CN212231092U

CN212231092U - Battery pack wrong connection prevention protection device

Info

Publication number: CN212231092U
Application number: CN202021607457.6U
Authority: CN
Inventors: 张兰芳; 张亚; 姚丹丹
Original assignee: Hefei Boas Battery Technology Co Ltd
Current assignee: Hefei Boas Battery Technology Co Ltd
Priority date: 2020-08-05
Filing date: 2020-08-05
Publication date: 2020-12-25
Anticipated expiration: 2030-08-05

Abstract

The utility model discloses a battery pack mistake-proofing protection device, which at least comprises a driving power supply, a power supply control unit, a plurality of negative pressure induction units and a switch unit, wherein each connecting branch is connected with one switch unit in series, and the driving power supply is connected with the switch unit of each branch and is used for outputting a driving signal to switch on the switch unit or closing the output to cut off the switch unit; two ends of each single battery are connected with a negative pressure sensing unit in parallel, and the negative pressure sensing unit is used for detecting negative pressure and outputting a sensing signal; the power supply control unit is connected with each negative pressure sensing unit and used for outputting a closing enable signal to enable the driving power supply to close and output when any sensing signal is received, otherwise, the power supply control unit outputs the enable signal to enable the driving power supply to keep outputting. Adopt the technical scheme of the utility model, all concatenate the switch unit at every branch road, arbitrary quantity battery connecting wire connects the mistake, and all branch road switches are whole to be ended, thoroughly breaks off the contact between application circuit and the battery, can effectively protect the application circuit.

Description

Battery pack wrong connection prevention protection device

Technical Field

The utility model relates to a power battery application system especially relates to a group battery prevents connecing wrong protection device.

Background

Because the single voltage and capacity of the storage devices such as storage batteries and super capacitors and the power generation devices such as photovoltaic systems (for convenience of explanation, batteries and battery packs are used for replacement in the following) are low, the single voltage and capacity are difficult to be directly used in large systems, and in practical application, a plurality of batteries are often required to be connected in series to improve the voltage, and a plurality of batteries are often connected in parallel to improve the capacity.

Because the production environment, the process parameters, the raw materials and the like are difficult to be completely consistent, each produced single battery has differences. And the difference between the unit cells may be enlarged as time goes by due to the difference in use environment. Due to the chemical characteristics, each single battery has a safe operation range including parameters such as voltage, current, temperature, power, etc., and the operation range of each parameter greatly affects the service life of the battery, and in order to be safe and reliable and to extend the service life of the battery, each electrical performance parameter of the battery pack must be managed, and a dedicated Battery Management System (BMS) is required. Of these electrical parameters, the cell voltage is particularly important and must be limited to a reasonable range. After the single batteries are connected in series, the whole battery pack is charged and discharged together, the current or ampere hours passing through each battery are the same, and due to the difference between the single batteries, the situation that the single battery is fully charged firstly and the single battery is discharged firstly inevitably occurs. In order to ensure safety and battery life, the entire battery pack must be stopped from being charged as long as one battery is fully charged, and the entire battery pack must be stopped from being discharged as long as one battery is discharged, so the BMS must monitor each cell in real time. Due to the fact that the number of battery nodes is large, in the assembling process of an actual battery system, the phenomenon of wrong connection can occur inevitably when a wiring harness is connected, and particularly in application occasions where battery charging and discharging equipment, battery detection equipment and the like need to be connected and disconnected frequently.

Referring to fig. 1, a schematic diagram of a battery pack connection cord is shown, wherein the connection cord B1 is connected to the connection cord B2 in reverse. Since general electronic circuits cannot bear negative voltage, in order to ensure safe and reliable operation, the negative voltage generated by misconnection of the battery connecting wire must be protected before being connected to the application circuit.

In order to solve the problem of wrong connection of the connecting wires of the battery pack, the most common solutions in the prior art are as follows: a diode is connected in series at the input end of the battery connection line as shown in fig. 2. When the positive electrode and the negative electrode of the input end are correctly connected, the diode D1 is conducted; when the positive electrode and the negative electrode of the input end are connected wrongly, namely reversely, the diode D1 is blocked, the input voltage is completely dropped on the D1, and the application circuit at the rear part cannot see the negative voltage, so that the protection effect is achieved.

