Twenty-four pulse wave rectification fault detection circuit based on series resonance power supply system

Abstract

The utility model relates to the technical field of twenty-four pulse wave rectification fault detection, and specifically relates to a twenty-four pulse wave rectification fault detection circuit based on a series resonance power supply system. The detection circuit comprises a signal acquisition circuit and a thyristor fault detection circuit. The signal acquisition circuit comprises a voltage acquisition circuit and a current acquisition circuit. The voltage acquisition circuit and the current acquisition circuit are connected with a current diverter and a direct-current voltage sampling part of the 24-pulse rectification circuit; and the thyristor fault diagnosis circuit is connected in parallel with a thyristor of the twenty-four pulse wave rectification circuit. Real-time and accurate fault device positioning can be realized, the anti-interference performance of the circuit is enhanced, and the fault detection precision is improved.

Description

TECHNICAL FIELD

[0001] The utility model relates to twenty four pulse wave rectification fault detection technical field, concretely relates to a kind of twenty four pulse wave rectification fault detection circuit based on series resonance power supply system. TECHNICAL BACKGROUND

[0002] Twenty four pulse rectifier is the key equipment in many industrial production and electrical drive systems, and the performance requirements of these systems to the rectifier are extremely high, and any fault can lead to system downtime or performance degradation. In series resonance type induction heating power supply system, twenty four pulse rectification fault diagnosis is of great significance to ensure the safe operation of power supply system, reduce economic loss, improve energy utilization efficiency, protect power grid safety and promote technological innovation and development.

[0003] When a fault occurs in the controllable rectifier device (such as thyristor open circuit or thyristor aging, etc.), the rectifier circuit with disease will lead to serious distortion of the rectified waveform, and the ripple will increase, which will bring adverse effects to the load and power grid.

[0004] Due to the complex structure of twenty four pulse rectifier, and a large amount of harmonic and interference signals will be generated in the working process, for the key components such as thyristor, the traditional method usually needs to measure the resistance value of each component in the rectifier circuit such as rectifier diode, capacitor, etc. after power off to determine whether there is open circuit, short circuit or component damage. This makes it difficult to monitor the running state of the rectifier in real time, and due to the lack of effective real-time monitoring means, the fault may not be discovered and handled in time, thus aggravating the severity of the fault. SUMMARY

[0005] The utility model provides a kind of twenty four pulse wave rectification fault detection circuit based on series resonance power supply system, it can realize real-time, accurate fault device positioning, enhance circuit anti-interference, improve fault detection precision.

[0006] The technical scheme adopted by the utility model is: a kind of twenty four pulse wave rectification fault detection circuit based on series resonance power supply system, characterized by, the detection circuit includes signal acquisition circuit and thyristor fault detection circuit, the signal acquisition circuit includes voltage acquisition circuit and current acquisition circuit;

[0007] Voltage acquisition circuit and current acquisition circuit are connected with the current shunt of twenty four pulse wave rectification circuit and direct current voltage sampling part;

[0008] The DCV+ end and the DCV- end of the direct current differential signal in the voltage collection circuit are connected to ground through resistance R10 and resistance R11, resistance R11 is connected to the same phase end of amplifier U10A in series with resistance R12, one end of resistance R13 is connected to the opposite phase input end of amplifier U10A, and the other end is connected to ground, the output end of amplifier U10A is connected to the input end IN1 of isolation IC U11 in series with resistance R14, the IN2 end of the isolation IC U11 is connected to the IN3 end of the isolation IC U11 through a wire, the IN4 end of the isolation IC U11 is connected to ground, the OC2 end of the isolation IC U11 is connected to a power supply, the E1 end of the isolation IC U11 is connected to resistance R13 through a wire, the E2 end of the isolation IC U11 is connected to ground through resistance R15 and resistance R16, and the E2 end of the isolation IC U11 is outputted through resistance R16;

