Constant current source heating reducing circuit for resistance meter

Abstract

The utility model discloses a constant current source heating reducing circuit for a resistance meter, which belongs to the technical field of resistance measurement and comprises a constant current source circuit, a BUCK step-down circuit, a comparator circuit, an error amplifying circuit and a triangular wave generating circuit. The technical key points are as follows: a BUCK step-down circuit is added between a constant voltage source and a constant current source circuit, when large resistance is measured, an MCU controls an error amplifier, so that the PWM duty ratio output by a comparator is 0, and the output voltage of the BUCK is equal to the voltage of the constant voltage source and is the same as that of a traditional resistance meter; the voltage drop of a constant current source passes through an error amplifier and a triangular wave input comparator, the output PWM duty ratio is controlled to control the output voltage of a BUCK circuit, the output voltage of the BUCK circuit is controlled through the reasonably designed error amplifier to meet the requirement of the constant current source, the thermal power consumption is obviously reduced when a milliohm-level resistor is measured, and the measurement accuracy is improved. And the stability and the endurance of the portable product are improved, and the portable product has a good use prospect.

Description

Technical Field

[0001] The utility model belongs to the technical field of resistance measurement, in particular to a circuit for reducing the heating of a constant current source for a resistance meter. Background Art

[0002] Ohmmeter An instrument for measuring resistance.

[0003] As shown in the figure, when measuring milliohm resistance, the resistance meter often increases the current to increase the voltage drop of the measured resistance to improve the measurement accuracy.

[0004] Traditional resistance measurement circuits use a constant voltage source controlled by a MOS transistor to generate a constant current output, providing a constant current source for the resistance meter. Given the constant current source current, the resistance value can be calculated using Ohm's law by measuring the voltage across the resistor under test.

[0005] However, increasing the measurement current means that the thermal power consumption of the MOS tube of the constant current source increases.

[0006] At this time, in order to ensure the stability of the system, heat dissipation needs to be strengthened, but this is fatal for portable products. Portable products are small in size and have poor overall heat dissipation capabilities, which directly affects the size and battery life of the product.

[0007] In summary, existing portable products have certain drawbacks when measuring small resistances and do not meet people's usage requirements. To this end, we propose a circuit for reducing the heating of a constant current source for a resistance meter. Summary of the Invention

[0008] To overcome the shortcomings of the prior art, the present invention provides a circuit for reducing the heating of a constant current source for a resistance meter. A buck step-down circuit is added between the constant voltage source and the constant current source circuit. When measuring large resistances, the MCU controls the error amplifier so that the PWM duty cycle output by the comparator is 0, and the output voltage of the buck is equal to the constant voltage source voltage. Similar to a traditional resistance meter, when measuring small resistances, the voltage drop of the constant current source is used to pass through the error amplifier and the triangular wave input comparator to control the output PWM duty cycle and thus control the output voltage of the buck circuit. By designing a reasonable error amplifier to control the output voltage of the buck to meet the needs of the constant current source, the overall thermal power consumption is significantly reduced when measuring milliohm-level resistances, which can improve the stability and battery life of portable products and has a good prospect for use.

[0009] The technical solution adopted by the embodiment of the present application to solve the technical problem is:

[0010] A circuit for reducing the heating of a constant current source for a resistance meter, comprising a constant current source circuit, a buck step-down circuit, a comparator circuit, an error amplifier circuit, and a triangle wave generating circuit:

[0011] The constant current source circuit is used to provide a constant current for the resistance meter to measure the resistance to be measured;

[0012] The error amplifying circuit is used to amplify the voltage drop when measuring resistance;

[0013] The triangle wave generating circuit is used to generate a triangle wave signal;

[0014] The comparator circuit is used to compare the triangular wave signal and the amplified voltage drop and output a PWM signal;

[0015] The BUCK step-down circuit is used to receive a PWM signal and adjust the output voltage according to the PWM signal.

