TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to electrical switches and, more particularly, to a power disconnect switch for disconnecting a source of electrical power.

BACKGROUND OF THE INVENTION In many fields, such as in emergency medical vehicles, for example, it is desirable to monitor current draw from a power supply (such as the vehicle battery and/or alternator) and to have the ability to disconnect the power supply from its load if the current draw becomes too great . Because of the large currents normally involved in such applications, the power disconnect function has traditionally been provided by a large mechanical rotating switch. This type of switch, however, has several inherent limitations. First, because it is a mechanical device, it is subject to wear and such switches suffer from frequent breakdowns. Also, the contacts of such mechanical switches tend to become loose with repeated use, resulting in less current flow capability through the switch. Finally, perhaps the most important limitation of such switches is that they cannot be controlled by a button on the vehicle dash and are usually mounted under the driver's seat. Such a location of the

switch is very inconvenient for many reasons. For example, location of the switch under the driver's seat requires that the battery/alternator power line be broken near the battery/alternator location (usually in the engine compartment) and routed to the switch and then back again. This necessitates the addition of multiple long power cables. Furthermore, location of the switch under the driver's seat (out of sight of the driver) necessitates that the driver manually feel the switch position in order to use it, rather than visually perceiving it. Because of these limitations, dash mounted switches have been developed which employ a large solenoid to affect the power switching. Such solenoids have a large contact which is pulled by an electromagnetic field in order to open and close the circuit. The solenoid switches have a tendency to seize after repeated use, even though they include self-cleaning contacts which utilize the arcing produced during opening and closing to burn off any dirt on the contacts. The failure of the self- cleaning process to prevent all malfunctions results from the fact that the self-cleaning does not occur unless a minimum amount of current is flowing when the unit is switched on or off. Furthermore, and perhaps most significant when used in emergency vehicles with sensitive electronic equipment on board, the collapsing electromagnetic field of the solenoid produces an "inductive kickback" (or counter-EMF) which causes large spikes in the power supplied to the attached load. Such spikes can seriously damage sensitive electronic equipment.

There is therefore a need in the prior art for a power disconnect switch that may be remotely operated, that is relatively immune to mechanical failure, and which does not

produce large amounts of noise in the attached power supply when it is switched. The present invention is directed toward meeting these needs.

SUMMARY OF THE INVENTION

The present invention relates to a power disconnect switch which employs a plurality of high current relays which are simultaneously activated by the application of a control signal to an attached control terminal . By providing a plurality of relays arranged in parallel across the power disconnect switch, the current flowing through the switch is divided equally among the parallel branches. The resistance of each of the parallel branches is carefully controlled in order to provide a nearly equal resistance in each parallel current flow path. Such a balanced design prevents free wheeling and the creation of thermal noise caused by unbalanced current flow through the parallel paths. Additionally, the power disconnect switch of the present invention includes snubbing diodes which are operative to short any counter-EMF produced by the switching relays, as well as filtering capacitors which are operative to short circuit any spike voltages to ground.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electronic schematic diagram of a first embodiment power disconnect switch of the present invention.

FIGS. 2A-C are circuit board masks for creating a circuit board to mount the circuit of FIG. 1.

FIG. 3 is a perspective view of the first embodiment power disconnect switch of the present invention.

FIG. 4 is a partial cross sectional view of the first embodiment power disconnect switch of the present invention. FIG. 5 is a side elevational view of the first embodiment power disconnect switch of the present invention.

FIG. 6 is an electronic schematic diagram of a microprocessor control circuit of a second embodiment power disconnect switch of the present invention. FIG. 7 is an electronic schematic diagram of a third embodiment current monitoring power disconnect switch of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.