[0009] The DCI+ end of the direct current differential signal of the current collection circuit is connected to the same phase input end of amplifier OPA2277 through resistance R20, the DCI- end of the direct current differential signal is connected to ground and then connected to the opposite phase input end of amplifier OPA2277 through resistance R21, the output end of amplifier OPA2277 is connected to the same phase input end of amplifier U20B after being connected to resistance R23, the output end of amplifier U20B is connected to the opposite phase input end of amplifier U20B after being connected to capacitor C25, one end of resistance R24 is connected to the opposite phase input end of amplifier U20B, and the other end is connected to ground; amplifier U20B is connected to isolation IC U21 input end IN1 after being connected to resistance R25, the IN2 end of isolation IC U21 is connected to the IN3 end of isolation IC U21 through a wire, the IN4 end of isolation IC U21 is connected to ground, the E1 end of isolation IC U21 is connected to the negative electrode of capacitor C25, the E2 end of isolation IC U21 is outputted through resistance R15 and resistance R16, and the OC2 end of isolation IC U21 is connected to a power supply;

[0010] The thyristor fault diagnosis circuit is connected in parallel with the thyristor of the twenty-four-pulse rectifier circuit, the K1 end of the thyristor is connected to two reverse-parallel light-emitting diodes LS1 and LV2 in sequence after being connected to current-limiting resistor R11, resistance R12, resistance R13, resistance R14, resistance R15, resistance R16, resistance R17, and resistance R18, and then connected to the A1 end of the thyristor, the A1 end of the thyristor is connected to the input end A end of photo-coupler HW11 and the input end K end of photo-coupler HW12;

[0011] The collector C end of photo-coupler HW11 is connected to a power supply through resistance RS12, the emitter E end of photo-coupler HW11 is connected to the base of triode QS1 and connected to ground through resistance RS13, and the emitter of triode QS1 is connected to a power supply through resistance RS11;

[0012] The collector C end of the optical coupler HW12 is connected with the power supply through the resistor RS52, and the emitter E end of the optical coupler HW12 is connected with the base of the triode QS5 and grounded through the resistor RS53.

[0013] Further, the OC2 end of the isolation IC U11 is connected with the 5V power supply.

[0014] Further, the OC2 end of the isolation IC U21 is connected with the 5V power supply.

[0015] Further, the emitter of the triode QS1 is connected with the 5V power supply through the resistor RS11.

[0016] Further, the collector C end of the optical coupler HW12 is connected with the 5V power supply through the resistor RS52.

[0017] Further, the collector C end of the optical coupler HW11 is connected with the 5V power supply through the resistor RS12.

[0018] Further, the emitter of the triode QS1 is connected with the 5V power supply through the resistor RS51.

[0019] Further, the resistances R11, R12, R13, R14, R15, R16, R17 and R18 are all 20K.

[0020] Compared with the prior art, the utility model has the advantages that:

[0021] 1) The utility model can sample and signal process the direct current and direct voltage, detect the rectifier circuit working state, calculate the trigger angle of each thyristor in the circuit, detect the open circuit and short circuit fault and accurately locate the fault thyristor.

[0022] 2) The utility model carries out FFT spectrum analysis to the collected fault voltage and current signal through the signal processing circuit and carries out normalization processing to the data, extracts the fault characteristic vector, utilizes the PCA master and slave element analysis method to preprocess the data, establishes multiple Gaussian models according to the distribution of characteristic dimension, comprehensively captures the fault characteristics of equipment, reduces the detection time and improves the detection precision.

[0023] 3) The optical coupler in the thyristor fault detection circuit of the utility model realizes electrical isolation and signal regulation, reduces noise and interference and can improve the stability and reliability of open circuit and short circuit detection.

[0024] 4) The utility model can realize the rapid fault positioning of the system, reduce the detection time and improve the detection precision through the fault diagnosis method matched with the device, and realizes the visual fault monitoring by transmitting the rectifier circuit working state to the terminal in real time.

[0025] 5) The utility model discloses a rectifier fault detection circuit, which detects the trigger angle of thyristor and faults such as short circuit and open circuit, wherein the cooperation of the two groups of photocouplers and current limiting resistors realizes electrical isolation and signal regulation, reduces noise and interference, and improves the stability and reliability of open circuit and short circuit fault detection. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 It is a system overall block diagram.

[0027] Figure 2 It is a twenty-four pulse induction heating power supply system topology diagram.

[0028] Figure 3 It is a signal acquisition circuit.

[0029] Figure 4 It is a thyristor fault detection circuit schematic diagram.

[0030] Figure 5 It is a single thyristor trigger angle calculation process.