However, the technical scheme still has the following technical defects:

firstly, the diode is conducted to have forward voltage drop, and the voltage of the input end of the application circuit cannot truly reflect the terminal voltage of the battery; meanwhile, the power consumption of the diode is large, and the diode can generate heat in large-current application to influence the stability of a system;

secondly, in the BMS, the current generally needs to flow in both directions, such as a bidirectional active balancing circuit of the battery, and the diode has unidirectional conductivity and cannot be applied in the occasion;

in addition, the technical scheme is only suitable for a single-string (two connecting wires) battery, and in the occasion of multiple-string batteries, negative voltage still exists when the connecting wires are connected in a wrong way. Referring to fig. 3, in the case where the connection lines B3 and B1 are misconnected, although D2 and D3 are blocked, there must be a load, i.e., each Batn (n) in the application circuit>0) There is always a resistance between node and Bat0, only the applied circuit has different load size, so the capacitance C₂And C₃The terminal voltage of the circuit still has negative value, and the subsequent application circuit is damaged.

Another solution in the prior art is to connect a P-MOS transistor (or an N-MOS transistor at the negative input end) in series with the positive input end of the battery connection line, as shown in fig. 4. When the positive and negative electrodes of the input end are correctly connected, the gate voltage of the PMOS Q1 is lower than the source voltage, Q₁Conducting; when the positive and negative electrodes of the input end are connected wrongly, namely are connected reversely, the gate voltage of the PMOS Q1 is higher than the source voltage, and Q is₁Shut down and input voltage all drops to Q₁In addition, the application circuit at the rear can not see the negative voltage, thereby playing a role in protection.

However, the technical scheme still has the following technical problems:

firstly, the driving voltage of a branch switch (MOS) is taken from a single battery, and the single battery has a low voltage, which makes the starting difficult, especially for a lead-acid battery, a lithium titanate battery, a nickel-hydrogen battery, a super capacitor, etc. of a low-voltage platform, which are difficult to apply. Even the lithium battery with higher platform voltage cannot be applied in the full SOC range, even if the lithium battery can be started, the on-resistance is also higher, and during overcurrent, the loss and the line voltage drop can be increased. In the application occasions with more battery strings, the withstand voltage requirement of the MOS used by each branch switch is higher, the gate starting voltage threshold of the MOS transistor is also higher, and the problem of difficult starting is more serious;

secondly, the technical scheme is also only suitable for a few paths, and when the number of the battery strings is more (more than or equal to 3), negative voltage still occurs when the battery is connected in a wrong way as shown in fig. 5, and a subsequent application circuit is damaged. Such as: b1 and B4 are in wrong connection, Q2 and Q4 are blocked, but Q1 and Q3 are still conducted, so that the voltage of a battery (B4-B3) is added between Bat1-Bat3, and Bat1 is higher than that of Bat3 by the voltage of one battery, and a subsequent application circuit is damaged; meanwhile, due to the fact that negative pressure occurs between the Bat3 and the Bat1, a subsequent application circuit is short-circuited after being damaged, so that a loop is formed by the B4-Q1-application circuit (short-circuited after being damaged) -Q3-B3-B4, the (B4-B3) battery is short-circuited, and a dangerous situation occurs. Even if the application circuit is not damaged because of protection, the application circuit typically has a diode in the anti-parallel integrated between the battery nodes, also shorting the battery (B4-B3) through the two diodes D2, D3.

Therefore, it is necessary to provide a technical solution to solve the technical problems of the prior art.

SUMMERY OF THE UTILITY MODEL

In view of this, it is necessary to provide a protection device for preventing misconnection of a battery pack, in which each branch is serially connected with a switch unit, any number of battery connection lines are misconnected, all branch switches are turned off, and the connection between an application circuit and a battery is completely disconnected, so that the application circuit can be effectively protected.

