Note: Descriptions are shown in the official language in which they were submitted.
3~
PHF . 81. 5 98
The invention relates to a method of forming
a supply bridge which can he sub~ected to high overloads
and is constituted by two identical branches each com-
prising a resistor.
The possibility of such overloads occurs, for
example, in telephony on the supply bridges of a sub
scriber's line, each branch of which comprises in series
between the central battery and the said line a resistor
of high value, a protecbive thermistor and, as the case
may be, a primary winding section of the subscriber's
transformer or a consta~t current generator~ The said
resistor may assume values from 55 to 205 J~ for resistance
~alues in continuous operation of the branch varying in
acaordance with the rele~ant countries from 150 to 400 J~ .
The overloads from ~arious sources occurring on
the line may be inter alia~pulses of high Yoltage of the
order of 5000 V, but of short duration (1 msec3, resulting
from a lightning stroke, or permanent voltages of a~out
250 V at industrial fxequencies due to the magnetic induc-
tion caused by a dissymm2try on the high-voltage lines
or due to contact with the :Low-voltage lines.
If it is assum~d, for example, that a permanent
overload of 300 V effecti~e at the frequency of 50 Hz is
applied to the line, each branch of the bridge ha~ing a d.c.
impedance of 150 IL (standard value for the French network)
is tra~ersed by a current of 2 A, which dissipates a power
of 200 W in each resistor ha~ing a value of, for exarnple,
50 Jq. When the response timç of the protective thermistor
in series with the said resistor is of the order of a
second, the corresponding energy can thus attain the high
~alue of 200 Joules.
The resistors used hitherto in the supply
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~ ~ ~3~ 6
r~ 1 _9~ 2 1.9.1982
brl(lges of telephone lines cannot guarantee the safe-ty
~r reliabili-ty which is required ~hen -they are subjec-ted to
S~IC]I limitations.
The wire-wound resistors have a high reliability
5 1~1l t tlley are inflammable. Their use proves to be dangerous
re~uires a constant vigilance.
The layer resistors are non-inflammable but
tl~cir reliability i5 very poor. They adversely affect the
rcliability of the system and especially -the availability at
l~ tlle Level of the individual subscriber's equipment~
The invention has for its object to provide a
nethod of forming a supply bridge which permits of obtaining
the properties of non-inflammability and reliability which
are re~uired when this bridge is subjected to the accidenti-
l5 al o~-erloads occurring during operation. This method is
characterized in that the resistors of the said supply bridga
are formed simultaneously by silk-screen printing of a
thick resistant layer on the same insulating support having
the thickness re~uired for withstanding the said overloads,
' each ol' these resistors being interposed between two connec-
tion strips of a conducting material deposited in the same
manner and connected by welding to two metal wires of the
same ne-twork fixed on the said support, whilst moreover each
of the resis-tors can be adjusted to a predetermined value by
25 removing throughout its leng-th a part of the resistant
layer between the said connection strips.
The said insulating support is constituted, for
e~ample, by a ceramic material and, taking into accoun-t the
said overloads, a thermal calculation has been carried out
30 to determine its thickness lying between 2 and 3 mm.
The ~ollowing description with reference to the
accompanying drawings, given by way of example, permits of
understanding more clearly how the invention can be realised.
Figure 1 shows diagrammatically a supply bridge
o~ a telephone line,
Figure 2 is a plan view of the cons-truc-tion of
the resistance bridge according to the invention,
Figures 3a and 3b illustrate -two methods of
3~S~;
rlT~ 598 3 1 . 9.19~2
c~d~justing a resistor of -the bridge,
Fig. Il shows -the data and results o~ a -thermal
calculation carried out on a ceramic sample having a given
thicl~ness ~Figure 4a ) subjec-ted in a clirec-tion at right
angles -to one of its faces to a cumulative test of -thermal
shoc~s having a constan-t amplitude which e~hibits a
~-ariation wi-th time which is shown in Figure 4b and -to which
corrcsponds -the theoretical variation of -temperature shown
in ~`igures 4c and 4d.
Figure 1 shows -the configuration o~ a conven-
tional supply bridge o~ a subscriber's line, one o~ the
branches of which comprises, after the positive terminal ~
of the central battery connected to earth, a resistor 1 in
series with a protective thermistor 2 and a primary winding
section 3 of the trans~ormer, which transmits the speech
currents and the secondary winding 4 of which is connected
to the e~change. The other branch comprises, after the
negative terminal 6' of the battery brought to the poten~al
-E9 the symmetrical corresponding elements 1', 2' and 3'.
The junction points of the thermistor 2 and the primary
winding section 3 on the one hand and of the -thermistor 2'
and -the primary winding section 3' on the other hancl are
interconnected through the capacitor 5 which blocks the
direct current and constitutes a short circuit at the vocal
~5 frequencies, whilst the two other ends of the primary
winding sections are connected to the wires 7 and 7',
respectively, of the subscriber's line.
~ he res~stors 1 and 1' are constitu-ted in most
cases by very reliable wire-wound resistors, which in the
case of overload rarely take fire by themselves, but are
heated red-ho-t without becoming inoperative, which involves
the risk that the surrounding electronic material takes
fire.
