Message 21853 in TECH.WW
From: G8MNY
Date: Mon, 04 May 26 06:59:00 Z
Newsgroups: TECH.WW
Subject: 88mH Coils, Explained!
Message-ID: <58835_GB7CIP>
Path: N2MH4|N3HYM|IW2OHX|GB7CIP
R:260504/0704Z 30879@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260504/0703Z 3833@N3HYM.#FRDK.MD.USA.NOAM BPQ6.0.24
R:260504/0702Z 5795@IW2OHX.ILOM.ITA.EU BPQ6.0.25
R:260504/0659Z @:GB7CIP.#32.GBR.EURO #:58835 [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 600ohm 2 wire telephone line system is quite lossy. This is due to
too high a capacity between the wires and the copper resistance, this causes
high loss and also higher loss at higher frequency than ideal.
Simple Line A Loaded Line section
600R === === === === 600R 1200R === :::: === 1200R
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 and has several effects...
2/ substantially reduces the line loss below the cut off frequency
3/ and flattens the comms band frequency response.
Normal 600ohm 88mH Loaded 1200ohm
Line Loss (e.g. 20 miles) Line Loss
dB dB
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 and 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
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 or
iron core, they are quite high Q, and 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 a max of 20mW (+13dBm) 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
\_ _/
~~~
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
and have half the 88mH test inductance values below.
Centre Centre
)::( o )::(o\ / L Cancels
)::( WRONG )::( X so just a
o )::(___ o )::(_/ \___ few mH
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!
)::( o )::( o o )::(o\ / L Cancels
)::( 22mH OR )::( 22mH WRONG )::( X so just the
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!
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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