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1. * GB780118 (A)
Description: GB780118 (A) ? 1957-07-31
Electric meter circuits
Description of GB780118 (A)
COMPLETE SPECIFICATION
Electric Meter Circuits
We, WESTERNELECTRIC COMPANY, INCOR
PORATED, of 195, Broadway, New York City,
New York State, United States of America, a Corporation of the State
of New York,
United States of America, do hereby declare the invention, for which
we pray that a patent may be granted to us, and the method by which it
is to be performed, to be particularly described in and by the
following statement :
This invention relates to the art of electrical measurements and more
particularly to meter circuits providing a meter with an electrically
suppressed zero.
Prior to this invention, zero suppression was obtained in various
ways, all of which had certain disadvantages. Bridge circuits
containing non-linear resistance arms are known. These bridges are
balanced at a predetermined impressed voltage and give an indication
of variations from the predetermined voltage. This effectively
provides the indicating meter with an electrically suppressed zero.
Examples of this type of circuit are disclosed in the United States
Patent of C. S. Bradley No. 280,563 granted 3rd
July, 1883 and in British Patent Specification 2,630 of 1886. The
principal objections to apparatus of this character are their large
size, their cost and their inconvenient cumbersomeness.
Measurements with circuits providing electrically suppressed zeros are
also made by applying an opposing voltage, thereby balancing out the
major part of the voltage being measured and employing a sensitive
voltage indicator for indicating the difference. The meter may either
be calibrated in impressed volts or in the difference volts. In either
case, the stability of the reference source used for opposing the

2. principal part of the voltage to be measured enters into the accuracy.
These reference voltages are generally quite unstable and require
frequent adjustment and calibration. Moreover, this arrangement is
also objectionable from the standpoint of weight, cost and bulk.
A third method consists of backwinding the spring of an otherwise
conventional meter so as to bias the needle against its initial stop.
The impressed voltage is raised to some predetermined limit before
deflection of the meter begins, thereby supplying a suppressed zero by
mechanical means. This arrangement, while satisfactory for some
purposes, is objectionable for at least three reasons. First, the
current through the meter is frequently substantially larger than the
normal rated current for the meter.
Second, mechanical instability arises by reason of the increased
bearing friction caused by backwinding the spring. Backwinding also
causes adjacent convolutions of the spring to come into contact,
thereby adding further mechanical instability. Third, upon suddenly
dropping the current to zero, the impact of the pointer against the
stop sometimes causes the pointer to bend. It is also known to connect
two sets of metal rectifiers arranged in parallel but with opposite
polarity, in series with an a.c. indicating instrument, the rectifiers
having an ohmic resistance which rapidly falls with increasing
current, so that the instrument with which it is connected has a
non-linear response.
This invention provides a suppressed zero meter circuit comprising a
galvanometer, having in series therewith a non-linear conductive
device having a substantially constant relatively low dynamic
resistance over a major part of the range of the meter and a very high
resistance for the initial part of the range, whereby energy falling
within the initial part of the range produces a very small response in
the meter whilst energy falling in the major part of the range
produces a nearly linear response. The nonlinear device may be in the
form of a diode of the dry type and is connected in' series with the
galvanometer. For cuiTents-,below a predetermined limit, for example,
corresponding to the avalanche breakdown point of the non-linear
device, very little deflection of the meter takes place. Above this
point, the current-voltage characteristic of the nonlinear device is
substantially linear over the major part of the range of the
galvanometer.
Thus, relatively large voltages may be impressed on the meter circuit
to produce only very small meter deflections up to the avalanche point
Beyond this point, small changes in circuit voltage will cause
relatively large deflections of the indicating instrument which are
nearly linearly proportional to the voltage changes. The dry type
diode is of small size and is very stable, so that zero suppression