In order to solve the technical problem existing in the prior art, the technical scheme of the utility model as follows:

a battery pack misconnection prevention protection device at least comprises a driving power supply, a power supply control unit, a plurality of negative pressure induction units and switch units, wherein each connecting branch is connected with one switch unit in series, the switch units are controlled by the driving power supply, the driving power supply is controlled by the power supply control unit and is connected with the switch units of each branch and used for outputting driving signals to enable the switch units to be switched on or switched off to enable the switch units to be switched off;

two ends of each single battery are connected with a negative pressure sensing unit in parallel, and the negative pressure sensing unit is used for detecting negative pressure and outputting a sensing signal; the power supply control unit is connected with each negative pressure sensing unit and used for outputting a closing enable signal to enable the driving power supply to close and output when any sensing signal is received, otherwise, outputting the enable signal to enable the driving power supply to keep outputting.

As a further improvement, the switching unit is implemented by using a MOS device.

As a further improvement scheme, the power supply control unit is realized by adopting an MOS tube, a triode, a comparator or an AND gate.

As a further improvement scheme, the negative pressure induction unit is realized by adopting an optical coupler, a PhotoMOS or a relay.

As a further improvement, the power supply control unit is integrated in the driving power supply.

As a further improvement, the switching unit at least comprises a first MOS transistor Q1, a first resistor R1 and a second resistor R2, and the source and the drain of the first MOS transistor Q1 are connected in series in the connection branch; the driving power supply outputs a driving signal as a driving voltage V_CAnd the other end of the first resistor R1 is connected with the grid of the first MOS transistor Q1 and one end of the second resistor R2And the other end of the second resistor R2 is connected with the source electrode of the first MOS transistor Q1, and the drain electrode of the first MOS transistor Q1 is connected with the battery end.

As a further improvement, the power control unit at least includes a second MOS transistor Q2 and a third resistor R3, one end of the third resistor R3 is connected to the power supply terminal VCC, the other end of the third resistor R3 is connected to the gate of the second MOS transistor Q2 and to the output terminal of the negative voltage sensing unit, the drain of the second MOS transistor Q2 is connected to the enable terminal of the driving power supply, and the source of the second MOS transistor Q2 is grounded.

As a further improvement, the negative pressure sensing unit at least comprises an optical coupler device U1 and a fourth resistor R4, an input end of the optical coupler device U1 is connected in parallel with two ends of the single battery, and when a connecting line of the single battery is reversely connected, an output end of the optical coupler device U1 is conducted to generate a sensing signal; the fourth resistor R4 is connected in series in an input loop of the optical coupler U1.

As a further improvement, the fourth resistor R4 is a PTC resistor.

Compared with the prior art, the utility model discloses following technological effect has:

1. each branch is connected with a switch unit in series, any number of battery connecting wires are connected in a wrong way, all branch switches are completely cut off, the connection between the application circuit and the battery is thoroughly disconnected, and the application circuit can be effectively protected;

2. the driving voltage of the branch switch is from a completely independent driving power supply, is irrelevant to the type and the SOC state of the battery, can be quickly started and conveniently adjusted, and is suitable for batteries of different types and different SOC states of the battery; meanwhile, the driving voltage of the branch switch is only determined by the driving power supply and can be properly adjusted within a safety range, so that the on-resistance of the branch switch is reduced, and the loss and the temperature rise are reduced;

3. the switch unit adopts an MOS switch, allows current to flow in two directions and has no limitation on the type of a subsequent application circuit; the conduction voltage drop of the branch switch is small, and the rear-end application circuit can obtain the real battery voltage;

4. the voltage-resistant protection circuit can be theoretically applied to battery packs of any string number, as long as the total voltage of the battery packs is within the safe voltage-resistant range of the branch switch, and the voltage-resistant protection circuit can be conveniently expanded.

Drawings

Fig. 1 is a schematic diagram of a connection line of a battery pack in a wrong connection manner.

Fig. 2 is a schematic diagram of one of the prior art embodiments.

Fig. 3 is a schematic diagram of the reverse connection of multiple batteries in one embodiment of the prior art.

Fig. 4 is a schematic diagram of a second prior art embodiment.

Fig. 5 is a schematic diagram illustrating the reverse connection of a plurality of batteries according to a second embodiment of the prior art.

Fig. 6 is a schematic block diagram of the battery pack fault-connection-prevention protection device of the present invention.

Fig. 7 is a schematic circuit diagram of a preferred embodiment of the battery pack fault-connection-prevention protection device of the present invention.

Fig. 8 is a schematic circuit diagram of another embodiment of the switch unit of the present invention.

