Patent Application
20100208406
The present invention relates to a circuit protection device (or a circuit protection element). More specifically, it relates to a circuit protection device comprising a bimetal switch and a PTC component as well as an electrical circuit (or an electrical apparatus) having such a circuit protection device. Such a circuit protection device may be used as a protection device in electrical circuits, which use various high voltage or high current batteries, in electric vehicles, cordless vacuum cleaners, power tools, wireless stations, and the like, for example.
Circuit protection devices are incorporated into circuits for various types of circuits in order to protect electrical/electronic apparatuses and/or electrical/electronic parts incorporated in such circuits in case voltage higher than the rated voltage is applied to and/or current higher than the rated current flows through the circuits.
As such a circuit protection device, using a bimetal switch and a PTC component which are connected in parallel has been proposed (see the Patent Reference below). With such a circuit protection device, under normal operating conditions, i.e. with voltage at or below the rated voltage and current at or below the rated current, substantially all the current flowing through the circuit will pass through contacts in the bimetal switch; if, for example, there is an overcurrent, the bimetal part of the bimetal switch will rise in temperature to separate and open the contacts, and the current will be diverted to the PTC component. As a result, the PTC component will go into a high-temperature, high-resistance state and trip due to the overcurrent, substantially cutting off the current flow through the PTC component. At this point, the high temperature of the PTC component maintains a bimetal part at high temperature, so that the bimetal switch maintains the opened state. In other words, the latch condition of the bimetal switch is maintained. In such a circuit protection device, it is said that arcs do not occur at the contacts of the bimetal switch since there is no need to switch the current.
The present inventor, having closely studied the above circuit protection device, has noticed that in a circuit protection device wherein the PTC component is merely connected in parallel to the bimetal switch, arcs may be generated at the contacts in the bimetal switch, and in the worst case, the contacts may be welded. When such welding occurs, the device does not function as a circuit protection device and cannot protect the electrical/electronic apparatus and/or electrical/electronic parts incorporated into the circuit. Therefore, the problem to be solved by the present invention is to provide a circuit protection device as above mentioned with a further improved potential to protect circuits.
The present invention provides a circuit protection device comprising a bimetal switch and a PTC component, characterized in that:
the bimetal switch and the PTC component are electrically connected in parallel; and
the PTC component has a resistance after its activation that is equal to or less than 1.1 times (i.e. 1.1×) as large as a specific (or intrinsic) resistance of an electric circuit into which the circuit protection device is to be incorporated, which specific resistance is calculated based on a rated voltage and a rated current of the electrical circuit according to the following equation (1):
Rated Voltage/Rated Current=Specific Resistance (1).
When the circuit protection device according to the present invention is incorporated into an electrical circuit, the formation of a welded portion at the contacts in the bimetal switch may be further suppressed. As a result, the circuit protection function of the circuit protection device is further enhanced. Thus, the present invention further provides an electric circuit into which the circuit protection device according to the present invention is incorporated.
Having studied why welding occurred at the contacts in the bimetal switch, it has been concluded that the following concept is a possibility. However, the present invention described above and below is based on facts experimentally confirmed by the present inventor. Therefore, this concept is one of possibilities which are able to explain the present invention, and whether the concept is appropriate or not does not unduly limit the concept of the present invention.
The opening action of the contacts in a bimetal switch is macroscopically an instantaneous action. However, when such action is observed microscopically, it can be seen as an action in which the contacts gradually separate from each other in a very short period within such separation of the contacts. At the beginning of this very short period, a rated current flows between the contacts, and at the end of such very short period, current is interrupted between the contacts. In other words, the resistance between the contacts at the beginning of the very short period is substantially in the state of zero, and at the end of such period, the resistance has increased to be infinite. Therefore, the current flows and the resistance increases greatly within this very short period, so that electric power is consumed between the contacts.
On the other hand, it is possible to think that the phenomenon of the bimetal contact welding occurs through temperature rising of the contacts as a result of the effect of excessive voltage and/or excessive current. Therefore, when the energy consumed between the contacts becomes great, the risk of welding occurring will increase.
In view of electric power being a measure of the energy, the present inventor has therefore reached an idea in that the occurrence or non-occurrence of welding between the contacts can be determined by using the electric power, in particular its maximum value, consumed between the contacts in the above mentioned very short term as a measure. This electric power consumption between the contacts and its maximum value are calculated as explained below with reference to the circuit in
A predetermined circuit A is considered in
As described above, the predetermined circuit is configured such that the predetermined voltage is applied to the electrical apparatus B so that a predetermined current flows through the circuit. Such voltage and such current are called a rated voltage Vr and a rated current Ir, respectively. Such rated voltage and such rated current mean that, when the voltage Vr is applied to the circuit, the current Ir flows through the circuit, and therefore the circuit has a resistance of Vr/Ir.
