U.S. patent number 3,697,774 [Application Number 05/173,571] was granted by the patent office on 1972-10-10 for thyristor circuits for applying a voltage to a load.
This patent grant is currently assigned to Grigsby-Barton, Inc.. Invention is credited to Joseph E. Pascente.
United States Patent |
3,697,774 |
Pascente |
October 10, 1972 |
THYRISTOR CIRCUITS FOR APPLYING A VOLTAGE TO A LOAD
Abstract
Thyristor circuits are disclosed for applying a voltage to a
load wherein substantial improvements are provided with respect to
increasing the operating life of switch contacts employed therein.
A reduction in the amount of radio frequency interference generated
in such circuits upon circuit switching may also be obtained. Such
improvements are provided by the appropriate use of one or more
inductors in the circuits, which may be employed together with
certain other circuit components.
Inventors: |
Pascente; Joseph E. (Norridge,
IL) |
Assignee: |
Grigsby-Barton, Inc. (Rolling
Meadows, IL)
|
Family
ID: |
22632623 |
Appl.
No.: |
05/173,571 |
Filed: |
August 20, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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108269 |
Jan 21, 1971 |
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Current U.S.
Class: |
361/13;
361/10 |
Current CPC
Class: |
H01H
9/547 (20130101) |
Current International
Class: |
H01H
9/54 (20060101); H01h 009/30 () |
Field of
Search: |
;307/136,252B,305
;317/11R ;200/144R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schaefer; Robert K.
Assistant Examiner: Smith; William J.
Parent Case Text
The present invention relates generally to improved circuits for
applying an electrical voltage to a load, and more particularly to
such circuits employing a thyristor for performing switching or
phase control functions. This application is a division of Ser. No.
108,269, filed Jan. 21, 1971, assigned to the assignee hereof, and
now abandoned.
Claims
What is claimed is:
1. A circuit for applying a voltage to a load, comprising a
thyristor having a control terminal and a pair of load terminals,
first circuit means for coupling a voltage source to a load through
said pair of thyristor load terminals, and second circuit means for
coupling said thyristor control terminal to one of said thyristor
load terminals to provide a triggering signal for said thyristor
derived from said voltage source, said second circuit means
including contact switching means and an inductor connected in
conductive series circuit relation between said control terminal
and said one thyristor load terminal for preventing arc discharge
in said contact switching means upon contact closure.
2. The circuit of claim 1 wherein said contact switching means
comprises a pair of mechanical contacts normally having an arc or
glow discharge occurrence therebetween upon closure, and said
inductor being of sufficiently high Q to prevent the occurrence of
said discharge.
3. The circuit of claim 2 wherein said contact switching means
comprises a reed switch having said pair of mechanical
contacts.
4. The circuit of claim 1 wherein said contact switching means
comprises a pair of mechanical contacts and said second circuit
means further comprises a series connected resistor and capacitor
in parallel circuit relation to said contacts.
5. The circuit of claim 1 wherein a series connected capacitor and
inductor combination is connected in parallel circuit relation to
said thyristor load terminals.
6. The circuit of claim 4 wherein said contact switching means
comprises a reed switch having said pair of mechanical
contacts.
7. A circuit for applying a voltage to a load, comprising a
thyristor having a control terminal and a pair of load terminals,
means for applying the voltage source to the load through said pair
of thyristor load terminals, a capacitor circuit coupled across
said pair of thyristor load terminals, and means for coupling said
thyristor control terminal to one of said thyristor load terminals
through mechanical contact switching means conductively series
connected to an inductor to provide a triggering signal for said
thyristor derived from said voltage source, said inductor
preventing high frequency are discharge in said contact switching
means upon contact closure.
8. The circuit of claim 7 wherein said switching means comprises a
hermetically sealed reed switch normally producing a glow discharge
therein due to high frequency current components flowing
therethrough, and said inductor being such as to inhibit said
current components to prevent the occurrence of said glow discharge
in said switch.