3. may be provided without substantially increasing either the cost or
the bulk of the instrument.
The invention also comprises a suppressed zero meter circuit for
alternating current comprising an alternating current meter having in
series therewith a pair of substantially identical dry diodes
connected with opposed polarity in series with the meter, each diode
having an avalanche breakdown point at a current small compared to
that required for a full scale deflection of the meter and having a
substantially constant dynamic resistance for currents greater than
the current at said point
The invention may be better understood by reference to the
accompanying drawings in which:
Fig. 1 discloses a voltmeter circuit in accordance with this invention
for measuring direct voltages:
Fig. 2 is similar to Fig. 1 but includes shunt and series resistors
for the voltmeter and a protector device to protect the voltmeter
against accidental damage from reverse voltages;
Fig. 3 discloses a slight modification of the invention wherein a
plurality of nonlinear devices are connected in series for use in
higher voltage circuits;
Figs. 4, 5 and 6 are characteristics of a preferred non-linear device
suitable for the practice of the invention;
Fig. 7 discloses a typical suppressed zero meter scale;
Fig. 8 discloses the invention as used for measuring current supplied
to the load of a direct-current circuit; and
Figs. 9 and 10 show the invention adapted for measuring alternating
voltages.
Referring now to Fig. 1 it will be noted that the circuit comprises
simply a galvano meter 1 connected in series with a non-linear
conductive device 2. This latter device must have at least two ranges
of widely different dynamic resistance the initial range being of very
high resistance relative to the other range or ranges. The other range
or ranges must have a substantially constant dynamic resistance
whereby the response of the meter will be substantially linear. The
initial range may also be of substantially constant dynamic
resistance. A device which is especially adapted for the practice of
the invention is a commercially available silicon diode which has been
found to have a nearly constant dynamic resistance over the major
portion of its current range. The reverse current characteristic is
preferred for this purpose and consequently the polarity of the device
2, as symbolically illustrated in Fig.
1, is such as to give the least conduction for voltages of the
polarity indicated at test terminals T1 and T2. As this voltage is
slowly increased from zero, substantially all of the applied voltage

4. appears as a voltage drop across the device 2. When the avalanche
breakdown point is reached, the device 2 begins to rapidly increase
its conduction. Up to this point the meter 1 has shown only a very
small deflection. After this point is reached, the meter is found to
deflect very rapidly for a small change of applied voltage. Moreover,
the deflection has been found to be substantially linear with respect
to changes in supplied voltage above that point. This linearity adds
greatly to the usefulness of the invention.
To further understand this invention, reference may be made to Figs.
4, 5 and 6.
The portion of the characteristic just mentioned is that part denoted
as the reverse characteristic shown in the third quadrant of
Fig. 4. Here it will be noted that voltage changes from zero to
slightly above five volts will produce a current change in the order
of only two or three milliamperes. In this range the dynamic
resistance is very high and approximates the ohmic resistance.
After this point is reached, it will be noted that the current
increases almost linearly with only small changes in supply voltage.
This is represented by the substantially linear portion between points
A and B on the reverse characteristic of Figs. 4 and 6 where the
dynamic resistance is very much lower.
Fig. 6 shows on an enlarged scale only that part of the reverse
characteristic of Fig. 4 between applied voltages of 5 to 5.6 volts.
Fig. 6 also shows the reverse characteristics for two other
temperatures. The curve just described is for a temperature of 8705C.
The other two curves are for 60-C. and --30"C..
as shown. It will be noted that all three curves are nearly parallel
over their linear ranges and that the temperature effect is not too
great even for wide temperature changes.
Compensation. where desired. may be effected by using copper wire for
part of a resistor 4, referred to below with reference to Fig. 3.
taking into account the effect of the copper conductor in the coil of
meter 1
Diodes suitable for use in the practice of this invention are quite
small in size and are easily mounted within the enclosure of
commercially available instruments. Thus, their use does not in any
way increase the bulk nor substantially increase the weight of the
instruments as they are presently available.
It will also be noted that the desired characteristic is obtained
without requiring the galvanometer coil to carry currents
substantially in excess of its normal rating.
The invention has been described in relation to the reverse
characteristic between the points A and B of Fig. 4. It is also
evident that, under certain conditions, the portion of the forward