[0016] Preferably, the error amplifier circuit includes a DC amplifier, the non-inverting input end of the DC amplifier is connected to a resistor and a constant current source circuit in sequence, the output end of the DC amplifier is connected to a resistor, a resistor, a resistor, a resistor and a positive electrode of a 5V power supply in sequence, and the inverting input end of the DC amplifier is connected to the emitter of the transistor and the output end of the DC amplifier;

[0017] The base of the transistor is connected between the resistors and connected to the positive electrode of the power supply through the resistors, and the collector of the transistor is connected between the resistors;

[0018] The end of the resistor away from the resistor is connected to the resistor and the base of the transistor in sequence, the emitter of the transistor is connected to a 5V power supply, the collector of the transistor is connected to one end of the resistor and the drain of the MOS transistor, and the source of the MOS transistor is grounded;

[0019] The 5V power supply is further connected to resistors and resistors in sequence and then grounded. The other end of the resistor is connected between the resistors and resistors and is connected to the comparator circuit.

[0020] Preferably, the triangular wave generating circuit includes a comparator, wherein the output end of the comparator is sequentially connected to a resistor and a 5V power supply, the non-inverting input end of the comparator is sequentially connected to a resistor and a 5V power supply, the non-inverting input end of the comparator is further connected to a resistor and then grounded, the inverting input end of the comparator is connected to a capacitor and then grounded, a resistor is connected between the inverting input end of the comparator and the output end of the comparator, and a resistor is connected between the non-inverting input end of the comparator and the output end of the comparator;

[0021] The inverting input terminal of the comparator is connected to the comparator circuit;

[0022] One end of the resistor connected to the 5V power supply is also connected to a capacitor, and the capacitor is grounded.

[0023] Preferably, the constant current source circuit includes a diode, a MOS tube, a current limiting resistor, a test resistor, a sampling resistor and a negative electrode of the power supply, which are sequentially connected to the positive electrode of the 5V power supply;

[0024] The source of the MOS tube is connected to a current-limiting resistor, and the drain of the MOS tube is connected to a diode;

[0025] The drain of the MOS tube is also connected to a capacitor, and the capacitor is grounded;

[0026] A voltage stabilizing diode is connected between the drain of the MOS tube and the test resistor;

[0027] The gate of the MOS tube is connected to the resistor and the output end of the operational amplifier in sequence;

[0028] The inverting input terminal of the operational amplifier is connected to a resistor, the other end of the resistor is connected between a test resistor and a sampling resistor, and a capacitor is provided between the inverting input terminal of the operational amplifier and an output terminal of the operational amplifier.

[0029] In summary, the present invention has at least one of the following beneficial technical effects:

[0030] This utility model describes a circuit for reducing the heat generated by a constant current source for a resistance meter. A buck (Buck) step-down circuit is added between a constant voltage source and a constant current source circuit. When measuring high resistances, the MCU controls the error amplifier, causing the PWM duty cycle output by the comparator to be zero. The buck's output voltage equals the constant voltage source voltage, similar to a traditional resistance meter. When measuring low resistances, the constant current source's voltage drop is used to pass through the error amplifier and a triangular wave input comparator, controlling the output PWM duty cycle and thus the buck's output voltage. By designing a properly designed error amplifier to control the buck's output voltage to meet the needs of the constant current source, the overall thermal power consumption is significantly reduced when measuring milliohm-level resistances, improving the stability and battery life of portable products and promising application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a circuit block diagram of the utility model;

[0032] Figure 2 It is a schematic diagram of the principle of the utility model;

[0033] Figure 3 This is the triangle wave generating circuit diagram of the utility model;

[0034] Figure 4 This is the error amplification circuit diagram of the utility model;

[0035] Figure 5 This is a constant current source circuit diagram of the utility model;

[0036] Figure 6 It is a circuit diagram of the comparator in the utility model;

[0037] Figure 7 This is a circuit diagram of the BUCK step-down circuit in the utility model;

[0038] Figure 8 This is a schematic diagram of the principle of a traditional resistance meter. DETAILED DESCRIPTION

[0039] The embodiments of the present application solve the problems described in the background art by providing a circuit for reducing the heating of a constant current source for a resistance meter.