Therefore, when Vr/Ir=Rf, the circuit which is said to have its rated voltage and its rated current of Vr and Ir respectively corresponds to a circuit wherein E=Vr, and an electrical apparatus having a resistance of Vr/Ir as its resistance (Rf) is incorporated in
In the circuit shown in
In this case, when it is assumed that the current flowing through the circuit A is I and the whole circuit is considered, the following is obtained:
I=E/(Rf+Rb).
When it is assumed that the electric power consumed between the contacts of the bimetal switch is P, the following is obtained:
P=I×Vb.
Since Vb=E−Vf, the following is obtained:
P=I×(E−Vf)
Since Vf=I×Rf, the following is obtained:
P=I×(E−I×Rf)=IE−I2×Rf
When the electric power consumption of P which is expressed as a function of I is differentiated with I, the following is obtained:
P′=E−2I×Rf.
Since P′ becomes 0 when the electric power consumption reaches the maximum, the following is obtained:
P′=E−2I×Rf=0.
When the current I at this point is assumed to be Imax, the following is obtained:
Imax=E/(2Rf)=(½)×(E/Rf).
When the Vb at this point is assumed to be Vmax, the following is obtained:
Vmax=E−Vf=E−Imax×Rf=E−[E/(2Rf)]×Rf=E/2.
In the above state, E/Rf is equal to the value of the current flowing through the circuit when Rb is zero. It is impossible that Rb becomes zero. However, when Rb may substantially be ignored in comparison with Rf, that is, when the contacts of the bimetal switch are sufficiently touching to be in a closed state, Rf>Rb so that there is no problem in that Rb is assumed to be substantially zero. Such a state may be considered to be a state where the rated voltage Vr is applied as the voltage E to the circuit A and the rated current Ir is flowing through the circuit A.
Thus, when the electric power consumption between the contacts of the bimetal switch becomes the maximum, the following is obtained:
Imax=E/(2Rf)=(½)×(Vr/Rf)=(½)×Ir
Vmax=E/2=(½)×Vr
Therefore, when the electric power consumption between the contacts becomes the maximum, the current flowing between the contacts becomes a half of the rated current, i.e. a current of Ir/2, and the voltage applied between the contacts becomes a half of the rated voltage, i.e. a voltage of Vr/2.
In brief, the voltage and the current, when the electric power consumption becomes the maximum between the contacts of the bimetal switch which has been incorporated in series in a predetermined circuit as a circuit protection device, will be half values of the rated voltage and the rated current respectively of the circuit. Therefore, the resistance between the contacts at this point is (Vr/2)/(Ir/2)=Vr/Ir=Rated Voltage/Rated Current. In other words, this resistance is a specific resistance of the circuit.
It is presumed that, in the vicinity of this resistance, the electric power consumption between the contacts becomes the maximum, as a result of which arcing is likely to be generated between the contacts thereby increasing the possibility of the contact welding; thus this resistance (=Vr/Ir) is also tentatively called “arcing resistance”. It is noted that the term “arcing resistance” used in this specification is used only as a matter of convenience, and it should be kept in mind that, although it is presumed that there is some sort of correlation with the occurrence of the arcing between the contacts, it cannot necessarily be presumed that the resistance between the contacts becomes this arcing resistance upon the occurrence of the arcing.
The inventor has studied further and discovered experimentally that, when the resistance of the PTC component electrically connected in parallel to the bimetal switch is equal to or less than the arcing resistance, the possibility of welding occurring between the contacts is reduced. The “resistance of the PTC component” as stated herein is a resistance (an electrical resistance) of the PTC component after it has been connected in parallel to the bimetal switch. When solder was used to connect a commercially available PTC component to the bimetal switch, the PTC component is in the state wherein it already tripped once because of the heat caused by connecting with solder (i.e. in the state after thermally tripping), and the present specification calls the resistance of the PTC component in this state as the “resistance of the PTC component”. This resistance is also called as a “resistance after activation” in the technical field related to the present invention. It is noted that when electric welding is used to connect the PTC component to the bimetal switch, it may be possible that the heat conducted to the PTC component is not enough to cause tripping. In such a case, the PTC component will trip for the first time when an overcurrent flows through the PTC component for the first time (i.e. the PTC component tripping electrically), and the resistance afterwards corresponds to the “resistance after activation” as described above. In the present invention, therefore, the “resistance after activation” means the resistance after the commercially available PTC component has tripped for the first time either thermally or electrically. The PTC component before solder connecting (i.e. the commercially available PTC component itself) has a resistance smaller than such resistance (this smaller resistance is also called as a “basic (or reference) resistance” in the related art).