Referring to FIG. 1, a first embodiment power disconnect switch of the present invention is illustrated and indicated generally at 10. The power disconnect switch 10 includes three terminals. A terminal Tl connects to the power supply which is to be connected or disconnected by the switch 10, such as the battery terminal of an emergency vehicle. A terminal T2 couples to the load which is drawing current from the battery connected to terminal Tl. By placing the switch 10 between these two terminals, it is possible to disconnect the load from the power supply by opening the switch 10. The third terminal T3 is a trigger terminal which is operative to either close or open the power disconnect switch 10. Placing a signal on terminal T3 which is negative with respect to the voltage applied to terminal Tl will operate to open circuit the power disconnect switch 10.

The power disconnect switch 10 includes four parallel current flow paths between the battery terminal Tl and the load terminal T2. Current flowing between each of these four paths is controlled by respective relays RLY1, RLY2, RLY3, and RLY4. Each of the relays RLY1-4 is designed to safely carry at least as much as 1 quarter of the total rated current for the power disconnect switch 10. It will be appreciated by those skilled in the art that the power disconnect switch 10 may be designed with any plural number of relays, the number of relays chosen being a factor of the current handling capabilities of each individual relay and the total current handling capability desired for the power disconnect switch 10 as a whole. Each of the relays RLY1-4 includes an input terminal 30 which is coupled to the battery terminal Tl. Additionally, each of the relays RLY1-4 includes an output terminal 87 which is coupled to the load terminal T2. Each of the relays includes a respective electromagnet having a first terminal 85 which is coupled to the negative trigger terminal T3 and a second terminal 86 which is coupled to the battery terminal Tl. When a voltage is placed on the negative trigger terminal T3 which is negative with respect to the battery voltage applied to terminal Tl, current will flow through the respective electromagnets, creating an electromagnetic field which is operative to close the respective relays, thereby allowing current to flow between the battery terminal Tl and the load terminal T2 through the four relays

RLY1-4. As will be explained in greater detail hereinbelow, the resistances of the current flow paths through the relays RLY1-4 are equalized such that equal portions of the total current will flow through each of the relays RLY1-4.

The power disconnect switch 10 further includes diodes Dl-4 which are coupled between the terminals 85 and 86 of respective relay electromagnets. These snubbing diodes Dl-4 operate to short circuit any counter-EMF voltages produced by the relays RLY1-4. In a preferred embodiment, each of the relays RLY1-4 comprise a model VF7-11F12 PC board mount relay manufactured by Potter and Brumfield of Princeton, Indiana. Additionally, each of the snubbing diodes Dl-4 comprises a model 1N4001 rectifier diode manufactured by Motorola Incorporated of Phoenix, Arizona. Finally, the power disconnect switch 10 includes capacitors Cl-4 which are operative to short circuit any transient spike voltages to the terminal T3, thereby only allowing pure direct current signals to be transferred between the battery terminal Tl and the load terminal T2. It will be appreciated by those skilled in the art that because the power disconnect switch 10 of FIG. 1 may be operated by the presence or absence of a negative trigger signal at the terminal T3, such a power disconnect switch 10 may be activated or de-activated from a location remote from the power disconnect switch 10. For example, the power disconnect switch 10 may be located in the engine compartment of an emergency vehicle, however, the switch 10 may be activated by means of an appropriate trigger switch placed on a dashboard within the passenger compartment of the emergency vehicle. This allows the power disconnect switch 10 to be located near the battery/alternator, however it is not necessary for an operator of the emergency vehicle to exit the vehicle in order to activate or de-activate the power disconnect switch 10.