[0031] Figure 6 It is a direct current voltage and current fault diagnosis method flow. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0033] The utility model provides a kind of twenty-four pulse rectifier fault detection circuit based on series resonant power supply system, as shown in Figure 3 And Figure 4 As shown, it includes signal acquisition circuit and thyristor fault detection circuit, the signal acquisition circuit includes voltage acquisition circuit and current acquisition circuit;

[0034] Voltage acquisition circuit and current acquisition circuit are connected with the current shunt of twenty-four pulse rectifier circuit and direct current voltage sampling part;

[0035] The DCV+ end and the DCV- end of the DC voltage differential signal in the voltage collection circuit are connected to ground through the voltage division of the resistor R10 and the resistor R11, and the resistor R11 is connected to the non-inverting input terminal of the amplifier U10A in series with the resistor R12. The resistor R13 is connected to the inverting input terminal of the amplifier U10A at one end and connected to ground at the other end; the output of the amplifier U10A is connected to the input terminal IN1 of the isolation IC U11 in series with the resistor R14, the IN2 end of the isolation IC U11 is connected to the IN3 end of the isolation IC U11 through a wire, the IN4 end of the isolation IC U11 is connected to ground, the OC2 end of the isolation IC U11 is connected to a 5V power supply, the E1 end of the isolation IC U11 is connected to the resistor R13 through a wire, the E2 end of the isolation IC U11 is connected to ground through the resistor R15 connected by a wire, and the E2 end of the isolation IC U11 is outputted through the resistor R16, as shown in Figure 3 .

[0036] The DCI+ end of the DC current differential signal in the current collection circuit is connected to the non-inverting input terminal of the amplifier OPA2277 through the resistor R20, the DCI- end is connected to ground and then connected to the inverting input terminal of the amplifier OPA2277 through the resistor R21, the output end of the amplifier OPA2277 is connected to the non-inverting input terminal of the amplifier U20B in series with the resistor R23, the output end of the amplifier U20B is connected to the inverting input terminal of the amplifier U20B in series with the capacitor C25. The resistor R24 is connected to the inverting input terminal of the amplifier U20B at one end and connected to ground at the other end. The amplifier U20B is connected to the input terminal IN1 of the isolation IC U21 in series with the resistor R25, IN2 and IN3 are directly connected, IN4 is connected to ground, the E1 end is connected to the negative electrode of the capacitor C25, and the E2 end is outputted through the resistors R15 and R16; the OC2 end of the isolation IC U21 is connected to a power supply, as shown in Figure 3 .

[0037] The thyristor fault diagnosis circuit is connected in parallel with the thyristors of the twenty-four-pulse rectifier circuit, the K1 end of the thyristor is connected to the current-limiting resistors R11, R12, R13, R14, R15, R16, R17 and R18 with a resistance of 20K in sequence, then connected to two reverse-parallel light-emitting diodes LS1 and LV1, and then connected to the A1 end of the thyristor. The A end of the input terminal of the optocoupler HW11 connected to the K end of the input terminal of the optocoupler HW12. The collector C end of the optocoupler HW11 is connected to a 5V power supply through the resistor RS12, the emitter E end is connected to the base of the transistor QS1, and is connected to ground through the resistor RS13. The emitter of the transistor QS1 is connected to a 5V power supply through the resistor RS11. The collector C end of the optocoupler HW12 is connected to a 5V power supply through the resistor RS52, the emitter E end is connected to the base of the transistor QS5, and is connected to ground through the resistor RS53. The emitter of the transistor QS1 is connected to a 5V power supply through the resistor RS51, as shown in Figure 4 .

[0038] The utility model discloses a structure for detecting twenty-four pulse rectifier circuit in series resonance induction heating power supply system carries out signal acquisition and thyristor fault detection. Figure 2 As shown in the figure, the series resonance twenty-four pulse wave induction heating power supply is composed of three-phase electric input, twenty-four pulse rectifier circuit, filter circuit, inverter circuit and series resonance load. Four groups of three-phase alternating current output by transformer are connected to three-phase air break QS1, QS2, QS3 and QS4 respectively.