As described previously, when incorporating into a predetermined circuit a circuit protection device in which a bimetal switch and a PTC component connected in parallel, current which has flowed through the bimetal switch can be shunted to the PTC component upon opening of the bimetal switch. For example, in the case wherein a ratio of the resistance of the PTC component to the specific resistance (or the arcing resistance) of the circuit is equal to or less than 1.1, preferably equal to or less than 1.0, more preferably equal to or less than 0.9, most preferably equal to or less than 0.4, particularly equal to or less than 0.3, and more particularly equal to or less than 0.2, for example equal to or less than 0.15, a half of the current that would have flowed between the contacts without the presence of the PTC component can be shunted to the PTC component when the electric power consumption between the contacts becomes the maximum. As a result, the possibility of arcing between the contacts is decreased. In the most preferred embodiment, the ratio of the resistance of the PTC component to the specific resistance (or the arcing resistance) of the circuit is equal to or less than 0.2, and for example equal to or less than 0.15. It is noted that in the circuit protection device according to the present invention, the basic resistance of the PTC component is preferably equal to or less than ⅘, and more preferably equal to or less than ⅔ of the specific resistance (or the arcing resistance), and for example equal to or less than ½ of the specific resistance.
Conversely, when the resistance of the PTC component is larger than the arcing resistance of the circuit, for example twice larger than the arcing resistance, the arc generation may not be sufficiently suppressed even if a PTC component has been incorporated in the circuit and welding may occur.
In the circuit protection device according to the present invention, the bimetal switch used in the circuit protection device is a switch using a bimetal element, and a well-known one may be used. This is a switch configured such that, when current flowing through the bimetal switch exceeds a predetermined current value and becomes excessive, the contacts which are touching each other will separate by means of generated heat.
Among such bimetal switches, those, for example, that use platinum, gold, silver, copper, carbon, nickel, tin, lead, or an alloy of these metals (for example tin-lead alloy) as a contact material thereof may be given as examples of ones that are particularly preferred for the use in the circuit protection device according to the present invention. Above all, a bimetal switch using silver as the contact material is particularly preferred. It is noted that bimetal switches having a relatively narrow gap between the contacts may suitably be used in the circuit protection device of the present invention. Such bimetal switch having a gap of preferably 0.4-4 mm, in particular 2 mm or less, more preferably 0.7-2 mm, particularly preferably 0.8-1.5 mm, and for example around 1 mm may suitably be used in the circuit protection device.
The PTC component that is connected in parallel to the bimetal switch in the circuit protection device according to the present invention may be a conventional PTC component that is itself used as a circuit protection device, and its electrically conductive element may be made of a ceramic or of a polymer material. A particularly preferred PTC component is one that is called a polymer PTC component, and a PTC component having an electrically conductive polymer element in which an electrically conductive filler (for example carbon, nickel, or nickel-cobalt filler) is dispersed in a polymer material (for example a polyethylene, a polyvinylidene fluoride, etc.) can be suitably used.
When the circuit protection device according to the present invention is incorporated into a predetermined circuit and performs its intended function normally, substantially all of the current flowing through the circuit will pass through the bimetal switch. Therefore, in the circuit protection device according to the present invention, the resistance of the PTC component has a resistance value that is at least 10 times, preferably at least 50 times, more preferably at least 100 times, and particularly preferably at least 300 times as large as the resistance that the bimetal switch inherently has (normally 0.5-20 milliohms).
The circuit protection device 1 according to the present invention comprises a PTC component 2 and a bimetal switch 4, which are electrically connected in parallel, or if not electrically connected in parallel, are configured so that they can be thus connected. A ratio of the resistance of the PTC component 2 to the specific resistance Rf of the circuit 3 is for example one or less, preferably one-half or less, more preferably ⅓ or less, for example ¼ or less, and in particular ⅛ or less. Further, the PTC component has a resistance of for example at least 10 times, and preferably at least 100 times as large as the resistance that the bimetal switch 4 inherently has.
Specific embodiments of the circuit protection device 10 according to the present invention are shown as schematic side views in
In the illustrated embodiment, the PTC component 12 comprises a PTC element 23 and electrode layers 22 and 24 disposed on both sides thereof, and an insulating layer 26 is present between the PTC component 12 and the lead. The PTC element and the electrode layers on both sides thereof may be similar to those used in the known PTC components, and the electrode layers may have leads on their outer surfaces. In such embodiment, such PTC element forms, in place of the shown PTC element 23, a PTC component together with the electrode layers (preferably metal foil electrodes) on the both sides of the PTC element, and also forms leads on the above mentioned outer surfaces in place of the shown electrode layers 22 and 24, which leads are connected to the leads 10 and 10′ respectively. By incorporating the PTC component 12 and the bimetal switch 16 in the electrical circuit as described above, an electrical circuit is configured wherein these are electrically connected in a parallel relationship. It is noted that in the shown embodiment, another insulating layer 26′ is disposed between the bimetal switch 16 and the lead 10′.