9. The circuit of claim 7 wherein said switching means comprises a
hermetically sealed reed switch normally producing a glow discharge
therein due to high frequency current components flowing
therethrough, and a further capacitor circuit coupled across said
reed switch for causing said current components to by-bass said
switch to prevent the occurrence of said glow discharge
therein.
10. The circuit of claim 7 further comprising an additional
inductor connected in series with a capacitor in said capacitor
circuit, and the series combination being coupled across said pair
of thyristor load terminals.
11. In a thyristor circuit for applying a voltage to a load,
comprising a thyristor having a control terminal and a pair of load
terminals, commutation circuit means including a capacitor
connected across said thyristor load terminals, load circuit means
coupled to said thyristor load terminals for responsively switching
a voltage source to the load in accordance with the state of said
thyristor, gate circuit means for selectively coupling said
thyristor control terminal to one of said thyristor load terminals
through switch contacts to provide a triggering signal derived from
said voltage source to effect the state of said thyristor, the
improvement wherein said gate circuit means comprises an inductor
conductively connected in series circuit relation to said switch
contacts between said control terminal and said one thyristor load
terminal so that the signal derived from said voltage source is
conducted to said control terminal through said inductor upon
closure of said switch contacts, and said inductor prevents high
frequency arc discharge between said contacts upon closure.
12. In the thyristor circuit of claim 11, said inductor having a
sufficiently high Q to prevent glow discharge between said switch
contacts upon closure.
13. In the thyristor circuit of claim 11, said gate circuit means
further comprising a series connected resistor and capacitor
combination connected in parallel circuit relation to said switch
contacts to prevent glow discharge between said contacts upon
closure.
14. In the thyristor circuit of claim 11, said commutation circuit
means including an inductor connected in series with said
capacitor.
15. A circuit for applying a voltage to a load, comprising a triac
having a gate, an anode and a cathode, a commutation circuit
including a capacitor connected across the anode and cathode of
said triac, means for coupling the anode and cathode in circuit
relation to the load to apply a voltage source thereto in
accordance with the state of said triac, and a series circuit
combination of an inductor and a reed switch conductively connected
between the anode and the gate of said triac to provide a
triggering signal thereto.
16. The circuit of claim 15 wherein said inductor has a
sufficiently high Q to prevent glow discharge between the switch
contacts of said reed switch upon closure.
17. The circuit of claim 15 further comprising a series connected
resistor and capacitor combination connected in parallel circuit
relation to said reed switch to prevent glow discharge between the
contacts thereof upon closure.
18. The circuit of claim 15 wherein said commutation circuit
includes an inductor connected in series with said capacitor.
19. A circuit for applying line voltage to a load, comprising a
thyristor having a pair of load circuit terminals and a gate
terminal; a load circuit coupled to said load circuit terminals for
applying the line voltage to the load in response to the thyristor
being in a conductive state; and a gate circuit including switch
contacts and an inductor connected in conductive series circuit
relation between said gate and the line voltage for supplying, from
the line, a gate driving current through said inductor to effect
said conductive state of the thyristor, said inductor preventing
high frequency arc discharge between said contacts upon
closure.
20. A circuit for applying a voltage source to a load, comprising a
thyristor having an anode, a cathode, and gate terminal; first
circuit means coupled to said anode and cathode for applying said
source to the load in response to the thyristor being in a
conductive state; and second circuit means including switch
contacts and an inductor for selectively applying a gate drive
signal derived from said source to said gate terminal through said
inductor to effect said conductive state of the thyristor, said
inductor having a sufficiently high Q to prevent glow discharge
between said contacts upon actuation.
21. A circuit for applying a voltage to a load, comprising a
thyristor having a control terminal and a pair of load terminals,
first circuit means coupled to said load terminals for applying a
voltage source to the load in response to the thyristor being in a
conductive state; second circuit means including switch contacts
and an inductor connected in conductive series circuit relation
between said control terminal and one of said load terminals; and
means storing a charge in response to said voltage source and
discharging through said second circuit means upon closure of said
switch contacts, said inductor preventing high frequency arc
discharge between said contacts upon closure.