[0040] Example:

[0041] A circuit for reducing the heating of a constant current source for a resistance meter, such as Figure 1-Figure 7 As shown, it includes a constant current source circuit, a BUCK step-down circuit, a comparator circuit, an error amplifier circuit and a triangle wave generating circuit:

[0042] Constant current source circuit: used to provide a constant current for the resistance meter to measure the resistance to be measured;

[0043] Error amplifier circuit: used to amplify the voltage drop when measuring resistance;

[0044] Triangle wave generating circuit: used to generate triangle wave signal;

[0045] Comparator circuit: used to compare the triangle wave signal and the amplified voltage drop and output a PWM signal;

[0046] BUCK step-down circuit: used to receive PWM signal and adjust the output voltage according to the PWM signal.

[0047] like Figure 4 and Figure 6 As shown, the error amplifier circuit includes a DC amplifier U3A, the non-inverting input terminal of the DC amplifier U3A is connected to the resistor R18 and the constant current source circuit in sequence, the output terminal of the DC amplifier U3A is connected to the resistor R13, the resistor R9, the resistor R6, the resistor R5 and the positive electrode of the 5V power supply VDA in sequence, and the inverting input terminal of the DC amplifier U3A is connected to the emitter of the transistor Q5 and the output terminal of the DC amplifier U3A;

[0048] The base of the transistor Q5 is connected between the resistor R13 and the resistor R9, and is connected to the positive electrode of the power supply VAT through the resistor R10. The collector of the transistor Q5 is connected between the resistor R9 and the resistor R6.

[0049] One end of resistor R6 away from resistor R9 is sequentially connected to resistor R8 and the base of transistor Q4. The emitter of transistor Q4 is connected to the 5V power supply VDA. The collector of transistor Q4 is connected to one end of resistor R15 and the drain of MOS transistor Q6. The source of MOS transistor Q6 is grounded.

[0050] The 5V power supply VDA is also sequentially connected to resistor R14 and resistor R19 and then grounded. The other end of resistor R15 is connected between resistor R14 and resistor R19 and is connected to the comparator circuit.

[0051] When the error amplification circuit is in use and measuring a large resistor, set SD to high level. VFB = 0V < VRAMPmin = 0.38V, the comparator U2 outputs a low level, MOS transistor Q6 is fully turned on. At this time, VAT_5V = VDD_5V. When the test resistor R2 in the constant current source circuit is not connected, R2 = +∞, which is the same as the situation of measuring a large resistor.

[0052] When measuring a small resistor, set SD to low level. When Q6 is not turned on, VFB = 2V. At this time, the period of the triangular wave is 85KHZ, and the period is about 12mS. Assuming that the rise and fall of the triangular wave are symmetric, the high-level time Ton of the PWM output by U3 = 12 - [(VFB - 0.38×6) / (3.75 - 0.38)]×2 = 6.23mS, D = Ton / T = 0.52, VAT_5V = 0.52×VDD_5V = 2.56V. The maximum output voltage in the BUCK mode is Vat_5Vmax = 2.56V. When Q4 is turned on, VFB = 2.5V. Calculate Vat_5Vmin = 1.85V according to the same principle above.

[0053] As Figure 3 shown, the triangular wave generating circuit includes comparator U4B. The output end of comparator U4B is sequentially connected to resistor R21 and the 5V power supply VDA. The non-inverting input end of comparator U4B is sequentially connected to resistor R21 and the 5V power supply VDA. The non-inverting input end of comparator U4B is also connected to resistor R24 and then grounded. The inverting input end of comparator U4B is connected to capacitor C11 and then grounded. A resistor R23 is connected between the inverting input end and the output end of comparator U4B. A resistor R25 is connected between the non-inverting input end and the output end of comparator U4B.

[0054] The inverting input end of comparator U4B is connected to the comparator circuit.

[0055] The end of resistor R21 connected to the 5V power supply VDA is also connected to capacitor C7, and capacitor C7 is grounded.