[0039] Four groups of diode and RC parallel circuit are arranged at the output of twenty-four pulse rectifier circuit, which optimizes the circuit performance, improves the circuit stability and provides stable input signal for DC voltage and current sampling. It comprises four three-phase controllable rectifier circuits, which are composed of first rectifier thyristor VT11-VT16, second rectifier thyristor VT21-VT26, third rectifier thyristor VT31-VT36 and fourth rectifier thyristor VT41-VT46; the rectifier output is connected to large inductance L1 and L2 and large capacitor C1 for filtering, and then connected to thyristor VT51, VT52, VT53 and VT54 to form a single-phase H-bridge inverter circuit, and the inverter output is connected to inductance L3, capacitor C1, inductance L5 and inductance L4 in sequence; the twenty-four pulse rectifier circuit is provided with four three-phase controllable thyristor rectifier circuits, and the first three-phase voltage is connected to six rectifier thyristors VT11, VT12, VT13, VT14, VT15 and VT16, and each thyristor is provided with a resistance-capacitance RC absorption circuit, that is, the thyristor is connected in parallel with the corresponding resistance and capacitor, and the output is connected in parallel with diode D1, capacitor C1 and resistance R1.

[0040] The filter circuit is connected in parallel with large capacitor C1 after the rectifier output, and the current shunt FL is connected in series after the filter circuit. The current shunt after the filter circuit can calculate the current value in the circuit by measuring the voltage drop of the shunt, which provides a reliable signal source for fault analysis of the rectifier circuit, reduces the fault misjudgment rate, and thus realizes high-precision fault diagnosis result. According to the influence of the fault state of the thyristor on the output DC voltage and current waveform of the circuit, the fault state voltage waveform is obtained. The fault detection unit of the twenty-four pulse DC power supply series resonance power supply for fault diagnosis through Gaussian mixture model algorithm can more comprehensively capture the fault characteristics of the equipment, reduce the diagnosis time and enhance the reliability of the evaluation result.

[0041] The series resonance load is composed of inductance L3, capacitor C2, inductance L5 and inductance L4.

[0042] Figure 1The utility model discloses a general structure block diagram, and the induction heating power supply system includes three-phase twenty-four pulse rectifier circuit, filter circuit, inverter circuit and series resonance load, and the utility model mainly carries out fault detection to twenty-four pulse rectifier thyristor, and signal acquisition is carried out to rectifier output and adopts the fault diagnosis method of matching to make real-time accurate positioning and diagnosis to rectifier circuit thyristor fault state.

[0043] Figure 2 As shown in the series resonance type power supply topology structure diagram, 380V three-phase alternating current is input into twenty-four pulse rectifier circuit through three-phase transformer, and the output DC voltage is processed through the filter link and then inverts, and the load is LC series resonance type. The utility model mainly carries out signal acquisition and thyristor fault detection to rectifier circuit. Three-phase alternating current 380V voltage is converted into 4 groups of alternating current input voltage with 30 degrees phase difference through phase-shifting transformer and is rectified respectively through three-phase full-bridge, and the input voltage is provided for the inverter link after filtering. The DC voltage after rectification and the DC current after LC filtering are sampled, and the current sampler is used to sample at the rectifier negative pole, and the signal is 0-75mV.

[0044] Figure 3 The circuit principle diagram of DC voltage and current collection in STM32. The circuit power supply is 12V DC power supply, and is converted into 12V, 12V, 5V through 3 groups of DC-DC isolation conversion, and is used to power the DC voltage processing circuit, DC current processing circuit and CPU. Since the sampling DC signal is 0-75mV, the signal strength is low, and the operational amplifier adopts bipolar power supply, and the negative power supply is generated by ICL7660. The DC sampling voltage is input by DCV+, DCV-, is converted to the appropriate range range after voltage division and isolation and obtains DC_V, and is input to the CPU. Similarly, the DC sampling current is input by DCI+, DCI-, is amplified and isolated to obtain DC_I, is converted to the appropriate range after conversion and is input to the CPU. The CPU carries out data processing and analysis to the input signal, and the specific fault diagnosis operation method is as shown in the attached Figure 4 , and the analysis result is output to the terminal in real time through the serial port. When there is a fault in the circuit, an ALM signal is output to facilitate the warning.