Description
The present embodiment of the invention relates to such circuits
utilizing a switch having mechanical contacts, such as a
hermetically sealed reed switch, as a control element for
selectively switching or applying a switching or triggering signal
to the control or gate terminal of a thyristor.
The use of hermetically sealed reed switches as control elements
for thyristors, such as silicon controlled rectifiers (SCR's) and
triacs, has become conventional in many types of circuits. Such
reed switches have generally been considered as offering many
advantages over other forms of control devices, and indeed they do,
since, in addition to their small size, they can typically be
actuated by passing A.C. or D.C. through a small winding
thereabout, or by the placement of a small magnet in proximity
thereto. While, at the same time, they can maintain complete
electrical isolation between the control signal input and the
switched power output of the circuit. Moreover, it has also been
generally considered that a long operating life was assured the
thyristor-reed switch combination by the minimal volt-ampere
switching load placed on the reed switch by the SCR or triac
triggering requirements, which are well within the normal ratings
of conventional reed switch designs. This is discussed, for
example, in the General Electric SCR Manual, 4th Edition, 1967, pp.
143-144, published by the Semiconductor Products Department of
General Electric Company, Syracuse, New York.
However, it has been found that although the rate or percentage of
failures of such reed switch circuits is generally low, it is still
high enough to be significant in many situations. Also, the
operating life of reed switches in such circuits appears to be
substantially less than would normally be expected from a
consideration of the circuit parameters. This is especially a
problem where a large number of such circuits are employed in a
single, complex apparatus, thus tending to reduce significantly its
overall reliability.
Such switch failures have heretofore been generally attributed to
merely defective reed switch constructions, since the applied
voltages and loads normally employed with such thyristor-reed
switch circuits would not be expected to cause gate currents which
could exceed the switch contact ratings. However, the measurement
of the thyristor gate current in such circuits and the observation
of the reed switch upon switch closure has lead to my recognition
and belief that the switch failures have not generally been due to
latent switch defects, but to contact destruction from burning,
pitting, sputtering or other forms of contact degradation.
Furthermore, I believe that such contact degradation is due to
arcing within the reed switch prior to complete switch closure
caused by extremely high frequency gate currents (i.e., in the
gigahertz range) which may be generated in the thyristor gate
circuit. Such high frequency currents as I have detected may be
generated by the negative resistance effects produced by the
electrical discharge through the gas within the hermetically sealed
reed switch, and the initial high current surge for the discharge
appears to be produced in the gate circuit upon or during switch
closure by the abrupt discharging of the capacitor circuit
typically connected across the thyrister load terminals for
commutation purposes, as well as of the inherent junction
capacitance of the thyristor. In A.C. circuits I believe this
action occurs to the greatest extent when by chance the reed switch
is actuated for closure at a moment in time when the line voltage
to the thyristor circuit is at or near its peak, so that the
commutation capacitor for reducing the dv/dt across the thyristor,
and the thyristor itself, have a relatively large charge. However,
it also occurs during the other portions of the line voltage cycle,
and the degree of arcing appears to be proportional to the
instantaneous amplitude of the line voltage at the moment of
switching.
More particularly, the damaging and sputtering of the reed switch
contacts caused by this arcing phenomenon results in substantially
decreasing the useful operating life of the reed switch, and
consequently of the circuitry of the device or apparatus with which
it might be employed. Also, where the switch contacts are perhaps
close to their extremes in operating tolerances, or where their
operation may be otherwise marginal, this phenomenon may cause the
switch to fail in a relatively short operating time after its
initial operation. For A.C. operation, the shortening of the useful
contact life may be related to the probability of switch closure
during portions of the voltage cycle when its magnitude is
sufficiently high to cause the greatest discharge and arcing
intensity.
