US 3582718 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
O United States Patent 1111 3,5 2,713
 Inventor Gordon Barr Spellman 3,434,038 3/1969 Vette 323/4 [21 I A I N n gg FOREIGN PATENTS o. I221 f H8119 1,409,515 7/1965 France 3 1 7/14s.5  Patented June I, 1971 Primary Examiner-Lee T. I-Iix (73] Assignee Cutler-Hammer, Inc. Attorney-Hugh R. Rather Milwaukee, Wis.
 CIRCUIT FOR IMPROVING RELAY ABSTRACT: A transistor circuit for controllmg energization :giggf LIMITING of a DC RELAY COIL FORMS A LOW IMPEDANCE PATH THROUGH THE TRANSISTOR UPON INITIAL APPLICA-  U.S.Cl. 3l7/148.5, TION ()F VOLTAGE AND ALLOWS A FAST COIL C 323/4 RENT RISE THAT INCREASES THE OPERATING SPEED  Int. Cl. H0lh 47/32, o THE ELECTROMAGNETIC RELAY UNDER A G05f BROAD RANGE OF TEMPERATURE CONDITIONS  Field of Search 317/123 WITHOUT EXCEEDING COIL CURRENT LIMITATIONS 1485' 149; 323/4 9 As the current increases to a predetermined value, the [s6] Rderences Cited transistor control circuit is operated to provide a current limit action. The circuit is also designed so that it will continue to UNITED STATES PATENTS allow relay operation, although with an increased operating 3,1 14,872 12/1963 Allard 323/4 time, provided there is minimum required pickup voltage, in 3,235,775 2/1966 Winston 317/1485 the event any one of the components fails.
PATENTED m 1m. 3582.718
,. A i r1 fi BC R2.
fig. Z k
Y Y V Y V Inventor arden BarrSpcllman CIRCUIT FOR IMPROVING RELAY PERFORMANCE WITH CURRENT LIMITING BACKGROUND OF THE INVENTION Control circuits that allow operating current to flow but limit current increase caused by a short circuit or other malfunction in the load circuit have been known heretofore.
A common type of current limiter is a resistor in series connection with the output of the electric power source. Such resistor limits the current to a safe value if a short circuit occurs in the load circuit but has the disadvantage that it dissipates too much power at high operating current values.
To reduce the power dissipation, a transistor type current limiter circuit has been used. This type of current limiter for a resistance load is shown in FIG. 82-] of the Motorola Semiconductor Circuits Manual, Motorola, Inc., 1964. As shown therein, normal operating current flows through the transistor emitter-collector circuit to the load, but in the event of a short circuit at the load causing an increase in current, a diode connected across the emitter-base circuit clamps the voltage thereat to limit the current thereby to protect the transistor.
The present invention is an improvement thereon.
SUMMARY OF THE INVENTION This invention relates to an electronic circuit that improves relay performance by increasing its operating speed, limits the current in the event of a malfunction and is fail-safe.
An object of the invention is to provide an improved electromagnetic device operating circuit.
A more specific object of the invention is to provide improved means to increase the operating speed of an electromagnetic relay in a broad temperature range without exceeding coil current limitations in a fail-safe manner.
Other objects and advantages of the invention will hereinafter appear.
BRIEF DESCRIPTION OF THE DRAWING FIG. I shows a circuit diagram of a relay operating circuit constructed in accordance with the invention; and
FIG. 2 shows a circuit that may be substituted for a component in FIG. I to enhance its fail-safe character.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is shown a relay operating circuit constructed in accordance with the invention.
As shown therein, a switch SW is arranged to connect a relay coil RC and the collector and emitter of an NPN transistor T1 and a resistor R1 in series in that order from the positive terminal to the zero voltage or negative terminal of a direct current electric power source of 28 volts or the like. Resistor R1 is provided with a predetermined low value of resistance such as 3.6 ohms or the like since the load current flows therethrough.
A control circuit for the transistor extends from the junction between switch SW and the relay coil through a resistor R2 to the base of the transistor. This resistor is provided with a relatively higher value of resistance such as 900 ohms or the like to limit the base current of the transistor. As will be apparent, this resistor and resistor R1 allow base-to-emitter current to flow when the switch is closed to turn the transistor on all the way to saturation.
The transistor is provided with current limiting means that limits the transistor conduction in the event of a continuous increase in the load current. This means comprises a Zener diode ZD connected from the base of the transistor to the zero or negative side of the source, that is, across the base-emitter junction of the transistor and series resistor R1. This Zener diode is provided with a breakover voltage rating such as 3.3 volts or the like. Consequently, it normally blocks current flow but breaks over to allow current flow therethrough when the voltage thereon increases above a predetermined limit such as 3.3 volts. It will be apparent that increasing load current causes the sum of the voltage drops across the base-emitter junction and series resistor R1 to reach the Zener voltage. This causes breakover of diode ZD to shunt current from the base-emitter junction of the transistor to clamp it at that value and to limit the current in the collector-emitter circuit of the transistor.
The circuit of FIG. I is also provided with means for conducting some of the relay coil current in parallel with the transistor to reduce the current and resulting heat in the transistor. This means comprises a resistor R3 connected across the collector-emitter circuit of the transistor. This resistor also contributes to the fail-safe character of the circuit as hereinafter described. This resistor is provided with a value of 47 ohms or the like to allow energization of the relay in the event of transistor fails to operate.
For exemplary purposes, the relay is shown in FIG. 1 as having a normally open contact 1 and a normally closed contact 2. When relay coil RC is energized, contact I closes and contact 2 opens in the usual manner and vice versa when the coil is deenergized.