[0056] Upon power-up, capacitor C11 is uncharged, V+ > V-, and the comparator outputs a high level. At this point, the voltage at the non-inverting input, V1, equals 5 × R22 / [(R23 + R25) / / (R2 + R22)] ≈ 3.75V. After C11 charges and reaches V1, the V->V+ comparator outputs a low level, and capacitor C11 discharges through R23. At this point, the voltage at the non-inverting input, V2, equals 5 × (R23 / / R24) / [(R23 / / R24) + R22] ≈ 0.38V ( / / represents the parallel operation of the resistors). When V+ > V-, the comparator outputs a high level. The frequency is determined by the capacitor charging curve; the actual frequency is 85 kHz. The minimum voltage of the output triangle wave is 0.38V, and the maximum voltage is 3.75V.

[0057] like Figure 5 As shown, the constant current source circuit includes a diode D2, a MOS tube Q1, a current limiting resistor R1, a test resistor R2, a sampling resistor R7 and a negative electrode AGND of the power supply, which are connected in sequence to the positive electrode of the 5V power supply VAT;

[0058] The source of the MOS transistor Q1 is connected to the current limiting resistor R1, and the drain of the MOS transistor Q1 is connected to the diode D2;

[0059] The drain of the MOS tube Q1 is also connected to the capacitor C1, and the capacitor C1 is grounded;

[0060] A voltage stabilizing diode D1 is connected between the drain of the MOS tube Q1 and the test resistor R2;

[0061] The gate of the MOS tube Q1 is connected to the resistor R3 and the output terminal of the operational amplifier U1B in sequence;

[0062] The inverting input terminal of the operational amplifier U1B is connected to the resistor R4 , the other end of the resistor R4 is connected between the test resistor R2 and the sampling resistor R7 , and a capacitor C6 is provided between the inverting input terminal of the operational amplifier U1B and the output terminal of the operational amplifier U1B.

[0063] When the resistor is not connected, V- = 0V, V+ = VREF, op amp U1 outputs a high level, MOSFET Q1 is fully conductive, and VAA_CS = VAT_5V. When resistor RT is connected, V+ = VREF, V- = IT × RF, where IT = VREF / RF. The op amp output controls the conduction level of MOSFET Q1 to achieve a constant current output. The current flowing through RT can be controlled by adjusting the values of VREF and RF.

[0064] Taking the measurement of a 1mΩ resistor as an example, the constant current source circuit current is 1A, and the sampling resistor is 100mΩ. Without using the buck circuit, using a 5V supply, and ignoring the voltage drop across the wire, the power dissipation of MOS transistor Q1, Pmax, is (5 - 0.101) × 1 = 4.809W. Under the same conditions, using the buck circuit, the power dissipation of MOS transistor Q1, Pmax, is ≤ (2.56 - 0.101) × 1 = 2.459W. Reducing thermal power consumption by half is crucial for the stability and battery life of portable products.

[0065] like Figure 2 As shown in the figure, this circuit adds a BUCK step-down circuit between the constant voltage source (power supply) and the constant current source circuit.

[0066] Working principle: When measuring large resistance, the MCU controls the error amplifier circuit to make the PWM duty cycle output by the comparator circuit 0. The output voltage of the buck circuit is equal to the constant voltage source voltage, the same as a traditional resistance meter.

[0067] When measuring low resistances, the voltage drop across the constant current source circuit is used to generate a triangular wave through the error amplifier and triangular wave generator circuits. This is then fed into the comparator to control the PWM duty cycle, thereby controlling the output voltage of the buck circuit. By designing a properly designed error amplifier circuit, the buck circuit's output voltage is controlled to meet the requirements of the constant current source circuit. Given the current drawn by the constant current source circuit, the resistance of the resistor under test can be calculated using Ohm's law by measuring the voltage across the resistor under test.