Additionally, from my observation of the high frequency components
of current which appear to be present in the thyristor gate circuit
upon switching, I believe that the gate circuit of thyristor
switching and phase control circuits, generally provide a
substantial contribution to the total radio frequency interference
(RFI) that such circuits typically produce. This contribution to
RFI would then be present in conventional thyristor switching and
phase control circuits employing, for example, a triac having a
switch connected in its gate circuit for triggering the triac in
response to a given signal.
Accordingly, it is one object of the present invention to improve
the operating life of a mechanical switch, such as a conventional
hermetically sealed reed switch, in thyristor circuits used for
applying a voltage to a load.
It is another object of the invention to provide improved thyristor
circuits producing a minimum of radio frequency interference.
It is a further object of the invention to provide an improved
circuit for applying a voltage to a load impedance, which circuit
employs a thyristor-reed switch combination wherein the reed switch
has a substantially greater operating life than has heretofore been
the case, thus significantly increasing the reliability of the
circuit.
These and other objects of the invention are more particularly set
forth in the following detailed description and the accompanying
drawings of which:
FIG. 1 is a schematic diagram showing a thyristor switching circuit
in accordance with one embodiment of the present invention
including certain optional components; and
FIG. 2 is a graphical representation showing voltage and current as
a function of time at certain points in the circuit of FIG. 1.
In general, referring to FIG. 1, there is shown a circuit for
applying an A.C. voltage V.sub.L across terminals 10 and 12 to a
load impedance 14 through a thyristor circuit 16. The thyristor
circuit 16 is a selectively controlled switching circuit and
comprises a thyristor, illustrated as triac 18, having a control or
gate terminal 20 and a pair of load terminals 22 and 24, which may
be referred to hereinafter as the anode and cathode, respectively.
First or load circuit means, illustrated as conductive connections
26 and 28, couples the voltage source V.sub.L to the load 14
through the pair of triac load terminals 22 and 24. Second or gate
circuit means, designated generally as 30, couples the triac gate
terminal 20 to one of the triac load terminals, such as anode 22 as
shown, to provide triggering signals for the triac which render the
same conductive in a manner well known to the art, applying the
voltage V.sub.L to the load 14. To limit the dv/dt for commutation
purposes, and to enable the circuit to operate with inductive
loads, a shunt capacitor 32 is connected across the triac 18 as is
also known; but in the present circuit, for reasons to be
hereinafter explained, the capacitor is connected in series with an
inductor 34 rather than a resistor as is conventional in such
circuits.
The second or gate circuit means 30 includes switching means,
illustrated as a reed relay 35, connected in series circuit
relation to the triac gate terminal 20 and the triac anode 22, and
an inductor 36 is also connected in series circuit relation to the
switching means 35 in the gate circuit. The gate circuit 30 further
comprises a series connected combination of a resistor 37 and a
capacitor 38 in parallel circuit relation to the reed relay 35. The
action of the series inductor and the shunt RC network
substantially increases the life of the reed switch by eliminating
the chance of contact sputtering or degradation by high frequency
current components in the initial discharge of capacitor 32 and the
junction capacitance of triac 18. Further, this same increase in
switch life may alternatively be achieved without the use of the
shunt RC network 37,38 by employing an inductor 36 with a
sufficiently high Q, as will be discussed in greater detail
hereinafter.