5. characteristic between the points C and D in the first quadrant of
Fig.
4 may also be used. This would provide zero suppression in a circuit
requiring much larger currents but at lower voltages. It will be
noted, however, that this forward characteristic is by no means as
linear as is the reverse characteristic. Consequently, the reverse
characteristic is preferred under ordinary circumstances. Acomparison
of Figs.
5 and 6 will further show the better linearity obtainable from the
reverse characteristic, as compared with the forward characteristic.
In comparing these figures, as well as comparing the first and third
quadrants of Fig. 4, it must be kept in mind that the two
characteristics are drawn to different scales so that the reverse
characteristic tends to appear much less linear than it would if it
were drawn to the same scale as the forward characteristic.
In Fig. 2 a protector diode 3 has been added as well as a series
resistor 4 and a shunt resistor 5. The protector diode 3 is poled in
the opposite direction from the suppressor diode 2 so that, should the
supply voltage be accidentally reversed, the meter will not be damaged
by a flow of excess current. The diode 3 should have a reverse voltage
rating higher than the voltage to be applied to terminals T1 and T2.
Resistors 4 and 5 may be made adjustable in accordance with
conventional practice. As used in this invention, it is convenient to
adjust resistor 4 to give a minimum deflection of the meter at the
lower end of its scale. The shunt resistor 5 may thereafter be
adjusted to give a full scale deflection for the maximum voltage to be
measured.
In the event that the voltage to be measured is in excess of that
which may be withstood by a single diode or non-linear resistor 2,
additional elements such as diodes 21 and 22 may be added in series as
shown in
Fig. 3.
The kind of zero suppression that may be obtained by the use of the
invention is illustrated by the meter scale shown in Fig. 7.
In comparing the scale of Fig. 7 with the reverse characteristic shown
in Figs. 4 and 6, it should be noted that at zero current the pointer
rests at the normal meter zero point
O of Fig. 7, corresponding with the origin 0 of Fig. 4. As the voltage
to be measured is increased from zero to 45 volts, the voltage changes
across the diode element 2 by an amount corresponding to the distance
from the point 0 to point A on the characteristic curve of Fig. 4. The
relatively small current represented by the horizontal distance from
the vertical axis to point A of the curve causes a small meter
deflection from zero to point A corresponding to the 45-volt point of

6. Fig. 7. Thereafter, an increase in voltage causes only a relatively
small voltage drop across the diode or non-linearelement 2 but the
current increases rapidly and almost linearly with the applied voltage
as indicated by the substantially linear scale in Fig. 7. Thus, for
example, if the voltage is increased from 45 volts to 55 volts, the
current will increase from point A to point B of the characteristic in
Fig. 4, corresponding to points A and B of the scale in Fig. 7. It
must be understood that these curves and the voltmeter scale shown in
Fig. 7 are merely illustrative of those that may be obtained. The
percentage of the entire meter deflection that may be used is
determined very largely by the particular diode selected, the voltage
range which is to be measured and the meter selected. A great variety
of characteristics are readily obtainable. Diodes suitable for this
purpose are readily obtained covering reverse voltage ranges from 3
volts to about 300 volts.
The invention may be easily used for measuring currents by employing a
resistance shunt such as resistor 6 in Fig. 8. The meter circuit
comprising galvanometer 1 and suppressor element 2 may correspond to
those shown in Figs. 1 and 2. The voltage drop across shunt resistor
6, as is well known, is proportional to the current supplied to the
load. The total current may be measured by a less sensitive instrument
8 connected in series with the rheostat 7 and the power supply. The
galvanometer 1, although a more sensitive instrument, will require a
substantial deflection of meter 8 before the voltage drop across shunt
resistor 6 is large enough to cause the suppressor element 2 to
increase its conduction. Thereafter, galvanometer 1 will deflect to
accurately show the change in current supplied to the load and may be
calibrated to read the actual value of this current.
Figs. 9 and 10 show the invention applied to the measurement of an
alternating current.
Here the galvanometer 9 is adapted for measuring alternating currents.
Connected in series with this galvanometer are two oppositely poled
asymmetrical non-linear conductors 2 and 2A having substantially
identical characteristics. A symmetrical nonlinear conductor may be
used in their stead providing it has a characteristic similar to that
shown in Fig. 6. It will be understood, of course, that a symmetrical
conductor would have identical characteristics for cur rents
flowing-in either direction.
For the alternating-current circuits shown in Figs. 9 and 10, no
substantial current flows through the meter 9 so long as the
alternating voltage source is below a predetermined limit. This limit
is caused to match the avalanche breakdown point, for example, near
the point A shown in the characteristic of Figs. 4 and 6. For applied
voltage greater than this amount, a suppressor element begins