When switch SW is closed, current flows from the positive side of the source therethrough and through resistor R2, the base and emitter of the transistor and resistor R1 to the zero or negative side of the source. This current in the base'emitter junction turns the transistor full on. As a result, a low impedance path is formed through the collector-emitter circuit thereof and switch SW and resistor R1 to cause fast increase of the energizing current for correspondingly fast energization of relay coil RC. This causes fast operation of the relay contacts.
When the transistor collector-emitter current reaches a predetermined value above that required for the aforesaid fast operation of the relay, Zener diode ZD avalanches. This occurs when the base-emitter voltage and the resistor R1 voltage rise to the Zener voltage value. The Zener diode shunts current from the base-emitter junction which results in the transistor conduction being clamped so that it cannot increase above the value where the Zener diode breaks over. Thus, current limiter action has taken place.
The fail-safe character of the circuit of FIG. 1 may be enhanced by substituting the parallel resistors R4 and R5 of FIG. 2 for resistor R1. If resistors R4 and R5 are each given a resistance value twice that of resistor R1, the total resistance of the parallel circuit will be equal to the resistance of resistor R1. In this manner, the resistance of the circuit remains unchanged but if one resistor R4 open-circuits, the circuit can still be operated although the resistance value of this portion of the circuit has doubled. In the aforementioned example wherein resistor R1 has a value of 3.6 ohms, resistors R4 and R5 would each have a value of 7.2 ohms. Thus, although one resistor might fail, the other resistor maintains circuit continuity at 7.2 ohms. This would permit use of the circuit until the fault is corrected.
Whenever the circuit is in operation, some current flows through transistor TI and through resistor R3 in shunt of the transistor. This resistor dissipates some of the wattage that would otherwise heat the transistor. It also limits the collectorto-emitter voltage on rapid circuit tum-off. Moreover, it would allow operation of the circuit if transistor T1 fails and open-circuits.
Other fail-safe characteristics involve resistor R2 and Zener diode ZD. It will apparent that if resistor R2 open-circuits, the relay can still be energized. In this event, the transistor cannot be made to conduct but an increased operating time circuit extends through resistors R3 and R1 to allow operation of the relay. If the Zener diode fails and open-circuits, only the current limiting function has failed but the circuit can energize the relay.
The circuit above described has the advantages that it per mits reduction in designing in the areas of coil voltage tolerances and coil resistance change with temperature. Large wire size can be used in the coil to get faster operation without the usual increased heating involved in increasing the wire size.
The current limiting circuit allows applying, during the first few milliseconds of operation, an overvoltage to the coil that causes the contacts (of an existing design of relay) to close faster and with a much greater force, provided the iron is not completely saturated at nominal voltage. This increased force would allow increasing the initial contact pressures that in turn lead to greater rupture test capability.
The faster operation and high forces lead to lower bounce and longer relay life.
The applied temporary overvoltage allows operation of standard relays with increased magnetic gaps, increased wear allowance, and increased arc gaps. These result in increased DC voltage ratings and increased contact life.
While the circuit hereinbefore described is effectively adapted to fulfill the objects stated, it is to be understood that the invention is not intended to be confined to the particular preferred embodiment of electronic circuit for improved relay performance disclosed, inasmuch as it is susceptible of various modifications without departing from the invention.
1. An electronic system for improving relay performance comprising:
an electric power source;
a transistor having emitter and collector electrodes forming a main current conduction path and a base electrode forming with said emitter electrode a control current conduction path;
a sensing device for sensing the relay coil current;
means connecting the relay coil in a circuit for energization from said source under the control of said main current conduction path so that the coil current energizes said sensing means, the impedance of said circuit being low to afford fast operation of the relay whenever said circuit is rendered operative;
control means comprising means for providing base current to control turn-on of said transistor and breakover means operable when the voltage drop across said sensing means and said control current conduction path reaches a predetermined value for limiting the turn-on of said transistor to a predetermined value of coil current;
and shunt impedance means connected in parallel to said main current conduction path for conducting current in response to the induced voltage of the relay coil when said circuit is opened.
2. The invention defined in claim 1, wherein said sensing means comprises:
a resistor of low resistance value connected to the emitter side of said transistor in said coil energizing circuit.
3. The invention defined in claim 1, wherein said means for providing base current to control turn-on of said transistor comprises:
a resistor of relatively high resistance value;
and means connecting sand resistor between said source and said base electrode.
4. The invention defined in claim 1, wherein aid means connecting the relay coil in a circuit for energization from said source under the control of said main current conduction path so that the coil current energizes said sensing means comprises:
a series circuit connected to said source and including the relay coil and the emitter and collector electrodes of said transistor and said sensing device with the relay coil connected to the collector electrode and the sensing device connected to the emitter electrode.
5. The invention defined in claim 4, wherein said breakover means comprises:
a Zener diode connected across the base and emitter electrodes and said sensing device to respond to the sum of the voltage drops thereacross.
6. The invention defined in claim 4, wherein said means for providing base current to control turn-on of said transistor comprises:
a resistor connected across the relay coil and the collector to base electrodes of said transistor.
7. The invention defined in claim 6, wherein said shunt impedance means comprises:
a resistor connected from the collector to the emitter electrode of said transistor to conduct current in parallel with said main current conduction path when the transistor is conducting and also when it is turned off 8. The invention defined in claim 7, wherein said electric power source comprises:
a direct current source having its positive voltage side connected to the relay coil and its other side connected to said sensing device.
9. The invention defined in claim 1, wherein said transistor is an NPN conductivity type transistor.
10 The invention defined in claim 1, wherein said sensing device comprises:
two resistors connected in parallel for fail-safe purposes to allow operating current to flow in the event one of these resistors open-circuits.