More particularly, the reed relay 35 is of a conventional type
comprising a hermetically sealed reed switch 40 and a coil 42 which
is normally wound about the reed switch 40 and terminates in
terminal pair 44 to which a suitably switched energizing control
voltage V.sub.C is applied. Application of the appropriate voltage
V.sub.C to relay terminals 44 energizes the coil 42 which generates
a magnetic field which, in turn, causes the contacts of the reed
switch 40 to close. The reed switch 40 conventionally comprises two
conductive reeds 46 and 48 of magnetic material having respective
electrical contacts 50 and 52 disposed in mutually facing relation
at the overlapping adjacent ends thereof. The contacts 50 and 52
are typically composed of gold with rhodium, silver or tungsten
cladding or plating, or of gold on some other base metal. The
opposite ends of the respective reeds 46 and 48 are supported in
and extend through the opposing ends of an elongated glass housing
54 which hermetically seals the reed switch and maintains a
predetermined environment therewithin. Such conventional reed
switches typically contain a gaseous environment consisting
essentially of nitrogen at pressures ranging from 1 to 6
atmospheres. Also, such switches may sometimes have an evacuated
environment so that the space within the glass housing is at a
substantially reduced or very low pressure relative to one
atmosphere.
In conventional thyristor switching circuits of the general type on
which the circuit of FIG. 1 is an improvement, the elements 34 and
36 would typically be resistors rather than inductors and the RC
circuit 37, 38 would be omitted. When such conventional thyristor
circuits are triggered by closure of the reed switch, especially at
or near the peak or maximum line voltage, I have observed white
arcing or lightning-like flashes, containing possibly a trace of
red, within the glass reed switch enclosure 54. This arcing or
flashing appears to occur after the reed relay is energized, but
before the contacts 50 and 52 of the reed switch are completely
closed. Such arcing results in substantial degradation or
deterioration of the reed switch contacts, such as from burning and
pitting, and accounts for otherwise unexpected failures of such
circuits in view of the generally random relation between the
energization of the reed relay and the instantaneous valve of the
line voltage.
With the addition of the inductance provided by the inductor 36 to
the conventional aforementioned thyristor circuit, the arcing or
flashing previously observed in the reed switch is eliminated, but
is replaced by a blue glow discharge. While it is believed that the
contact degradation from the glow discharge will be less than that
caused by the arcing, still the sputtering off of the plating or
cladding of the switch contacts may well occur to some extent
within such an ionic glow discharge. Thus, the presence of the
inductor 36 in the gate circuit of the triac 18 will increase the
useful operating life of the reed switch 35, and consequently of
the thyristor circuit, since it causes the highly damaging arcing
phenomenon to be replaced by a less damaging glow discharge
phenomenon upon energization of the reed relay.
Now, with the further addition of the series resistance-capacitance
circuit 37, 38 in shunt with the reed switch 40, the glow discharge
is eliminated, even upon reed switch actuation at the peak line
voltage. Consequently, no sputtering or contact degradation is
believed to take place with these additional components, and the
useful operating life of the reed switch is now merely a function
of its mechanical construction.
As a further improvement, I have also found that the resistor 37
and capacitor 38 can be eliminated, while still preventing the glow
discharge in the reed switch 40, by selecting a coil for inductor
36 with appropriate frequency and Q characteristics. For example, a
10 mh coil (70F 102Al of J.W. Miller Company, Compton, California)
having a minimum Q of 41 at 250 kHz., a resonant frequency not less
than 0.52 MHz., and a D.C. resistance of about 101 ohms, performed
satisfactory in the gate circuit of a triac switching circuit
controlled by a conventional reed switch operating at the peaks of
100 volts r.m.s. line voltage. No arcing or glow discharge was
observed, even without any RC circuit across the switch. For
relatively low Q coils, however, the RC circuit as illustrated will
generally be required. Thus, if desired, the Q and frequency
characteristics of the coil to eliminate the RC network across the
switch may be empirically determined for any specific circuit
construction by trying coils of increasing Q until a sufficiently
high Q is obtained, when the glow discharge within the reed switch
upon closure is eliminated. The coil may thus function to choke or
damp the high frequency current components which appear to be
produced in the gate circuit, and to eliminate the cause of the
ionic glow discharge between the reed switch contacts during
closure.