7. conduction thereby causing deflection of the meter 9. It will be
understood that for one phase of the alternating voltage, one of the
elements 2 or 2A will be conducting in its forward direction and will
therefore have an exceedingly small drop across its terminals. The
zero suppression characteristic for this phase is provided by the
companion element. Upon the reversal of the voltage phase the two
elements 2 and 2A reverse their functions. For example, for the
connection shown in Fig. 9, if the voltage of the upper terminal of
the source is increasing in the positive direction, element 2 would be
conducting in its forward direction while element 2A is operating on
its reverse characteristic and would permit substantially no current
to flow in the meter circuit until after the voltage has increased to
a point corresponding to point A of its reverse characteristic. Upon a
reversal of polarity during the next phase, element 2A would be
conducting in its forward direction while element 2 would act as the
suppressor operating on its reverse characteristic.
The invention has been described as applicable to both direct voltage
and direct current measurements and to the measurement of alternating
voltages. It is obvious that it may also be used as an alternating
current measuring device by employing a current shunt such as shunt 6
shown for the direct current circuit of Fig. 8. Moreover, the
invention is readily extended to either the voltage coil or current
coil of a wattmeter whereby a wattmeter may be given a suppressed zero
power characteristic. When applied to the voltage coil the connections
are of the type shown in Figs. 1, 2 or 9 and when applied to the
current coil of a wattmeter the circuit would be of the type shown in
Fig. 8.
It will be evident that this invention provides unique advantages
heretofore unattainable in the suppressed zero metering art. The
diodes used have been found to be highly stable and the results
readily reproducible.
They are commercially available with reverse voltage ratings ranging
from 3 volts to about
300 volts. Zero suppression may be applied to a meter with very small
additional cost.
The resulting structure is of very light weight
and may be self-contained within the meter case. Nloreover, the
instrument coil is not required to carry current in excess of its
normel rating.
What we claim is:
1. A suppressed zero meter circuit comprising a ga1anomc-tcr having in
series therewith a non-linear conductive device having a substantiall-
constant relatively low dynamic resistance over a major part of the
range of the meter and a very high resistance for the initial part of

8. the range, whereby energy falling within the said initial part of the
range produces a very small response in the meter whilst clergy
falling in the said major part of the range produces a nearly linear
response.
2. A circuit according to Claim 1 wherein the non-linear device has a
voltage current characteristic including two current ranges of wideIy
different substntially constant dyna- mic resistance, the lower
resistance portion extending over a maicr part of the range of the
meter.
3. A circuit according to Claim 1 or 2 wherein the nonlinear device is
an avalanche breakdown dry diode. the avalanche breakdown point being
at the start of the said substantially constant dynamic resistance
portion.
4. A circuit according to Claim 3 in which a second diode having a
reverse voltage rating higher than the voltage of the circuit in which
the meter circuit is to be connected is connected in series with the
meter so as to give protection against accidental reversals of
voltage.
5. A suppressed zero meter circuit for alternating current comprising
an alternating current meter having in series therewith a pair of
substantially identical dry diodes connected with opposed polarity in
series with the meter, each diode having an avalanche breakdown point
at a current small compared to that requited for a full scale
deflection of the meter and having a substantially constant dynamic
resistance for currents greater than the current at said point.
6. A meter circuit according to Claim 3 wherein a plurality of diodes
are arranged in series.
7. A suppressed zero meter circuit arranged and adapted to operate
substantially as described with reference to and as shown in Fig. 1 or
Fig. 2 or Fig. 3 or Fig. 8 or Fig. 9 or Fig. 10 of the accompanying
drawings.