In order to mitigate this great disadvantage,
according to the present inven-tion, these wire-wound
resistors are replaced by the resistance bridge shown iIl
plan view in Figo 2 and constituted by the two resistors 1
~g3~
I'lll` ~l.~ 4 I.9.1982
l`ormed simultaneously by silk-screen printing
~ ic~ er of a re~istant ink deposited between the
( onllcction s-trips 8 and 9 on the one hand and 8' and 9'
oll rlle ~-ther hand obtained by processes of silk-screen
~>rllll Ll~f, o~` conductive links on the same ceramic support
IO, l~le thickness of which has been calculated so that :it
is (~p~ble o~ witllstQnding thermal shocks of durations at
lenst e~llal to the response time of the thermistors 2and 2'.
The resistors 1 and 1', the dimensions of which
l ~lrc ~ between -the connection strips and L, have values R =
I~O ~ /L, the sheet resistance R of the layer being -the
cluo~ient of its resistivity and its thickness. The dimensions
sed Inost frequently are ~ = 7 mm and L = 10 mm.
A connector composed o~ eight metal wire~
5 ~lenoted by reference numerals 11 to 14 and 11' to 1~' and
reg~llarly arranged at the standard relative distance of
~4 Inm is fi~ed on the ceramic support, four of them,
denoted by reference numerals 11, 14 and 11', 14', being
connec-ted by welding to the connection strips 8, 9 and ~,
~ 9', respectively. The four other wires are preserved because
they contribute partly to the dissipation of the heat
accu~lulated in the ceramic materlal during the thermal
shocks to which it has to be subjec-ted; thus, they act as
radiators.
Figure 3 illustrates two methods of adjusting
the value of the resistors obtained by removing a part of
the resistant layer. The firs-t method generally used
consists (Figure 3a) in that a recess 15 is provided in the
resistant la~er 1. The configuration of the current lines
30 causes a constriction of these lines 16 on the lower side
of the recess, which results in a stronger heating in this
zone, which heat is $ransmitted to the subjacent ceramic
material. Such a method cannot be used for applications, in
~hich the ceramic material is subjected to strong thermal
35 shocks. The second method according to -the invention is
illustrated in Figure 3b. The aforementioned e~pression of
the value R of the resistor shows that this value can be
modified by varying one or the other of its dimensions.
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I'III ~l.59X 5 1.9.1g82
In the embodiment proposed, the dimension 1 is varied.
Firs-t a default value is calculated, which defines a
~idth ~alue L1, whils-t in a ~irst period o~ time a coarse
acljustment is effected by insulating the zone corresponding
-to L ~ L1 by cutting through the resistant layer 1 and the
conduc-tive connection strips 8 and 9 by means of sand-
bl~sting or by means of a laser along a horizont~l line 17
e~ceediilg its length c~. In a second period of time, the
connection wires 11 and 14 being connected to a rneasuring
lO device not sho~v-n, a fine adjustmen-t is effected by cu~ing
by means of a laser bea~ssuccessive resistant la~er parts
18, 19, 20 insulated in -the same manner and spaced apart by
about 200/um until the desired valuè is obtained~ This
adjusting method, which does not disturb the parallelism of
15 the current lines 21 in the resistor, thus prevents hot
points from being formed in the ceramic support.
Figure 4 shows the data and results of the
thermal calculation carried ou-t on a ceramic sample having
a thickness e, to which the thermal flux ~0 is appliod in
20 a direction at right angles to one of its faces (Figure 4a).
This sample has to withstand for 1610 seconds the cumulative
test of thermal shocks~ the cycle of which is composed on
two thermal pulses having an amplitude ~0 = 200 Joules
of a duration of 1 second spaced apart by 5 seconds and
25 follo~ved by a rest -time of 180 seconds (Figure 4b). The
problem consists in the resolution of the general equation
of heat propaga-tion dt = a d ~ in order to obtain in a
dx
given sample the temperature ~ in the ceramic material as a
30 funct:ion of the time -t and in the direction x, a = ~ /pCp
being the thermal diffusiveness~ ~ the conductivity,
~ the specific weight and Cp the specific heat. Under the
initial conditions and wi-thin the limits resul-ting from
~igures 4a and 4b, there is obtained:
~(x,t)= ~o ~ _n ~ _ _K2 a
n=1 Kn(eKn ~ ~eh ~ ~ h)
37~
l'lll ~1.~9~ 6 1 9.1982
[`I,ei~ the signal time, _ the Newton coefficient and n -the
tl1eorctically infinite number of solutions I~n of the equa-
tion l~ ~tgKne - ~ . Without entering into the details of the
rcs~llts of the calculation, Figure 4c, which illustrates
tllc ~-ariatio~ of the temperature as a func-tion of time 3
clcarl~ shows the cumulative effect ob-tained. In Fig. 4d,
~l1e rLme scale has been expanded in order to indicate from
~hc il~itial ins-tant the temperature increases corresponding
to Ll~C ~irs-t -thermal pulse for different thickness of the
ccrarnlc material increasing from e1 to e4. For the smallest
thiclcllesses~ the temperatures attained have maximum values
in a time shorter than the duration ~ of the thermal pulse,
E~perience has shown that in this case~ the cumulative test
of` thermal shocks is destructive. On -the contrary, the
li ceramic sample l~ithstands the test if its thickness is such
that the maximum tempera-ture is obtained above t= ~. In a
s~l~scriber's bridge of a telephone line~ r is not different
from the response time of the thermistor CTP in series with
the resistor in each branch of the bridge.
. 0
'
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