁÄÄRÄÄÁÄÄRÄÄÁÄÄÄ                ÄÄÄÁÄÄ((()ÄRÄÄÁÄÄÄ

The capacitance & resistance value of the wires can't be changed. Reducing the
load Z flattens the frequency response, but increases the loss (a useful trick
for short music circuits!). But adding series inductance spaced evenly along
the line, makes the line look like a cut off filter & has several effects...

     1/ increases the line impedance (matching transformers need for 600ê!)
     2/ substantially reduces the line loss below the cut off frequency
     3/ & flattens the comms band frequency response.

Normal 600ê                          Loaded 1200ê
Line Loss  (e.g. 20 miles)           Line Loss
dB                                   dB
30´                        ,'        30´                    Þ
  ³                      .'            ³                    Ý
20´                   .ú'            20´  Flatter response  Ý
  ³             __..-~                 ³ And Half the Loss Þ
10´------ÄÄÄÄ~~~                     10´                 __/
  ³                                    ³  -------ÄÄÄÄÄ~~~
 0ÅÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂ>Hz      0ÅÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂ>Hz
   100 200 500 1k  2k  5k  10k          100 200 500 1k  2k  5k  10k

The standard for cables between telephone exchanges (office) was to use a
loaded cable with an 88mH balanced coil every 2000 yards, starting 1000 yards
from the exchange & completing the last 1000 yard pi section with an added
capacitor or added capacitor & 44mH terminated inductor half section.

   1000 Yards       2000 Yards       1000 Yards
    ÄÄÄÂÄÄÄÄÄÄ((()ÄÄÄÄÄÄÄÂÄÄÄÄÄÄ((()ÄÄÄÄÄÂÄÄÄÄ         ÄÄÄÂÄÄÄÄÄÄ((()Ä
1200ê ===     ::::      ===     ::::    ===       OR     ===     ::::  1200ê
    ÄÄÄÁÄÄÄÄÄÄ((()ÄÄÄÄÄÄÄÁÄÄÄÄÄÄ((()ÄÄÄÄÄÁÄÄÄÄ         ÄÄÄÁÄÄÄÄÄÄ((()Ä
              88mH              88mH  (Made up to)               44mH
                                      (1 section)
   <- - full section - ->                             <- half section ->

BACK TO THE COILS
The coils consist of 2 identical bifilar wound windings on a common ferrite
core, they are quite high Q, & capable of keeping their inductance with up to
50mA of DC current flowing (which is a lot for a pot core!) which is why they
are quite large for the max of 10mW of AF signals on telecomms lines.

Wire colours are normally Black & White Red/Black & Red/White, & it should be
quite easy to identify the 2 line pairs.
           _..._
Pot Core  <_   _>===== 2 pairs
2cm tall  ³ ~~~ ³===== of wires
2.5cm dia ³     ³
          \_   _/
            ~~~
Some coils are ally can encapsulated, others plastic & some not at all.

INDUCTANCE VALUE
Watch out for the 44mH ones, as they look much the same other than labelling &
have half the 88mH test inductance values below.

       Centre                   Centre
  ______ Tap                      Tap ÚÄÄÄÄÄÄÄÄ¿
  )::( o                               )::(o\ /   L Cancels
  )::(                       WRONG     )::(  X     so just a
o )::(___                            o )::(_/ \___  few mH
 ³_______88mH                         ³___________ ?


If just 1 winding is used then you have 22mH. By paralleling the 2nd coil you
get half the DC resistance, but the phase must be right!

     _____          ÚÄÄÄÄÂÄÄ                   ÚÄÄÄÄÄÄÄÄÂÄÄ
 )::(              o )::( o                   o )::(o\ /   L Cancels
 )::( 22mH    OR     )::(  22mH        WRONG    )::(  X    so just the
 )::(_____           )::(                       )::(_/ \   odd mH
                    ÀÄÄÄÄÁÄ                    ÀÄÄÄÄÄÄÄÄÁÄÄ

USES
So with suitable capacitors they make useful AF filters etc.

They can be used as isolation transformers, not too well isolated, & no good
for LF response with the low L value.

One can also be used as an efficient voltage step up or a -rail inverter with
just 2 transistors in flip flop. Note the push pull gives nearly 100% output
all the time so smoothing is not needed in some applications!

+12V ÄÂÄÄÄÂÄÄÄÄÂÄÄÄÂÄÄÄÄ¿
 55mA ³   e\³ 2k2 2k2 ³/e
      ³+    ÃÄÄ´   ÃÄÄ´  2x 50V PNP
100u ===   /³ 22k 22k ³\    100mA
      ³   ³     \ /     ³
      ³   ³      X      ³
      ³   ³_____/ \_____³
      ³   ³             ³
      ³   ³  =========  ÃÄÄ´<ÃÄÂÄÄÂÄÄ> -11V 50mA
      ³   ÃÄ(((()Â(((()ÄÙ      ³  ³ -  (unregulated)
      ³   ÀÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄ´<ÃÄÙ ===100u
 0V ÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄ>


See my buls on "Passive CW Headphone Filter", "AF 2 Tone Test Osc Design" &
also "DC Power Conversions".


Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP


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