Fig. 9 is a schematic diagram of the present invention in a battery pack application system.

Fig. 10 is a schematic diagram illustrating the principle of the multiple batteries connected in reverse in the device of the present invention.

The following detailed description of the invention will be made in conjunction with the above-described drawings.

Detailed Description

The technical solution provided by the present invention will be further explained with reference to the accompanying drawings.

In an actual power battery system, a battery pack is formed by connecting a plurality of single batteries in series, each single battery is connected with an application circuit through an independent branch, and due to the fact that a plurality of connecting wires are arranged, the connection of a wiring harness is difficult to avoid, and the phenomenon of wrong connection can occur.

Referring to fig. 6, it is shown that the utility model relates to a battery pack prevents connecing wrong protection device's functional block diagram, it only shows the connection principle of battery cell, can expand and be applied to in the battery pack. The device at least comprises a driving power supply, a power supply control unit, a plurality of negative pressure induction units and a switch unit, wherein each connecting branch is connected with one switch unit in series; and once the connecting wire has the wiring error, the control switch unit is in a cut-off state, so that all branch switches are completely cut off, the connection between the application circuit and the battery is thoroughly disconnected, and the application circuit can be effectively protected.

The independent driving power supply is adopted to control the switch unit, the voltage of the battery is irrelevant, the state of the switch unit is completely determined by the output of the driving power supply, and the switch unit can be quickly started and closed as long as reasonable driving voltage is set. Meanwhile, the output voltage of the driving power supply controls all the switch units simultaneously, and the purpose of synchronously controlling all the branches is achieved.

The power supply control unit is arranged to control the driving power supply, and the output end of the power supply control unit is used for controlling the enabling end of the driving power supply.

The two ends of each single battery are connected with a negative pressure sensing unit in parallel, when a connection line has a wiring error, the battery end can generate negative voltage, and the negative pressure sensing unit is used for detecting the negative pressure and outputting a sensing signal; the output end of each negative pressure sensing unit is connected with the power supply control unit, the power supply control unit outputs a closing enable signal to enable the driving power supply to close and output when receiving any sensing signal, otherwise, the power supply control unit outputs the enable signal to enable the driving power supply to keep outputting.

In the above technical scheme, the switch unit can be realized by using an MOS device; the power control unit may be implemented using MOS devices. The negative pressure induction unit can be realized by an optical coupler, a PhotoMOS or a relay.

In addition, the power supply control unit is integrated in the driving power supply, and when an actual circuit is designed, an enabling end is designed in the driving power supply and is directly connected with the induction signal.

Referring to fig. 7, a schematic circuit diagram of a preferred embodiment of the device for protecting a battery pack against misconnection according to the present invention is shown, wherein the switch unit at least includes a first MOS transistor Q1, a first resistor R1 and a second resistor R2, and a source and a drain of the first MOS transistor Q1 are connected in series in the connection branch; the first resistor R1 and the second resistor R2 are voltage dividing resistors and adjust the voltage of the grid of the first MOS transistor Q1; the driving power supply outputs a driving signal as a driving voltage V_CThe diode is connected with one end of a first resistor R1, the other end of the first resistor R1 is connected with the grid electrode of a first MOS transistor Q1 and one end of a second resistor R2, the other end of the second resistor R2 is connected with the source electrode of the first MOS transistor Q1, and the drain electrode of the first MOS transistor Q1 is connected with a battery end.

In a preferred embodiment, a first diode D1 is further disposed between the output terminal of the driving power supply and the first resistor R1, so as to prevent mutual interference between the branches.

In a preferred embodiment, a zener diode Z1 is connected between the gate and the source of the first MOS transistor Q1, and is used for protecting the gate voltage of the switching transistor within a safe range and preventing breakdown. Further, the zener diode Z1 may be replaced with a component having a voltage clamping function, such as a TVS or an ESD diode.

In fig. 7, the NMOS used for the first MOS transistor Q1 may be replaced by a PMOS (the driving power supply may be also changed to a negative power supply synchronously), or an active switch such as a power transistor, an IGBT, a GTO, or a relay. In addition, a bidirectional switch can be used for replacement, as shown in fig. 8, two switch tubes are connected in series, and the stability of switch control is further improved.