Because current flows between the terminals Tl and T2

through a variety of parallel paths in the power disconnect switch 10 of FIG. 1, power is dissipated across the terminals Tl and T2, the relays RLY1-4, and the circuit board traces which couple these components together. Because of the multiple current flow paths, it is important that the impedance of each of these current flow paths be as nearly identical as possible. It is important to avoid unbalanced impedances between the current flow paths so that one current flow path does not turn on prior to the other current flow paths, which could require the relay in that flow path to momentarily handle more current than it is rated for. When this happens, the increased current flow causes the metal traces in that particular current flow path to heat up, thereby increasing their resistance, which in turn causes the other current flow paths to compensate by passing higher current. At this point, the other traces begin to heat up from their higher current draw while the original trace cools off because of its decreased current draw. The situation then reverses in succession, causing the circuit to oscillate between the current flow paths at high frequencies. Such oscillation caused by unbalanced impedances in the current flow paths creates a thermal noise in the power supply circuit which is detrimental to the performance of the electronic components coupled to the power supply. Therefore, it is important that the impedance of each current flow path be as nearly identical as practical. Because the inductance of each of the relays RLY1-4 is fairly consistent, the resistance of the circuit board traces which couple the relays to the input and output terminals Tl and T2 are the largest contributing factor.

In a preferred embodiment of the present invention, the

components illustrated in FIG. 1 are mounted on a circuit board made from FR4 fiberglass material which is copper clad on both sides. The copper cladding is preferabaly 3 ounce copper (i.e., 3.6 mils thick) . In designing the traces for the circuit board, several design requirements should be met. First, the traces should all have a matched impedance for each of the current flow paths. Second, the traces should be able to carry the rated amount of current for the power disconnect switch, which is 50 amps of continuous current for each current flow path in the preferred embodiment. The current handling capability of the traces is determined by the trace thickness and width. Because the distances of each of the current flow paths on the circuit board surface are not equal, it is necessary to adjust the width of the traces in order to balance the resistance between the various current flow paths. For instance, shorter traces can be made thinner than longer traces in order to achieve the same trace resistance. It is preferred to maintain a 10 percent maximum difference in the resistance between current flow paths (i.e. ,±5 percent of the design value) . Referring to FIG. 2A, there is illustrated a silk screen layer 100 for a first embodiment circuit board of the present invention. The silk screen layer 100 specifies the component mounting locations for each of the components in the power disconnect switch 10 of FIG. 1. The silk screen layer 100 is screened onto the top of a two layer circuit board. FIG. 2B illustrates the top side metalization pattern 102 of the circuit board, while FIG. 2C illustrates the bottom side metalization pattern 104 of the circuit board. The two layer metalization pattern illustrated in FIG. 2B-C produce four current flow paths

through the power disconnect switch 10 which have substantially equal resistance, thereby ensuring minimal noise generation by the power disconnect switch 10.

Referring to FIG. 3, the power disconnect switch 10, mounted on an appropriate circuit board, such as the circuit board defined by FIGS. 2A-C, is preferably mounted into a metal enclosure 106 which has extended mounting flanges 108 which may be used for convenient mounting of the power disconnect switch 10 in the engine compartment of the emergency vehicle. The battery terminal Tl and the load terminal T2 are formed by threaded bolts which are passed through the circuit board prior to installation into the container 106. The container 106 preferably does not have a top surface. Rather, the circuit board is inserted into the container 106 with the terminals Tl and T2 facing upwards, and the entire interior of the container 106 is filled with a suitable potting compound, both below and above the circuit board to the top edge of the container 106. The negative trigger terminal T3 will also protrude through the top of the potting compound. It is preferred that the potting compound used within the enclosure 106 have a very high resistivity so that there is no current flow through the potting compound. Additionally, because of the large amounts of current which will typically flow through the power disconnect switch 10, the potting compound must have adequate heat sinking capability to sink heat from the components of the power disconnect switch 10 to the ambient air. It is further desired that the potting compound be very homogenous, in order to prevent cracking of the potting compound through thermal expansion as it warms during operation of the power disconnect

switch 10. The potting compound should also be pliable to some extent so that it does not tend to chip or crack when dropped or subjected to severe vibration. Potting compounds meeting these requirements will provide adequate protection to the circuit board of the power disconnect switch 10 from weather, temperature, dust, contamination, and corrosion. A preferred potting compound is manufactured by Key Laboratories of Clearwater, Florida, and is composed of a model BF2 hardener mixed with a model 303 black resin. The two components are combined in a ratio of 2:1 of resin to hardener and mixed for six minutes prior to pouring into the enclosure 106.

