Source: http://www.google.com/patents/US5633540?ie=ISO-8859-1&dq=5,893,120
Timestamp: 2014-03-14 17:38:52
Document Index: 635092743

Matched Legal Cases: ['Application No. 08', 'Application No. 08', 'Application No. 08', 'Application No. 08', 'Application No. 08', 'Application No. 08']

Patent US5633540 - Surge-resistant relay switching circuit - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsIn a relay switching panel, a switching circuit operates to selectively connect and disconnect resistive, capacitive and inductive loads to an AC power source with substantially no arcing. The switching circuit comprises a pair of relays, preferably connected in parallel, with one of such relays having...http://www.google.com/patents/US5633540?utm_source=gb-gplus-sharePatent US5633540 - Surge-resistant relay switching circuitAdvanced Patent SearchPublication numberUS5633540 APublication typeGrantApplication numberUS 08/673,666Publication dateMay 27, 1997Filing dateJun 25, 1996Priority dateJun 25, 1996Fee statusPaidAlso published asDE19781824T0, DE19781824T1, WO1997050163A1Publication number08673666, 673666, US 5633540 A, US 5633540A, US-A-5633540, US5633540 A, US5633540AInventorsJames M. MoanOriginal AssigneeLutron Electronics Co., Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (42), Non-Patent Citations (6), Referenced by (39), Classifications (16), Legal Events (5) External Links: USPTO, USPTO Assignment, EspacenetSurge-resistant relay switching circuitUS 5633540 AAbstract In a relay switching panel, a switching circuit operates to selectively connect and disconnect resistive, capacitive and inductive loads to an AC power source with substantially no arcing. The switching circuit comprises a pair of relays, preferably connected in parallel, with one of such relays having a triac, connected in series therewith. With the relays open, an air gap isolates the power source and the load. In closing the relays in sequence, one relay provides a conductive path from the power source to the triac. After a suitable delay to allow the relay contacts to stabilize in the closed position, the triac is triggered to provide a conductive path from the power source to the load, and a large current surge flows to the load. After the current surge has subsided, the second relay is closed to provide a direct conductive path between the power source and the load. Next the first relay and the triac are removed from the circuit to ensure full conduction through the second relay. Another sequence of opening and closing the relays and the triac in a particular order can be employed to provide substantially arcless switching during turn off. As a result of this arrangement, the switching circuit is low cost, compact and reliable over an extended period of time.
What is claimed is: 1. A switching circuit for switching power between a power source and a load, said switching circuit comprising:(a) first and second electrically controllable switches connected in series to electrically connect and disconnect said power source and said load; (b) a third electrically controllable switch connected in parallel with said series connected first and second electrically controllable switches; (c) control means connected to said first, second and third switches; said control means being operable for sequentially closing said first, second and third switches in that order to connect said power source to said load; (d) said control means being operable for thereafter opening said first and second switches and retaining said first and second switches open until an opening sequence is begun; (e) said control means being further operable for reclosing said first switch and said second switch while said third switch remains closed to initiate an opening sequence; (f) said control means being further operable for opening said third switch after reclosing said first and second switches during said opening sequence; and (g) thereafter reopening said first and second switches. 2. The apparatus as defined by claim 1 whereby said first switch is reclosed prior to said second switch.
13. A switching circuit for switching power between a power source and a load, said switching circuit comprising:(a) first and second electrically controllable switches connected in series to electrically connect and disconnect said power source and said load; (b) a third electrically controllable switch connected in parallel with said second electrically controllable switch; (c) control means connected to said first, second and third switches; said control means being operable for sequentially closing said first, second and third switches in that order to connect said power source to said load; (d) said control means being operable for thereafter opening said second switch and retaining said second switch open until an opening sequence is begun; (e) said control means being further operable for reclosing said second switch while said first switch and said third switch remain closed to initiate an opening sequence; (f) said control means being further operable for opening said third switch after reclosing said second switch during said opening sequence; and (g) thereafter reopening said first and second switches. 14. The apparatus of claim 13 wherein said load comprises an inductive load.
17. A method for switching power between a power source and a load, said method comprising the steps of:(a) providing a switching circuit comprising a first switch and a controllably conductive device connected in series with one another and connected in series with said power source and said load, and a second switch connected in parallel, with said series connected first switch and said controllably conductive device; (b) connecting said power source to said load by closing said first switch and waiting for a first predetermined time period and after said first predetermined time period, rendering said controllably conductive device conductive and waiting for a second predetermined time period and thereafter closing said second switch; thereafter rendering said controllably conductive device nonconductive and opening said first switch until an opening sequence is begun; (c) and disconnecting said power source from said load by reclosing said first switch device and rendering said controllably conductive device conductive again to initiate an opening sequence, and opening said second switch and then rendering said controllably conductive device nonconductive again and reopening said first switch. 18. The method of claim 17 wherein said second switch is closed after a delay of from 70 to 135 milliseconds after rendering said controllably conductive device conductive.