I have further found that the presence of the inductor 36 in the
thyristor gate circuit substantially reduces or eliminates the
radio frequency interference (RFI) which appears to be typically
produced in the gate circuit of the triac. While the generation of
RFI in such circuits has been commonly attributed to the voltage
and current switching of the load (i.e., to the triac load
circuit), it has been my observation that a significant, if not
principal, contribution to the total RFI of the circuit is caused
by the gate circuit of the triac. And it is this component or
contribution which is now eliminated by the inductor 36 serially
connected with the reed switch 40 between the triac anode 22 and
the gate 20.
More specifically, without the inductor 36 or the RC circuit 37, 38
in the thyristor switching circuit of FIG. 1, and with a 100 ohm
current limiting resistor substituted for the inductor 36, a triac
gate current pulse at 20 is produced on reed switch closure at peak
line voltage (110 v.r.m.s.) which has a magnitude of approximately
1.0 ampere and a duration of 1.5 microseconds. And this occurs
regardless of the particular load 14, i.e., whether the load is of
relatively low resistance, such as is provided by a 15 amp. lamp,
or a high resistance, such as a 10 K ohm resistor. I noted that
when the reed switch 40 was pressurized, such as to 6 atmospheres,
the gate current in-rush was somewhat higher than when the reed
switch was of the so-called back-filled type having one atmosphere
pressure. However, in either case the addition of inductor 36
having a resistance equal to 100 ohms and an inductance of 10 mh in
series with the reed switch 40 reduces the initial peak current
surge to approximately 50 percent of that occurring without the
inductor and, further, reduces the duration of the pulse to
approximately 0.1 microsecond.
Referring, more particularly, to FIG. 2, there is graphically shown
in FIG. 2(a) a portion of the voltage V appearing across the triac
18 in the circuit of FIG. 1, but with resistors for the inductors
36 and 34 and without the RC circuit 37, 38. The load 14 is assumed
to be resistive. Also, it is assumed that the reed relay 35 is
energized by voltage V.sub.C so that the reed switch 40 closes when
the voltage V is at or near its maximum at the time t.sub.1. At the
same time, the gate current to the triac 18 is shown in FIG. 2(b)
where the gate current is shown to rise steeply at time t.sub. 1.
The voltage V across the triac 18 falls off rather sharply to a
minimum at time t.sub.2 which occurs approximately 0.1 microseconds
from t.sub. 1. During this 0.1 microsecond interval the gate
current rises to a peak of about 0.9 amperes. The gate current then
decreases gradually until time t.sub.3 approximately 1.5
microseconds after t.sub. 1, at which time the gate current drops
off rapidly to approximately zero. During the time interval between
t.sub.2 and t.sub.3 the voltage V across the triac 18 increases
gradually to about 2 volts and then decreases again to the normal 1
volt drop across the triac when it is in its conductive state.
FIG. 2(c) illustrates the gate current at the triac gate 20 when
the reed relay 35 is replaced by a semiconductor switch, such as
silicon controlled rectifier in a full-wave bridge circuit. As can
be seen, for turn-on at time t.sub.1 the gate current again rises
sharply to a value of 0.9 amperes and then decreases rapidly after
a duration of 0.1 microseconds. However, during the period t.sub.3
- t.sub.2 the gate current goes to approximately zero. Thus, it can
be seen that the switching of the triac 18 by means of the reed
switch 35 causes a different circuit operation than if the triac
were switched by some other means. The switching of the reed
switch, with its mechanical contacts, and the arcing phenomenon
occurring during the switch closure appears to be the cause of the
extended current pulse and its particular shape as shown in FIG.
2(b), which may be the result of the gas discharge characteristic
of the reed switch.
The graphs of FIGS. 2(a) and 2(b) correspond to circuits having
reed switches as described with environmental pressures of 6
atmospheres. For switches of the one atmosphere type, the wave
forms are similar, but the gate current pulses appear to diminish
in amplitude with the operating life of the switch. The wave forms
resulting from the so-called evacuated or "vacuum" reed switches
are essentially the same as those for the one atmosphere switches,
since they are probably not completely evacuated.