[0068] Finally, it should be noted that the above embodiments are merely examples for the purpose of illustrating the present invention and are not intended to limit the embodiments. Those skilled in the art will readily appreciate that other variations or modifications based on the above description are possible. It is not necessary and impossible to provide an exhaustive list of all embodiments. However, obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A circuit for reducing the heating of a constant current source for a resistance meter, characterized in that: Including constant current source circuit, BUCK step-down circuit, comparator circuit, error amplifier circuit and triangle wave generating circuit: The constant current source circuit is used to provide a constant current for the resistance meter to measure the resistance to be measured; The error amplifying circuit is used to amplify the voltage drop when measuring resistance; The triangle wave generating circuit is used to generate a triangle wave signal; The comparator circuit is used to compare the triangular wave signal and the amplified voltage drop and output a PWM signal; The BUCK step-down circuit is used to receive a PWM signal and adjust the output voltage according to the PWM signal.

2. A circuit for reducing heating of a constant current source for a resistance meter as claimed in claim 1, characterized in that: The error amplifier circuit includes a DC amplifier U3A, the non-inverting input terminal of the DC amplifier U3A is connected to the resistor R18 and the constant current source circuit in sequence, the output terminal of the DC amplifier U3A is connected to the resistor R13, the resistor R9, the resistor R6, the resistor R5 and the positive electrode of the 5V power supply VDA in sequence, and the inverting input terminal of the DC amplifier U3A is connected to the emitter of the transistor Q5 and the output terminal of the DC amplifier U3A; The base of the transistor Q5 is connected between the resistor R13 and the resistor R9, and is connected to the positive electrode VAT of the power supply through the resistor R10. The collector of the transistor Q5 is connected between the resistor R9 and the resistor R6. One end of the resistor R6 away from the resistor R9 is connected in sequence to the resistor R8 and the base of the transistor Q4. The emitter of the transistor Q4 is connected to the 5V power supply VDA. The collector of the transistor Q4 is connected to one end of the resistor R15 and the drain of the MOS transistor Q6. The source of the MOS transistor Q6 is grounded. The 5V power supply VDA is further connected to the resistor R14 and the resistor R19 in sequence and then grounded. The other end of the resistor R15 is connected between the resistor R14 and the resistor R19 and is connected to the comparator circuit.

3. A circuit for reducing heating of a constant current source for a resistance meter as claimed in claim 2, characterized in that: The triangular wave generating circuit includes a comparator U4B, wherein the output end of the comparator U4B is connected to a resistor R21 and a 5V power supply VDA in sequence, the non-inverting input end of the comparator U4B is connected to the resistor R21 and the 5V power supply VDA in sequence, the non-inverting input end of the comparator U4B is further connected to a resistor R24 and then grounded, the inverting input end of the comparator U4B is connected to a capacitor C11 and then grounded, a resistor R23 is connected between the inverting input end of the comparator U4B and the output end of the comparator U4B, and a resistor R25 is connected between the non-inverting input end of the comparator U4B and the output end of the comparator U4B; The inverting input terminal of the comparator U4B is connected to the comparator circuit; One end of the resistor R21 connected to the 5V power supply VDA is also connected to a capacitor C7, and the capacitor C7 is grounded.

4. A circuit for reducing heating of a constant current source for a resistance meter as claimed in claim 3, characterized in that: The constant current source circuit includes a diode D2, a MOS tube Q1, a current limiting resistor R1, a test resistor R2, a sampling resistor R7 and a negative electrode AGND of the power supply, which are connected in sequence to the positive electrode of the 5V power supply VAT; The source of the MOS transistor Q1 is connected to the current limiting resistor R1, and the drain of the MOS transistor Q1 is connected to the diode D2; The drain of the MOS transistor Q1 is also connected to the capacitor C1, and the capacitor C1 is grounded; A voltage stabilizing diode D1 is connected between the drain of the MOS tube Q1 and the test resistor R2; The gate of the MOS transistor Q1 is connected to the resistor R3 and the output end of the operational amplifier U1B in sequence; The inverting input terminal of the operational amplifier U1B is connected to the resistor R4 , the other end of the resistor R4 is connected between the test resistor R2 and the sampling resistor R7 , and a capacitor C6 is provided between the inverting input terminal of the operational amplifier U1B and the output terminal of the operational amplifier U1B.

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