As shown in fig. 7, the power control unit at least includes a second MOS transistor Q2 and a third resistor R3, one end of the third resistor R3 is connected to the power supply terminal VCC, the other end of the third resistor R3 is connected to the gate of the second MOS transistor Q2 and to the output terminal of the negative voltage sensing unit, the drain of the second MOS transistor Q2 is connected to the enable terminal of the driving power supply, and the source of the second MOS transistor Q2 is grounded. The second MOS tube Q2 is a standard switch circuit, when in normal state, the grid electrode is in high level, the switch tube is conducted, and the driving power supply enables to output a driving signal; once the sensing signal comes, the gate of the second MOS transistor Q2 changes to low level, the switching transistor is turned off, the driving power supply is turned off and enabled, and the driving signal is not output. The second MOS transistor Q2 described above is an NMOS, and similarly, the second MOS transistor Q2 may be replaced by a PMOS, a comparator, an and gate, etc., as long as it can implement line and function and synchronously adjust the active level.

The negative pressure induction unit at least comprises an optocoupler U1 and a fourth resistor R4, wherein the input end of the optocoupler U1 is provided with a photodiode, the photodiode is reversely connected with two ends of the single battery, when the connecting line of the single battery is reversely connected, the diode is conducted to emit light to drive a photosensitive switching tube at the output end of the optocoupler U1 to be conducted, namely, an induction signal is generated, and a low level signal is generated in the circuit; the fourth resistor R4 is connected in series in the input loop of the optical coupler U1 and used for adjusting the loop current.

In the technical scheme, each input branch is connected with an NMOS tube in series to serve as a branch control switch, the on and off of the branch switch are controlled by the driving power supply, when the driving power supply enables (outputs driving voltage), the branch switch is turned on, and when the driving power supply does not enable (prohibits from outputting driving voltage), the branch switch is turned off. In practical application, the voltage difference (Vc-V +) between the output voltage Vc of the driving power supply and the input positive voltage V + of the battery is designed to be larger than the gate starting voltage of the branch switching tube. The driving power is also controlled by a switch tube Q2(NMOS), and a pull-up resistor R3 is connected to a gate of Q2, and is turned on by default. An optocoupler is connected in parallel between the input connecting wires of the batteries, the cathode of an optocoupler input diode is connected with the positive electrode of a power supply, and the anode of the diode is connected with the negative electrode of the power supply through a current limiting resistor R0. When the battery connecting wire is normally connected, the optical coupler is not conducted, the control switch Q2 is conducted, the driving power supply is enabled, each branch switching tube is connected, and the battery is normally connected with the application circuit. When the battery connecting line is reversely connected, the optocoupler is switched on, the switch tube Q2 is switched off, the driving power supply is disabled, all branch switch tubes are switched off together, and the application circuit is disconnected with the battery.

Referring to fig. 9, it is shown that the utility model discloses prevent connecing wrong protection device is applied to the functional block diagram of group battery, every battery connection branch road all establishes ties a switch (NMOS), every section battery all connects parallelly connected one and connects reverse detection circuitry with the input that the optical coupling realized, the output of all opto-couplers is parallelly connected at control switch Qc input gate electrode department to realize line and function, any opto-couplers or many detect out that the battery connects reversely, can all draw down control switch Qc's gate drive voltage, shielding drive power supply output.

When all the battery connecting lines are normally connected, the cathode voltages of all the optocoupler input diodes are higher than the anode voltages, the optocouplers are all turned off, and the branch switches are turned on, as shown in fig. 9. Any two connecting wires are reversely connected or more than two connecting wires are wrongly connected, the voltage seen by at least one diode at the input end of the optical coupler can be changed into positive, so that the optical coupler is conducted, and the output of the driving power supply is shielded.