The addition to the circuit of the series resistance and capacitor
combination 37, 38 in shunt with the reed switch 40 appears to act
together with the inductor 36 to further prevent an initial high
current surge on closure of the reed switch contacts 50 and 52 and
may act as a by-pass filter for the high frequency current
components which I believe are generated in the gate circuit during
switch closure. It may be noted that although RC networks are
conventionally employed in some circuits across switch contacts to
prevent arcing on opening of the contacts (especially where power
is being removed from an inductive load), in the present circuit
the RC network is provided for the prevention of current surges on
closure of the contacts.
Although the inductor 36 may be employed in the triac gate circuit
without the optional RC circuit 37, 38 to obtain partial or full
advantages as hereinbefore discussed, the provision of the RC
circuit 37, 38 without the inductor 36 appears to have no effect or
improvement on the conventional circuit operation. Also, I have
noted that a 0.2 ampere high frequency noise or "hash" may still
appear in some cases on the leading and trailing edges of the gate
current pulses. However, most of this hash can be eliminated by
connecting a resistor in parallel with the inductor. In the
illustrated circuit, a 3,300 ohm resistor worked
satisfactorily.
The inductor 34 in the commutation circuit optionally replaces the
100 ohm resistor typically used in series with a 0.1 mfd.
capacitor, such as 32, in such circuits to further damp and impede
the high frequency components of the initial current discharged
from the capacitor 32 upon closure of the reed switch contacts 50,
52. Since the charge which is stored in the commutation capacitor
32 may significantly contribute to the initial current in-rush when
the reed switch is closed, the delaying action of the inductor 34
aids the action of the inductor 36, as a secondary measure,
assuring that no arcing will take place within the reed switch. The
use, if desired, of inductor 34, which may be of the same value as
the gate circuit inductor 36, in an RLC commutation circuit as
shown, also provides an additional advantage of improved
commutation or shut-off as compared with the typical RC commutation
circuit conventionally employed.
With respect to component values for the circuit of FIG. 1, the
inductor 36 preferably has an inductance of from 1 to 100 mh and a
resistance of approximately 100 ohms. The resistance of the
inductor 36 may be selected as in conventional designs to be just
greater than the peak supply voltage divided by the peak gate
current rating of the thyristor device, being typically 100 ohms.
The values of the resistor and capacitor in the RC circuit 37, 38
are preferably determined by selecting the value of resistor 37 to
minimize the fuse function i.sup.2 t to provide, for example, a 50
milliamp gate current for one microsecond. Thus, the capacitor 38
may preferably have a value of 0.001 mfd. and the resistor 37 a
resistance value of 100 ohms or 10 K ohms. I have found that
excellent results are obtained with either of the two alternate
values of resistance. However, in any particular circuit design,
the optimum component values may be determined empirically by
observing the gate current on an oscilloscope while switching the
gate current circuit on line voltage peaks.
Although the principles of the present invention were tried and
found to be applicable to the following reed switches employed for
testing, they are believed to be generally applicable to all reed
switches and their associated thyristor circuitry for applying a
voltage to a load impedance:
Hamlin, Inc., Lake Mills, Wis. MRR-5 6 atmos. MRO-2 evacuated MRR-2
1 atmos. Gordos Corp., Bloomfield, N.J. MR 138 6 atmos. MRO 88 1
atmos C.P. Clare, Chicago, Ill. pico reed 1 atmos. General
Electric, Owensboro, Ky. DR 123 1 atmos.
The principles of the present invention relative to improving the
operating life of reed switches are also believed to be applicable
to mechanical contact switches generally, such as, for example,
conventional relay or switch contacts disposed in the ambient air
environment.
Although several specific embodiments of the present invention have
been illustrated and described, various modifications and
applications thereof will be apparent to those skilled in the art;
accordingly, the present invention should be defined only by the
appended claims and equivalents thereof.
Various features of the invention are set forth in the following
claims.
* * * * *