Electricity: electrical systems and devices – Safety and protection of systems and devices – Arc suppression at switching point
Reexamination Certificate
2001-07-26
2003-12-30
Han, Jessica (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Arc suppression at switching point
C361S013000
Reexamination Certificate
active
06671142
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to relay contact protective circuits and, in particular, to fail-safe arc suppression control circuits that can protect relay contacts.
B. Description of the Related Art
A relay is used to perform automatic electrical switching. A relay typically comprises two parts: a coil and a magnetic switch. When an electrical current flows through the coil, a magnetic field is created in proportion to the current flow through the coil. At a predetermined point, the magnetic field is sufficiently strong to pull the switch's movable contact from its rest, or de-energized position, to its actuated, or energized position pressed against the switch's stationary contact. When the electrical power applied to the coil drops, the strength of the magnetic field drops, releasing the movable contact and allowing it to return to its original de-energized position. As the contacts of a relay are opened or closed, there is an electrical discharge called arcing, which may cause heating and burning of the contacts and typically results in degradation and eventual destruction of the contacts over time.
U.S. Pat. No. 3,912,941 to Passarella and U.S. Pat. No. 4,250,531 to Ahrens both teach an arc prevention circuit for relays that switch inductive loads. These circuits have the relay contacts drive a transistor amplifier which drives the load, thereby eliminating contact arcing induced by inductive voltage transients. But contact arcing can still occur. Also, the transistor amplifier, which is not a perfect switch, must be designed to dissipate heat, and it is heated the entire time the load is supplied with current. Thus, a large power transistor with elaborate heat sinking may have to be provided. Except in low power, low voltage situations, this is not a satisfactory solution.
U.S. Pat. No. 3,075,124 teaches inserting a semiconductor device in series with the relay contacts to suppress arcing, but such an arrangement also introduces the resistance of the semiconductor device into the load current flow path, causing excessive heating of the semiconductor device. This arrangement also cannot suppress all arcing, and it is dependent upon some minimal arcing to place its arc suppression mechanism into operation.
U.S. Pat. No. 3,184,619 to Zydney and U.S. Pat. No. 4,438,472 to Woodworth both teach a suppression circuit that momentarily shorts the relay contact with a transistor switch. But because the switch is not actuated before the contacts open, there is still the possibility of contact arcing with this arrangement.
U.S. Pat. No. 4,745,511 by Kugelman shorts the contacts momentarily with a MOSFET from the moment when the relay coil is first actuated until sometime after the contacts have opened, but this arrangement requires extra power supply connections and thus cannot be built into a relay without the addition of extra power supply pins to the relay housing. This arrangement also suppresses contact closure arcing, but that can be helpful to keep the contacts clean and operative.
U.S. Pat. No. 4,959,746 by Hongel teaches using an electronic MOSFET shunting switch triggered by actuation and deactuation of the relay coil to relieve both contact closure and contact opening arcing. However, Hongel does not teach designing such a system that permits flexibility in how the relay coil is driven by external circuitry. He also does not teach how such a system can be made fail safe for use with potentially destructive high voltages relative to contact spacing. There is no “fail safe” arrangement in Hongel's designs.
Thus, there is a need for a simplified and improved relay contact protective circuit that can be built into a standard relay housing without the need for extra power supply connections and with minimal impact upon the flexibility of relay utilization, particularly in the case of high voltages and close contact spacing.
SUMMARY OF THE INVENTION
The present invention provides improved functionality and reduced complexity for an electronic relay switch, and it permits lower voltage relays to be used in high voltage systems. Arc suppression is provided during the “breaking” of the circuit, and the arc suppression mechanism is verified before the relay is permitted to operate to avoid damage to the contacts and possible fire hazards. When an undesirable condition is detected, operation of the relay is prevented. In addition to avoiding damage to the contacts, damage to circuit components is prevented. In one embodiment, provision is made for full operability even when the relay coil connections are reversed.
Briefly summarized, the present invention may be characterized as a method for safely suppressing the arcing of a relay's contacts when the contacts open using a solid state switch connected in parallel with the relay's contacts as an arc suppressing device. This method comprises the steps of sensing the relay coil energization signal before its contacts close, and in response, energizing the solid state switch to short the relay contacts together. Then one tests whether the potential across the relay's contacts drops to a very low potential in response to this energization of the solid state switch before the relay's contacts close. If the potential across the relay's contacts is not very low, then one prevents energization of the relay coil and subsequent coil at least until contact closure energization terminates; but if the potential across the relay's contacts is very low, then one permits energization of the relay coil and subsequent contact closure, and one then senses deenergization of the relay before its contacts open, and in response, energizes the solid state switch again to short the relay contacts together long enough to suppress arcing that would otherwise occur when the relay's contacts open.
The energizing of the solid state switch in response to sensing energization of the relay before its contacts close can be a brief enough energization such that some contact closure arcing is permitted to occur to keep the contacts in good condition. The method may also include the steps of permitting energization of the relay whenever its contacts are closed, and preventing energization of the relay whenever its contacts are open, but permitting energization of the relay for a brief time, long enough to permit the contact closure, after the testing described above determines the potential across the solid state switch connected in parallel with the relay's contacts is very low. The steps of sensing energization or deenergization of the relay may be carried out by sensing a potential change at whichever end of the relay's coil is free to change its potential in response to an incoming energization signal, such that the method works regardless of which end of the coil of the relay may be connected to a fixed potential.
The invention may also be characterized as an arc suppression system for a relay having at least one pair of normally open contacts, having an energizing coil with at least two leads, and having at least two contact terminals and two coil terminals respectively electrically coupled to each of the contacts and to each of the coil leads. This system comprises a solid state contact shorting switch having two switch leads connected in parallel with the two contact terminals and having a control lead which, when energized with an input signal, causes the switch to effectively short circuit the two contacts of the relay. It further comprises a first timing pulse generator receiving an input signal from at least one of the two coil terminals and responding to a coil energizing signal by generating a first timing pulse which, when applied to the solid state switch's control lead, causes momentary conduction of the switch. It also comprises a circuit tester and coil switch having an input coupled to at least one of the two contact terminals independently of the switch leads of said solid state switch and arranged to signal, by its conductive or n
Beckert James J.
Roller Michael J.
Witt Paul A.
Foley & Lardner
Han Jessica
Omron Corporation
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