Referring now to FIG. 4, the power disconnect switch 10 is illustrated in partial cross section when installed within the enclosure 106. The terminal T2 is visible in the cross section of FIG. 4. As previously stated, the terminal T2 comprises a bolt which extends through the circuit board of the power disconnect switch 10. The terminal T2 makes contact with the circuit board 98 at a pad formed in the surface metalization of the circuit board 98. The contact pad is covered with a tin/lead solder mask over the circuit board copper layer in order to prevent corrosion. The terminal T2 makes contact with this contact pad through a washer 112. A split washer 114 makes contact with the other surface of the circuit board 98 and is secured thereto by a nut 116 which is threaded around the terminal T2. The nut 116 is preferably installed onto the terminal T2 using a precalibrated torque wrench in order to prevent unwanted compression of the circuit board 98, thereby preventing stress to the copper layers on the circuit board 98. The nut 116 secures the terminal T2 to the circuit board

mechanically and provides a platform for mounting of conductors to the power disconnect switch 10. The circuit board 98 is therefore positioned within the enclosure 106 such that the nut 116 is exposed above the top surface of the potting compound 110. 5 It is preferred that the potting compound cover the upper surface of the circuit board 98 by approximately 3/8 inch.

Referring now to FIG. 5, the power disconnect switch 10 is illustrated in a side elevational view with a current conductor 120 mounted to the battery terminal Tl. The conductor 120 is

10 mounted to the terminal Tl between two flat washers 122. A split washer 124 is then placed on terminal Tl over the upper flat washer 122, and a nut 126 is then threaded to the top of the terminal Tl and tightened in order to compress the remaining mounting components against the nut 116. Those skilled in the

,5 art will recognize that two separate terminal configurations are illustrated for the negative trigger terminal T3 in FIGS. 3 and 5. The exact physical configuration of any of the terminals Tl-3 of the present invention may be modified to suit a particular application.

20 Other functions may optionally be added to the power disconnect switch 10 of FIG. 1 in order to provide further features. For example, the microprocessor circuit 200 of FIG. 6 may be added to the power disconnect switch 10 of FIG. 1 in order to provide additional safety features. The circuit 200 has a

25 driver activated input terminal 202 which is coupled to a switch (not shown) which is easily accessible to the driver, such as on the dashboard of the emergency vehicle. The input 202 is a ground activated input. The circuit 200 includes a further input 204 which is coupled to the key ignition switch of the emergency

vehicle into which the power disconnect switch is located. The key ignition input 204 is positive activated, and is supplied with a positive voltage whenever the key ignition of the vehicle is activated. The inputs 202 and 204 are provided to the microprocessor Ul which in turn supplies the trigger signal to the power disconnect switch circuit 10 of FIG. 1. The trigger signal is provided on output terminal 206 which is coupled to the negative trigger terminal T3 of FIG. 1.

The microprocessor Ul runs a software program which analyzes the inputs 202 and 204 in order to make a determination as to whether to activate the output 206. For example, if the vehicle key ignition is on, as indicated by the presence of a positive voltage at input terminal 204, and the driver attempts to turn the power disconnect switch off, as indicated by the presence of a ground at the input 202, the microprocessor Ul will not ground the output 206, thereby preventing the power disconnect switch 10 of FIG. 1 from open circuiting. This is an important safety feature in an emergency vehicle, as the inadvertent open circuiting of the power disconnect switch 10 while important electronic medical instrumentation is operating can have life threatening consequences to the patient within the emergency vehicle. Therefore, the circuit 200 will not allow the power disconnect switch 10 to be open circuited when the key ignition switch of the vehicle is on. Furthermore, if the key ignition switch is turned off, as indicated by the absence of a positive voltage on the input terminal 204, and the driver attempts to close the power disconnect switch 10, as indicated by a ground signal at the input terminal 202, the microprocessor Ul will ground the output 206 (thereby closing the power disconnect

