Practical anti-interference detection circuit for navigation light dimmer

Airport navigation lights are important navigation aids that provide aircraft with information such as runway orientation, climb and descent angles, ensure safe takeoff and landing of aircraft, and protect the lives and property of passengers. Airport navigation lights Dimmers are control devices that adjust the brightness of lights according to airport weather conditions and takeoff and landing needs. On the one hand, they can maintain the main circuit current constant when the power grid and load impedance change within the allowable range, and on the other hand, they can switch the brightness level of navigation lights and adjust them to be stable within the specified time. The special characteristics of aviation transportation safety require that the dimmer has strong anti-interference ability, stable and reliable operation, and has open circuit, overcurrent, short circuit protection and open circuit protection measures.

1 Basic structure of dimmer

The dimmer is a typical closed-loop control system with the 80196 single-chip microcomputer as the core. The system uses the method of controlling the current of the strong electric circuit to achieve the control of the brightness of the lamp. The system adopts a modular structure, in which the 80196 single-chip microcomputer is the core module of the system, which combines the functions of digital regulator and system control and scheduling. The system uses the high-speed input/output of the 80196 single-chip microcomputer as the phase synchronization and trigger signal of the thyristor, and uses the 10-bit A/D converter of the 80196 as the current/voltage input interface of the system. The system is equipped with a digital current controller, and thus forms a current closed-loop system. In addition, the system also has a current and voltage detection module, a thyristor trigger pulse isolation amplifier module, a phase synchronization and contactor control module, a display and key control module, etc. The basic structure is shown in Figure 1.

Figure 1 Basic structure of a dimmer

2 Current balance detection principle and circuit improvement

Since the navigation lights are installed along the runway, the transmission line from the substation to the end of the airport is 2 to 4 kilometers long, so the method of boosting power transmission and reducing power consumption must be adopted. The inherent characteristics of the transformer require that the positive and negative trigger pulses of the thyristor circuit must be strictly symmetrical, otherwise it is very easy to cause the positive and negative half-cycle current of the transformer to be unbalanced, and an equivalent DC passes through the transformer, resulting in magnetic circuit saturation, the excitation reactance tends to 0 and a strong current is generated, which is equivalent to a power short circuit. If it is not repaired in time, the fuse will burn out, the thyristor and transformer will burn out, causing a large-scale and long-term power outage, which seriously threatens the increasingly busy flight transportation safety. Therefore, an online detection technology for the positive and negative current balance of the transformer is urgently needed. In addition, since the control site is near a high-voltage, high-current power grid, there is strong electromagnetic interference. Multiple high-power dimmers are put into operation or shut down at the same site, and various relays and contactors are often switched on and off, resulting in impact current in the power supply. Interference may enter the control system through various channels, causing distortion of detection signals, malfunctions, and program "runaway". In order to ensure the continuous and safe operation of the system, it is necessary to consider suppressing various interferences in the hardware design that completes certain functions.

2.1 Noise Interference Coupling Analysis

There are mainly the following ways of interference coupling.

1) Electrostatic coupling: Noise is coupled into the circuit through stray capacitance, as shown in Figure 2-a. En is the interference source, Cm is the parasitic capacitance, and Zi is the interference input impedance.

2) Electromagnetic coupling: The interference is coupled into the circuit through mutual inductance, as shown in Figure 2-b. In is the interference current, and M is the mutual inductance of the two circuits.

3) Common impedance coupling: The interference current is coupled into the circuit through the common impedance between two or more circuits. The common common ground connection is shown in Figure 2-c.

Figure 2 Several ways of interference coupling

4) Leakage current coupling: Due to poor insulation, interference is coupled into the circuit through insulation current as shown in Figure 2-d.

Among the above four types of interference, electrostatic coupling and electromagnetic coupling are proportional to the frequency of interference noise. The higher the high-frequency noise content in the power grid, the greater the coupling interference. Their common feature is that they can only couple when they share a common ground. Noise and interference come in from power supply, space, and process channels. The most difficult problem we face is the interference of process channels.

