Design and application of transient current detector

1 Introduction

The external critical resistance of pointer-type and light-spot galvanometers is large, the internal resistance is large, and the loss in the circuit is large. Moreover, when the current in the energized coil changes, the coil makes a damping motion, and it takes a certain amount of time to reach a stable position. The galvanometer has a slow response speed, so it cannot detect instantaneous current changes and instantaneous currents with small circuit loss requirements (such as LC oscillation currents), and is not suitable for measuring instantaneous circuit currents with small circuit resistance. Usually, multimeters can only measure the effective value of AC current and the magnitude of DC current. Therefore, neither galvanometers nor multimeters can meet the needs of measuring and observing instantaneous current changes. This design uses the principle of short-circuit current amplifier to amplify the detection current by 1:1, and can be combined with the auxiliary circuit to qualitatively detect the magnitude and change direction of transient currents with the help of light-emitting diodes.

2 Principle and Implementation

2.1 Short-circuit Current Amplifier

With the characteristics of high conversion rate, small input base current and drift current, and small drift voltage temperature coefficient of the integrated operational amplifier circuit, the principle of short-circuit current amplifier is used to amplify the current to be detected by 1:1, widen the required range of the internal resistance of the signal, improve the sensitivity of detection, and realize the detection of instantaneous current changes. Figure 1 shows an inverting input proportional operational amplifier circuit. The input signal Vi is connected to the inverting input terminal ∑ of the integrated operational amplifier through resistor R1, and the non-inverting input terminal ∑' is grounded through resistor R2. The output voltage VO is connected back to the inverting input terminal through the feedback resistor RF, forming a deep voltage negative feedback. In practical applications, in order to ensure that the two input terminals of the operational amplifier are in a balanced working state and avoid the input bias current from generating additional differential input voltage, the resistance of the inverting input terminal and the non-inverting input terminal to ground should be equal. In Figure 1, R2=R1∥RF should be set. Since the ideal op amp's L=L=0, there is no voltage drop on R2, VO=0, and since the ideal op amp's V+=V-, V-=0, Vi=Ii×R1, so the equivalent input resistance of the inverting input amplifier circuit is r1=V/Ii=Ii×R1/Ii=R1. If R1=0, the amplifier input resistance is zero. According to the value requirement of the balancing resistor, R2=R1∥RF, then R2=0, which constitutes a short-circuit current amplifier. The impedance of the current input is zero, and the output voltage VO changes linearly with the input current. As shown in Figure 1, because V∑∑=0, it is equivalent to a short circuit in the external circuit of the signal source, but it is actually not disconnected. The resistance between ∑ and ∑' is extremely large, and because the resistance of the ∑ point to ground reaches several megohms, the output current of the signal source can only form a loop through RF and IC, that is, VO=IORF.

2 Working principle of detection circuit

The circuit principle of transient current detector is shown in Figure 2. TL084 junction field effect tube input operational amplifier is selected. Each operational amplifier uses high voltage junction field effect tube and bipolar tube on a single integrated circuit, which is compatible with better matching, high conversion rate, small input base current and input drift current, and low drift voltage temperature coefficient. Integrated operational amplifier A and R1 form a short-circuit current amplifier, B and R2~R6, W1 form an inverting adder, which amplifies the output voltage V1 of operational amplifier A. R3, R4 and W1 form a circuit. If the output voltage VO of operational amplifier B is VO≠0 when input i=0, the movable contact of multi-turn potentiometer W1 can be moved to make VO=0. In fact, W1 is equivalent to the function of zero adjustment knob of pointer galvanometer. The voltage amplification factor of operational amplifier B is AV=-R6/R2=-40. R7~R17 are connected in series to generate 10 reference voltages. Each integrated operational amplifier is connected to a voltage comparator, and forms a level indication circuit with a resistor and a light-emitting diode. When there is an input current i, the output voltage V1 of operational amplifier A is -i?R1. This voltage is amplified by the amplifier circuit composed of B, R2~R6, and W1, and then compared with the reference voltage of the voltage comparator. The level indication circuit formed by the light-emitting diode synchronously reflects the relative size, direction and change law of the current flowing between terminals a and b. The five voltage comparators composed of operational amplifiers C~G have their inverting input terminals connected to the reference voltages 1.918 9 V, 1.465 8 V, 1.012 7 V, 0.559 6 V, and 0.106 5 V respectively, and the non-inverting input terminals are connected to the output VO of amplifier B, which is used for the comparison and display of the forward current (i.e., flowing in from terminal a and out from terminal b). The non-inverting input terminals of the other five voltage comparators are connected to the reference voltages, -0.106 5 V, -0.559 6 V, -1.012 7 V, -1.465 8 V, and -1.918 9 V, respectively, and the inverting input terminals are all connected to the output VO of the amplifier, which are used for comparison and display of negative current (i.e., flowing in from terminal b and out from terminal a).