switch 10) for only a predetermined period of time, after which the output 206 will be raised to a positive voltage level in order to open circuit the power disconnect switch 10. For example, the microprocessor Ul may be programmed to close the power disconnect switch 10 for only five minutes under these circumstances. The purpose of this precaution is to prevent the driver from running down the battery in the emergency vehicle by running electronic equipment when the key ignition switch is off (and therefore the vehicle alternator is not recharging the vehicle battery) . It will be appreciated by those skilled in the art that other important safety features may be implemented by changing the programming of the microprocessor Ul.

The circuit 200 of FIG. 6 is centered upon the microprocessor Ul, which is preferably a model PIC16C54 manufactured by Microchip of Chandler, Arizona. The driver input signal at terminal 202 is input to the RA1 input/output terminal of the microprocessor Ul through the resistor R60. The shunt R/C network R40/C60 provides filtering to the signal presented at terminal 202. The key ignition input at terminal 204 is applied to the input/output terminal RAO through the resistance R70, while the R/C network R80/C80 provides filtering for this input signal. Timing for the microprocessor Ul is provided by the crystal XI which is coupled to the OSCl and OSC2 inputs of the microprocessor Ul through an R/C network as is known in the art. The output of the microprocessor Ul is taken from input/output terminal RBO and is applied to the base of transistor Ql through the resistance R90. The transistor Ql is preferably a model TIP120 manufactured by Motorola of Phoenix, Arizona. The emitter of the transistor Ql is tied to ground and the output 206 is

taken from the collector the transistor Ql. It will be appreciated by those skilled in the art that the application of a positive voltage to the base of the transistor Ql will operate to couple the output 206 to ground. An automatic rebooting circuit is provided in the circuit

200 and comprises the R/C network R30/R50/C70 which is coupled to the MCLR input of the microprocessor Ul. If there is a loss of Vcc voltage, the rebooting circuit pulls immediately to ground and then back to the Vcc voltage in order to provide a rising edge signal which will clear the microprocessor and begin execution of its imbedded software routine from the start. The Vcc signal is provided to the circuit 200 by means of a zener regulator formed from the resistance R10 and the zener diode D10. The zener regulator converts between the V _DD signal which is typically 12 volts to the Vcc signal of +5 volts. The circuit

200 further includes filtering capacitors CIO, C20, C30, C90, and C101 which are coupled between the Vcc supply and ground.

A second optional feature which can be added to the power disconnect switch 10 of FIG. 1 is illustrated in FIG. 7 and indicated generally at 300. The circuit 300 provides a current monitoring function and operates to measure the voltage drop occurring between the battery terminal Tl and the load terminal T2. If the voltage drop between these two terminals is greater than a predetermined threshold voltage, then this is an indication that too much power is being dissipated within the power disconnect switch unit itself, which is indicative of a malfunction of the unit. The circuit 300 of FIG. 7 further monitors the power disconnect switch 10 to determine if current is flowing therethrough above a predetermined threshold, such as

the amount of current caused by the lights and other loads of the emergency vehicle being switched on. If the driver of the vehicle attempts to open the power disconnect switch 10 while this amount of current is flowing through the device, arcing may occur at the relay contacts, thereby damaging the relays. The circuit 300 therefore detects this condition and will not allow the power disconnect switch 10 to be open circuited until the current flowing through the switch has been lowered (e.g., by turning off the vehicle lights or other loads) . The circuit 300 of FIG. 7 employs a Hall effect sensor Ull which monitors the magnetic field produced by current flowing through a conductor coupled to terminal Tl of the power disconnect switch 10. The Hall effect sensor Ull is preferably a model UGN3503U manufactured by Allegro Microsystems, Inc. of Worcester, Massachusetts. The output of the Hall effect sensor Ull is input to an amplifier U2A, which is preferably a model TL082CPN manufactured by Texas Instruments of Dallas, Texas. The output of the amplifier U2A is input to a first comparator U3A and to a second comparator U3B. The comparators U3A and U3B are preferably a model LM358AN manufactured by National Semiconductor of Santa Clara, California. The threshold levels of the amplifier U2A and of the comparators U3A and U3B are set by a resistor network formed by connecting the resistors R121, R131, R141, R151, and R161 in series between the Vcc supply and ground. The comparator U3A is utilized to detect an over current situation within the power disconnect switch 10. In a preferred embodiment, the comparator U3A will only produce an output when the current sensed by the Hall effect sensor Ull exceeds 200 amperes in a preferred embodiment. The U3A comparator output