Referring to fig. 10, two connecting lines B1 and B3 are connected in reverse, the input voltage seen by the optocouplers OP2 and OP3 becomes positive, the output of the optocouplers is turned on, the gate voltage of the control switch Qc is pulled down, the output of the driving power supply is shielded, the battery connecting branch switch is closed, the connection between the battery and the application circuit is isolated, and the application circuit is protected. Because the connection error of the battery pack connecting wires is random, two adjacent connecting wires can be connected in error, and the input voltage of the optical coupler input circuit (the optical coupler and the current-limiting resistor) is the voltage of a single battery; the connection of several batteries can be crossed, or the potentials of two or more connecting wires are far from each other, in extreme cases, the connecting wires are always positive and negative, and the input voltage seen by the detection circuit is the voltage of the whole battery pack. This voltage value is large when the number of battery strings is large. In order to be compatible with the wrong connection condition of the battery connecting wires with different numbers, the current limiting resistance value input by the optical coupler must be ensured: the optical coupler can be switched on by one battery voltage, and the input current of the n batteries is within the safe current range of the optical coupler. Theoretically, the current of the optical coupler input loop is controlled within the safe current range, and the method can be suitable for battery packs of any number in series.

In order to be able to adapt to the uncertainty that in practice there may be a misconnection from a single battery to a plurality of batteries, in a preferred embodiment, the fourth resistor R4 is a PTC resistor. The current-limiting resistor is selected to be of a PTC type, the resistance value is small at normal temperature, and the optocoupler can be switched on when one battery voltage is available; when the input voltage value that detection circuitry sees is very big, the current value is also very big, and big current value makes current-limiting resistor R0 generate heat, and the resistance grow, and the electric current diminishes to guarantee that input current is in the safety range of opto-coupler.

The above description of the embodiments is only intended to help understand the method of the present invention and its core ideas. It should be noted that, for those skilled in the art, without departing from the principle of the present invention, the present invention can be further modified and modified, and such modifications and modifications also fall within the protection scope of the appended claims.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A battery pack misconnection prevention protection device is characterized by at least comprising a driving power supply, a power supply control unit, a plurality of negative pressure induction units and switch units, wherein each connecting branch is connected with one switch unit in series, the switch units are controlled by the driving power supply, the driving power supply is controlled by the power supply control unit and is connected with the switch units of each branch and used for outputting driving signals to enable the switch units to be switched on or switched off to enable the switch units to be switched off;

2. The battery pack fault-connection-prevention protection device as claimed in claim 1, wherein the switching unit is implemented by using a MOS device.

3. The battery pack fault-connection-prevention protection device according to claim 1 or 2, wherein the power control unit is implemented by using a MOS (metal oxide semiconductor) tube, a triode, a comparator or an AND gate.

4. The battery pack fault-connection-prevention protection device according to claim 1 or 2, wherein the negative pressure induction unit is implemented by an optical coupler device, a PhotoMOS or a relay.

5. The battery pack fault-connection prevention protection device according to claim 1 or 2, wherein the power control unit is integrally provided in the driving power supply.

6. The battery pack fault-tolerant protection device of claim 2, wherein the switch unit at least comprises a first MOS transistor Q1, a first resistor R1 and a second resistor R2, and a source electrode and a drain electrode of the first MOS transistor Q1 are connected in series in the connection branch; the driving power supply outputs a driving signal as a driving voltage V_CThe high-voltage power supply circuit comprises a first resistor R1, the other end of the first resistor R1 is connected with the grid of a first MOS transistor Q1 and one end of a second resistor R2, the other end of the second resistor R2 is connected with the source of a first MOS transistor Q1, and the drain of a first MOS transistor Q1 is connected with a battery end.

7. The battery pack fault-connection-prevention protection device as claimed in claim 6, wherein the power control unit at least comprises a second MOS transistor Q2 and a third resistor R3, one end of the third resistor R3 is connected to the power supply terminal VCC, the other end of the third resistor R3 is connected to the gate of the second MOS transistor Q2 and to the output terminal of the negative voltage induction unit, the drain of the second MOS transistor Q2 is connected to the enable terminal of the driving power supply, and the source of the second MOS transistor Q2 is grounded.

8. The battery pack fault-connection-prevention protection device as claimed in claim 7, wherein the negative pressure sensing unit at least comprises an optical coupler device U1 and a fourth resistor R4, an input end of the optical coupler device U1 is connected in parallel with two ends of a single battery, and when a connecting line of the single battery is reversely connected, an output end of the optical coupler device U1 is conducted to generate a sensing signal; the fourth resistor R4 is connected in series in an input loop of the optical coupler U1.

9. The battery pack fault-connection-prevention protection device as claimed in claim 8, wherein the fourth resistor R4 is a PTC resistor.