[0045] Figure 4 The thyristor fault detection circuit, A1 and K1 are connected at the A pole and the K pole of the thyristor, and when the forward voltage drop exists between the two ends of the thyristor, the optocoupler PC817 is turned on, and the PH1 input is low level, and when the thyristor is turned on or bears the reverse voltage drop, the optocoupler is not turned on, and the PH1 input is high level. The low level pulse width length of PH1 can be obtained by collecting, and the forward cut-off time of the thyristor is converted into the angle, and the trigger angle of the thyristor is obtained. The deviation of the measured trigger angle and the set trigger angle is calculated, and when the deviation exceeds the threshold value, the thyristor trigger is considered to be abnormal, and when the thyristor short circuit fault occurs, the LV1 is bright, and when the thyristor open circuit fault occurs, the LS1 is bright.

[0046] Figure 5 The thyristor trigger waveform diagram, 0 time AC voltage zero crossing, due to the characteristics of the optocoupler PC817, at t1, the voltage exceeds the on voltage, the optocoupler PC817 starts to conduct, at this time, PH1 is low, when the thyristor gate trigger pulse arrives at t2, the thyristor conducts, PH1 and K terminal circuit are short-circuited by the thyristor, and PH1 becomes high, therefore, the low level pulse width of PH1 is converted into an angle, that is Therefore, in practical application, the thyristor trigger angle θ is:

[0047]

[0048] Wherein, u P Is the sine AC voltage peak value.

[0049] Eight current limiting resistors are arranged in series in the circuit, the total resistance of the load is increased, and the current is reduced, which is crucial to protect the circuit components from excessive current damage. When the current is too large, the current limiting resistor can limit the flow of current to prevent the components from being damaged due to overheating. The current limiting resistor can share a part of the voltage, so that other circuit components can normally work within a safe voltage range. When selecting the current limiting resistor, the current is limited to the maximum current i max of the optocoupler PC817, and the resistance value of a single current limiting resistor is:

[0050]

[0051] The power of a single current limiting resistor is:

[0052]

[0053] Figure 6 The utility model is matched with a fault diagnosis method, builds a fault current voltage collection platform, collects original data in a circuit to form a data set, pretreats original data, and the original fault data contains data under normal and fault conditions. Frequency deviation and original waveform are distinguished, and only when the two are the same, normal data is judged, otherwise, fault data is judged. Thus, fault and normal data are distinguished. The DC voltage and current signals are subjected to FFT transformation, the amplitude obtained after voltage signal normalization is taken as fault characteristic data according to the analysis result, and the frequency domain characteristics under normal or fault arc are extracted. 4) the extracted characteristics form a characteristic matrix, and the characteristics are reduced in dimension according to the PCA method. The reduced data are input into a multi-Gaussian mixed model to detect whether a fault exists in the circuit.

[0054] The utility model discloses structure through the mode of software and hardware combination, makes the comprehensive effective fault detection in rectifier circuit part, and this combined detection unit has the advantages such as reducing fault diagnosis time, improve detection precision, can accurately identify fault type and position, to take measures in time to repair and maintain.

[0055] In the foregoing description, many specific details are set forth in order to provide a thorough understanding of the present application. However, the present application can be practiced in a variety of ways in which there are other specific details, and the present application is not limited to the details described herein.