drives the switching transistor Qll through the resistor R51 in order to produce two signals at the collector of the transistor Qll. The first output, labeled TRIP, may be used as a third input to the microprocessor Ul of FIG. 6 in order to cause the microprocessor Ul to ground the output 206 in order to open circuit the power disconnect switch 10. The second output from the collector of transistor Qll, labeled ERROR, may be used to illuminate an indicator light on the dash of the emergency vehicle to warn of the over current situation. The second comparator U3B produces an output when the current sensed by the Hall effect sensor Ull exceeds 30 amperes in a preferred embodiment. This output is transmitted through a pass transistor Q31 which provides drive for the power transistor Q21, which is preferably a model TIP125. The output of transistor Q21, labeled RLY, is applied to the similarly labeled input of the power disconnect switch 310 which causes the relays RLY1-4 to close. It will be appreciated by those skilled in the art that the power disconnect switch 310 is wired slightly differently than the power disconnect switch 10 of FIG. 1. The major difference between the two versions is that the power disconnect switch 310 of FIG. 7 includes a safety bypass switch SWl which allows the electronic circuitry 300 to be manually overridden by the operator if there is a failure in the electronic circuit 300. This is a failsafe safety device which allows the power disconnect switch 310 to be operated manually independently of the commands being given by the electronic circuitry 300.

The node 312 of the circuit 300 is further connected to the master disconnect input, labeled MD, which receives a ground

signal when the operator attempts to disconnect the power disconnect switch. The diode Dil guarantees that the user is providing a ground signal and not a positive voltage. Either the collector of transistor Q31 or the input MD can ground the node 312, thereby activating the transistor Q21. It will be appreciated by those skilled in the art that the preferred embodiment of the circuit 300 provides that the comparator U3A will trip the power disconnect switch when the switch exceeds the 200 amp limit, and the comparator U3B will latch the power disconnect switch closed whenever the current flowing through the device exceeds 30 amps. The reference signals used to determine the 200 amp and 30 amp references for the comparators U3A and U3B are provided by the resistor network. The resistance values in the embodiment illustrated in FIG. 7 have been chosen in order to provide a 170 amp difference between these two references. The exact point of the references may be adjusted by varying the adjustable resistor R131 in order to move the 170 amp window up or down. Furthermore, a 5 volt voltage regulator U41 is provided with the circuitry of FIG. 7 and is preferably a model LM78L05 5 volt voltage regulator manufactured by National Semiconductor of Santa Clara, California. The output of voltage regulator U41 provides the Vcc power supply to the circuit 300.

Those having ordinary skill in the art will recognize that the various optional features described hereinabove may be combined into a single power disconnect switch or they may be used individually in relation to a power disconnect switch. Furthermore, a power disconnect switch may be used without either of the optional features, such as the switch illustrated in FIG. 1. The selective combination of these optional features into a

single power disconnect switch is considered to be within the ability of one having ordinary skill in the art .

It will further be appreciated by those skilled in the art that the power disconnect switch 10 of the present invention may be used in a variety of applications in addition to emergency medical vehicles. For example, the switch of the present invention will find application in any automotive or marine application, such as boats, tow trucks, farm equipment, etc.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.