24. A method for disconnecting a load from a power source; said method comprising the steps of:(a) providing a switching circuit comprising a first electrically controllable switch and a controllably conductive device in series with the load and a second electrically controllable switch in parallel with said first electrically controllable switch and said controllably conductive device; said second electrically controllable switch being in the closed state when said load is connected to said power source; (b) disconnecting said power source from said load by closing said first electrically controllable switch and waiting for a predetermined period of time, and after said period of time rendering said controllably conductive device conductive and waiting for a second predetermined period of time; and thereafter opening said second electrically controllable switch, then rendering said controllably conductive device nonconductive and opening said first electrically controllable switch. 25. The method of claim 24 wherein said load comprises an inductive load.
FIELD OF THE INVENTION This invention relates to improvements in switching circuits for loads, including, for example, inductive loads, such as motor loads and magnetic low voltage transformers.
BACKGROUND OF THE INVENTION Relays for connection and disconnection of inductive, capacitive and resistive loads, and their combinations are well known. Such relays are disclosed in U.S. Pat. No. 5,309,068, entitled TWO RELAY SWITCHING CIRCUIT FOR FLUORESCENT LIGHTING CONTROLLER, in the name of Simo P. Hakkarainen et al, and assigned to the assignee of the present invention; and are also described in U.S. Pat. No. 4,855,612, U.S. Pat. No. 4,772,809, U.S. Pat. No. 4,209,814 and U.S. Pat. No. 3,864,604.
SUMMARY OF THE INVENTION In accordance with the present invention, a novel switching circuit with a novel operating sequence is provided to enhance the arcless switching of inductive, resistive and capacitive loads while protecting the series circuit thyristor from excessive surge currents and heating.
BRIEF DESCRIPTION OF THE DRAWING(S) FIGS. 1 and 2 are electrical schematics of prior art switching circuits;
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In applying power to loads such as an electronic ballast of the type used to control the operation of a fluorescent lamp, one finds that the ballast behaves as a capacitive load. Thus, each time power is applied to the load, for example, by closing a switch between the load and a line voltage source, there is an in-rush of current to the load which quickly subsides as the load charges up to line voltage. This 10 temporary current surge is a function of the number of electronic ballasts controlled by a single relay switch. For example, in the case of a full 16 ampere (steady-state) circuit of dimming ballasts, the current surge can approach 300 amperes. Though short-lived, perhaps only a few cycles, this level of surge can damage the contacts of even a relatively large relay having a high (e.g. 50 amp) current rating. The problem stems from the fact that each time a pair of relay contacts close or snap together, there is a tendency for them to bounce apart. When this bouncing occurs during a large current surge, the intervening gas or air ionizes and arcing occurs. Arcing also occurs during the opening sequence with an inductive load as previously described. The arcing has the effect of locally heating the conductive coatings on the relay contacts which eventually causes the relay to fail, either due to erosion of the contact material, or, more commonly, due to welding of the contacts in the closed position.
THE INVENTION The present invention is shown in FIG. 5 in which components similar to those of FIG. 3 have the same identifying numerals. It will be noted that the fixed delay circuits 18 and 20 have been removed (as compared to FIG. 3) and that their function is performed by a suitable microprocessor 100 or by suitable control devices which will control the switching of the electrically controllable switching devices RL3, RL4 and Q1, in accordance with the novel switching sequence of the invention. The prior art switching sequence of the circuit of FIG. 3 is shown in FIG. 6 in which the times t.sub.0, t.sub.1, and t.sub.2 represent the closing of the contacts C of switch RL3, the turning on of triac Q1, and the closing of the contacts D of switch RL4, respectively. Thereafter, at time t.sub.3, the contacts C of switch RL3 are opened and triac Q1 is turned off and the contact remains open and the triac remains off until the next closing sequence. In some prior art circuits, switches in the positions of RL3 and Q1 remain closed as long as contacts D of switch RL4 are closed (permitting possible damage to triac Q1 due to current surges on the line or overheating if the load current continues to flow through the controllably conductive device instead of flowing through the main relay, as discussed above).
FIG. 7 shows the novel operating sequence of the invention as controlled by microprocessor 100 in FIG. 5. In FIG. 7, it will be noted that the turn on sequence at times t.sub.0 through t.sub.4 is similar to that of the prior art FIGS. 3 and 6. A critical time delay of from 70 to 135 milliseconds is provided between the turning on of triac Q1 and the closing of contacts D of relay RL4 at times t.sub.1 and t.sub.2 respectively. This time is preferably 100 milliseconds and provides sufficient time to insure that capacitive current surges have dissipated before closing main contact D of relay RL4, while not permitting main thyristor current to flow so long that heat sinking would be necessary.
Significantly, a novel turn off sequence is employed in which, contacts C of relay RL3 close at time t.sub.5, followed by the turn on/closing of thyristor Q1, at time t.sub.6, followed by the opening of main contact D at t.sub.7. Thereafter thyristor Q1 is turned off/opened at t.sub.8 and contacts C of relay RL3 is opened at t.sub.9. The entire circuit then isolates the load and the power source by open contacts.
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OF PA, PENNSFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:MOAN, JAMES M.;REEL/FRAME:008049/0047Effective date: 19960624RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - About Google Patents - Send FeedbackData provided by IFI CLAIMS Patent Services©2012 Google