[0056] The above only is the preferred embodiment of the present application, and is not used to limit the present application, and for the person skilled in the art, the present application can have various changes and changes. Any modification, equivalent replacement, improvement etc. that is made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A twenty-four pulse rectifier fault detection circuit based on series resonant power supply system, characterized by, The detection circuit comprises a signal acquisition circuit and a thyristor fault detection circuit, and the signal acquisition circuit comprises a voltage acquisition circuit and a current acquisition circuit; The voltage acquisition circuit and the current acquisition circuit are connected with a current shunt and a direct current voltage sampling part of the twenty-four-pulse rectification circuit; The DCV+ end and the DCV- end of the direct current voltage differential signal in the voltage acquisition circuit are connected with the ground through voltage division of the resistor R10 and the resistor R11, the resistor R11 is connected with the resistor R12 in series and connected with the same phase end of the amplifier U10A, one end of the resistor R13 is connected with the opposite phase input end of the amplifier U10A, and the other end is connected with the ground, the output end of the amplifier U10A is connected with the resistor R14 in series and connected with the input end IN1 of the isolation IC U11, the IN2 end of the isolation IC U11 is connected with the IN3 end of the isolation IC U11 through a wire, the IN4 end of the isolation IC U11 is connected with the ground, the OC2 end of the isolation IC U11 is connected with the power supply, the E1 end of the isolation IC U11 is connected with the resistor R13 through a wire, the E2 end of the isolation IC U11 is connected with the ground through a wire and the resistor R15, and the E2 end of the isolation IC U11 is outputted through the resistor R16; The DCI+ end of the direct current current differential signal of the current acquisition circuit is connected with the same phase input end of the amplifier OPA2277 through the resistor R20, the DCI- end of the direct current current differential signal is connected with the ground and connected with the opposite phase input end of the amplifier OPA2277 through the resistor R21, the output end of the amplifier OPA2277 is connected with the resistor R23 and connected with the same phase input end of the amplifier U20B, the output end of the amplifier U20B is connected with the capacitor C25 and connected with the opposite phase input end of the amplifier U20B, one end of the resistor R24 is connected with the opposite phase input end of the amplifier U20B, and the other end is connected with the ground; the amplifier U20B is connected with the resistor R25 and connected with the input end IN1 of the isolation IC U21, the IN2 end of the isolation IC U21 is connected with the IN3 end of the isolation IC U21 through a wire, the IN4 end of the isolation IC U21 is connected with the ground, the E1 end of the isolation IC U21 is connected with the negative pole of the capacitor C25, the E2 end of the isolation IC U21 is outputted through the resistor R15 and the resistor R16, and the OC2 end of the isolation IC U21 is connected with the power supply; The thyristor fault diagnosis circuit is connected with the thyristor of the twenty-four-pulse rectification circuit in parallel, the K1 end of the thyristor is connected with the current limiting resistor R11, the resistor R12, the resistor R13, the resistor R14, the resistor R15, the resistor R16, the resistor R17 and the resistor R18 in sequence and connected with two reverse parallel light emitting diodes LS1 and LV2, and then connected with the A1 end of the thyristor, the A1 end of the thyristor is connected with the input end A end of the photo-coupler HW11 and the input end K end of the photo-coupler HW12; The collector C end of the photo-coupler HW11 is connected with the power supply through the resistor RS12, the emitter E end of the photo-coupler HW11 is connected with the base of the triode QS1 and connected with the ground through the resistor RS13, and the emitter of the triode QS1 is connected with the power supply through the resistor RS11. The collector C end of the optical coupling HW12 is connected with the power supply through the resistance RS52, the emitter E end of the optical coupling HW12 is connected with the base of the triode QS5 and grounded through the resistance RS53, and the emitter of the triode QS1 is connected with the power supply through the resistance RS51.

2. A twenty-four pulse rectifier fault detection circuit based on series resonant power supply system as claimed in claim 1 characterized in that, The OC2 end of the isolation IC U11 is connected with the 5V power supply.

3. A twenty-four pulse rectifier fault detection circuit based on series resonant power supply system as claimed in claim 1 or 2, wherein The OC2 end of the isolation IC U21 is connected with the 5V power supply.

4. A twenty-four pulse rectifier fault detection circuit based on series resonant power supply system as claimed in claim 3 wherein, The emitter of the triode QS1 is connected with the 5V power supply through the resistance RS11.

5. A twenty-four pulse rectifier fault detection circuit based on series resonant power supply system as claimed in claim 4 wherein, The collector C end of the optical coupling HW12 is connected with the 5V power supply through the resistance RS52.

6. A twenty-four pulse rectifier fault detection circuit based on series resonant power supply system as claimed in claim 5 wherein, The collector C end of the optical coupling HW11 is connected with the 5V power supply through the resistance RS12.

7. A twenty-four pulse rectifier fault detection circuit based on series resonant power supply system as claimed in claim 6 wherein, The emitter of the triode QS1 is connected with the 5V power supply through the resistance RS51.

8. A twenty-four pulse rectifier fault detection circuit based on series resonant power supply system as claimed in claim 7 wherein, The resistance R11, the resistance R12, the resistance R13, the resistance R14, the resistance R15, the resistance R16, the resistance R17 and the resistance R18 all have the resistance of 20K.

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