When an abnormal current flows through the electrical circuit so that the temperature of the bimetal switch 16 rises, the contacting state between the contact 18 and the terminal section 20′ is dissolved. At this point, the current flowing through the electrical circuit flows instantaneously from the lead 10 to the electrode layer 22 of the PTC component, and shunts thereafter to the lead 10′ via the PTC element 23 and the electrode layer 24. In this case, when the resistance of the PTC component after its activation is equal to or less than 1.1 times (×1.1) as large as the specific resistance of the circuit, a relatively large ratio of the abnormal current can be shunted to the PTC component at the moment when the bimetal switch opens, so that the possibility of an arcing occurrence, welding occurrence or the like in the vicinity of the bimetal switch contacts is greatly reduced. After this, the PTC component will be in the trip state and substantially cut off the current flow.
In
Using a PTC component, a bimetal switch, and an electrical element (a resistor, its resistance Rf=2.67Ω) which are commercially available and described below, a circuit was constructed shown in
PTC component: manufactured by Tyco Electronics Raychem, product name: RXE010, basic resistance: 2.6Ω, resistance after activation: 4.21Ω.
Bimetal switch: manufactured by Sensata Technologies, product name: Thermal Protector 9700K21-215, contact gap: 1 mm, resistance of bimetal switch: 11.6 mΩ.
DC 48V/18 A (therefore the specific resistance=2.67Ω=Rf) which is double the maximum contact rating of the bimetal switch was applied, and the wave profile of the current (current flowing through the bimetal switch) and the wave profile of the voltage (voltage between the both ends of the bimetal switch, i.e. the voltage drop across the bimetal switch) upon such application were measured with the ammeter A and the voltmeter V incorporated into the circuit shown in
From
Except that the commercially available PTC component to be used was changed to another PTC component as shown below, Example 1 was repeated:
PTC component: manufactured by Tyco Electronics Raychem, product name: RXE025, basic resistance: 1.5Ω, resistance after activation: 2.31Ω.
From
Except that the commercially available PTC component to be used was changed to a further PTC component as shown below, Example 1 was repeated:
PTC component: manufactured by Tyco Electronics Raychem, product name: RXE040, basic resistance: 0.67Ω, resistance after activation: 1.02Ω.
From
Except that the commercially available PTC component to be used was changed to a further PTC component as shown below and also that the rated voltage/current was 48 V-DC/20 A (thus, the specific resistance was 2.4Ω), Example 1 was repeated:
PTC component: manufactured by Tyco Electronics Raychem, product name: RXE135, basic resistance: 0.18Ω, resistance after activation: 0.3Ω.
In this Example also, the current and voltage measurement results were obtained which were similar to those of Example 3. Therefore, the circuit protection device in the present Example performed its function.
Except that the commercially available PTC component to be used was changed to a further PTC component as shown below, Example 1 was repeated:
PTC component: manufactured by Tyco Electronics Raychem, product name: RXE020, basic resistance; 1.8Ω, resistance after activation: 2.82Ω.
In this Example also, the current and voltage measurement results were obtained which were similar to those of Example 3. Therefore, the circuit protection device in the present Example performed its function.
The results of the above Examples are summarized in the following Table 1:
The circuit protection device according to the present invention can reduce the possibility of the arcing occurrence and the welding occurrence in the proximity of the contacts of the bimetal switch.
As known to those skilled in the art, the bimetal switch is a switch to act such that its contacts open due to heat, and comprises a bimetal element which is made of a heat sensitive material and at least one pair of mechanical contacts. Therefore, it would be understood by those killed in the art according to the above descriptions that other mechanical switch having contacts such as a relay (particularly an electromagnetic relay) can be used in place of the bimetal switch in the circuit protection device according to the present invention.
Thus, in the broadest scope of the present invention, the present invention provides a circuit protection device comprising a mechanical switch having contacts (for example, a relay, a bimetal switch or the like) and a PTC component, characterized in that:
the mechanical switch having contacts and the PTC component are electrically connected in parallel; and
the PTC component has a resistance after its activation that is equal to or less than 1.1 times (i.e. 1.1×) as large as a specific resistance of an electrical circuit to which the circuit protection device is to be incorporated, which specific resistance is calculated based on a rated voltage and a rated current of the circuit, according to the following equation (1):
Rated Voltage/Rated Current=Specific Resistance (1).
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-068813 | Mar 2007 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/054394 | 3/11/2008 | WO | 00 | 4/26/2010 |