When current i flows from terminal a, if the magnitude is 0.1 mA, the output voltage of amplifier B is VO=0.000 1 A×330 Ω×40＝1.32 V, which is higher than the reference voltage of voltage comparators E, F, G. They output high level, corresponding to LED3～LED5, which emit light; when i＝0.15 mA, VO＝0.00 015 A×330 Ω×40=1.98 V, which is higher than the reference voltage of voltage comparators C, D, E, E, G. These comparators output high level, corresponding to LED1～LED3 emit light. The number of LEDs is proportional to the magnitude of the detection current. When the input current changes from small to large, the order of the LEDs lighting up is LED5～LED4～LED3～LED1～LED1. When current flows from terminal b, the output voltage VO of amplifier B is negative, and comparators H, j, k, 1, M, which are responsible for negative current detection, output high level in turn, making the corresponding LED emit light. The larger the current, the lower VO, the more light-emitting tubes light up, and the order of lighting is LED7~LED8~LED9~LED10~LED11. In this way, the number and position of the light-emitting of the ten LEDs arranged in an arc (except the normally lit LED6) can qualitatively reflect the direction and magnitude of the detection current. And the change of the light-emitting tube and the current is displayed synchronously, which is very vivid and intuitive.

Since the output voltage V1 of the op amp A is equal to the product of the measured current i and R1, that is: V1=-iR1. V1max=-4.8 V. Take R1=330 Ω, then the maximum value of the measurable current is imax=V1max/R1=14.55 mA. The total resistance RS of the series connection of resistors R7~R17 is 22.07 kΩ. The voltage across R12 is V12 = (5 V + 5 V) R12 / RS = 10 × 470 / (22.07 × 103) V = 0.212 96 V. The reference voltages of op amps G and H are V12 / 2 = 0.106 5 V and -V12 / 2 = -0.106 5 V, respectively. The input voltage corresponding to the output voltage of 0.106 5 V is 0.106 5 / 40 = 0.002 7 V, which is greater than the input error voltage of TL084. Assume that the minimum current that can be detected is imin. Since imin × R1 × AV ≥ V12 / 2, imin ≥ V12 / (2R1 Av) = 0.106 5 / (330 × 40) A = 8.06 × 10-6 A, so the current detection range of the galvanometer is 8.06 × 10-6 A to 14.55 × 10-3 A. The reference voltage between two adjacent op amps such as op amps E and F that drive and display the magnitude of the current in the same direction is V=10 V×R10／RS=10 V×1 kΩ／(22.07 kΩ)V=0.453 1 V. Assuming the magnitude of the distinguishable input current is △i, then △iR1Av=V, so △i=V／(R1Av)=0.4531／(330×40)A=3.43×10-5A, so the current that can be displayed is 3.43×10-5A.

By closing the switch K2, the direction of the current change and the qualitative magnitude change of the current can be observed with the help of the light-emitting diode. By closing the switch K3, the magnitude of the current change can be observed with a voltmeter, but due to the inherent reasons of the voltmeter, the voltmeter cannot reflect the actual magnitude of the current with a higher frequency.

2.3 Current detector production Op

amps A~M use three op amp integrated circuits TL084, each of which contains four identical operational amplifiers. They share a common power supply and work independently of each other. The light-emitting diode LED6 is red or yellow, and the other LEDs are green. They are all φ6 high-brightness. All resistors are 1/8 W metal film resistors with an accuracy of 1±%. C1 uses an electrolytic capacitor with a withstand voltage of 25 V, and W1 uses a multi-turn potentiometer with a resistance of 1 kΩ. The power supply uses a ±5 V dual-output regulated power supply. Except for the potentiometer W1, LED and current-limiting resistors R18~R28, the other components are designed to be installed on a copper-clad board. The circuit board diagram is shown in Figure 3. All current-limiting resistors are welded together with the LED, which can reduce the lead wires. In order to adapt to the observation habits of the pointer galvanometer, LED1~LED11 can be installed on the panel in a fan-shaped arrangement, and LED6 is arranged in the middle, as shown in Figure 3. In order to facilitate students' observation, the external dimensions of the current detector can be appropriately larger, such as: 40 cm high, 25 cm wide, and 10 cm thick. As long as the components are correct and installed correctly, they can work normally without debugging.

3 Application

Before use, turn on the power switch K1, adjust W1 so that except for the LED6 in the middle row, the rest of the LEDs do not light up (i.e. zero adjustment). Connect the terminals a and b to the circuit to be tested. The demonstration experiment can be carried out like the pointer galvanometer.

For the demonstration of LC electromagnetic oscillation, select a coil with a large inductance and a small internal resistance with a magnetic core. The oscillation period should be large. The special self-inductance coil of the J2343 electromagnetic oscillation demonstrator should be used. Its maximum inductance is greater than 500 H and its resistance is less than 50 Ω. The capacitor should preferably be a 0.6μF CBB capacitor. If it is replaced by an ordinary capacitor with a withstand voltage greater than 25 V, the reverse leakage is more serious, which accelerates the energy loss and shortens the oscillation duration. The power supply is 6 V. Connect and operate according to the requirements of the demonstration experimental circuit, and you can clearly observe the damping oscillation with the same period. Since the LC oscillation circuit has a small damping, the number of oscillations can be observed more than 5 times, and the oscillation with a period of one tenth of a second can be observed, while the pointer galvanometer can generally only observe 2 periods, and it is difficult to respond to periods less than 1 s. The relationship between the oscillation period and the capacitance can be verified by replacing capacitors with different capacities.

Demonstration of electromagnetic induction phenomenon of a single wire: Use a 50-80 cm soft wire, connect the two ends to the galvanometer terminals a and b respectively, hold the middle part of the wire in the magnetic field of a horseshoe magnet and make it cut the magnetic lines of force, and the galvanometer will show that an induced current is generated. The relationship between the directions of the magnetic field, the wire movement, and the induced current, that is, the right-hand rule, can be very clearly verified, which solves the experimental demonstration problem of the movement of a single wire cutting the magnetic lines of force.

Demonstration of the principle of the generator: Connect the output end of the single-phase AC generator model to the input ends a and b of the galvanometer, so that the rotor of the generator starts to rotate slowly from the neutral plane position, and after one rotation, the LED shows that the magnitude and direction of the current change in one cycle. Gradually increase the speed, the faster the light-emitting tubes on both sides will alternate, and the more light-emitting tubes will light up, but they will always be synchronized with the rotation. When the speed reaches a certain level, the alternating flashes of the light-emitting tubes on both sides gradually speed up to the point where it is impossible to distinguish the direction change, and they are almost always shining, which just explains why 50 Hz AC passes through the light bulb without seeing the light flickering. This is exactly the observation effect that the pointer type galvanometer cannot achieve.

4 Conclusion

This instantaneous current detector overcomes the inherent disadvantages of pointer type and cursor type galvanometers, which have large losses and slow response speed in the circuit. It can detect instantaneous current changes and is suitable for detecting instantaneous currents in the circuit to be tested that require small losses and small circuit resistance. As a low-cost measuring instrument for related physical and electrical measurements, it can be used for demonstration experiments such as LC electromagnetic oscillation, single wire electromagnetic induction, and generator principles.