2.2 Circuit and Disadvantages of the Original Current Balance Detection Method

As shown in Figure 3, in order to make full use of the A/D conversion function of 80196, the design idea of ​​the original detection circuit is to collect the current signal into a small signal voltage through a transformer, use a diode to intercept the positive (or negative) half cycle, amplify it, and then convert it through the effective value converter AD536, then amplify and adjust the output resistance, and collect it with 80196A/D to compare the positive and negative cycle currents.

Figure 3 Original balance detection circuit

Since the digital ground of the 80196 is connected to the analog ground of a peripheral circuit , all peripheral links may receive interference, so the noise may enter the host along the A/D circuit. At the same time, the host may also directly couple the noise, often "freezing" due to interference. [page]

2.3 Improved positive and negative current detection method and circuit

In order to isolate interference, we separate the current, voltage detection and I/O interface circuits from the host with an optoelectronic isolator, that is, the microcomputer is powered by an independent power supply, while the A/D conversion and I/O interface are powered by another power supply. There is no common ground wire between them to prevent the digital circuit from directly coupling with interference; the nonlinear relationship between the input and output of the optoelectronic isolator cannot correctly reflect the size of the current and voltage signals, but it requires sufficient power to transmit signals, and can only transmit signals with a frequency less than 10KHZ, so it can further suppress peripheral circuit interference. At the same time, we use V/F and F/V conversion technology. As shown in Figure 4. The voltage signal generated by AD536 is converted into a frequency pulse signal by the V/F converter LM331, and then through the optoelectronic isolator, the same frequency signal is formed, which is shaped by the inverter and then converted into a voltage signal by the F/V conversion circuit, and then sent to the input end of the A/D conversion circuit in 80196 after high-frequency filtering and amplification. The V/F converter LM331 converts the input 直流电\'); companyAdEvent.show(this,\'companyAdDiv\',[5,18])"> DC voltage U1 and outputs a frequency pulse signal (frequency fout) whose frequency is proportional to the voltage. This conversion transforms the DC voltage transmission into a pulse signal transmission mode. The first advantage is that it can be transmitted to the microcomputer system after passing through the photoelectric isolator, which can effectively remove the magnetic field interference signal and the common mode interference signal; secondly, although the interference voltage signal generated when various high-power motors and electrical appliances start and stop can be coupled into the input end of LM331 through the power supply or power ground wire, it will cause the waveform of the output pulse to be distorted or affect the pulse amplitude, but it has little effect on the pulse frequency, and actually removes the interference from the power supply and ground wire.

Figure 4 Improved detection circuit

2.4 LM331 principle and usage introduction

The monolithic V/F, F/V converter LM331 is a product of the American National Semiconductor Company. The chip can work under single or dual power supply, with a linearity of up to 0.01%, high temperature stability, pulse output compatible with all logic output circuits, low power, wide dynamic range, and low price. Its internal circuit structure is shown in Figure 5.

The bandgap reference circuit generates a 1.9V DC voltage and sends it to both pins. Both pins are connected to Rs to form a reference current IR = 1.9Rs. The input voltage VIN of the input comparator is compared with VX. When VIN > VX, the single pulse timer is started and the frequency output transistor and the switch current source are turned on. The timer timing period t0 = 1.1RtCt. In this period, the current IR charges CL to make VX rise. When VX rises to VX > VIN, the current IR is turned off, the timer resets itself, and CL gradually discharges until VX < VIN. Then the comparator starts the timer again and starts the next cycle. The current flowing into CL is strictly equal to VX/RL≈VIN/RL.

Based on the principle of equal balance between positive and negative charges, the output signal frequency is strictly proportional to the input voltage.

The actual circuit is shown in Figure 6. The differences are that the R1 and C1 high-frequency filter circuits are added to the input end to remove the high-frequency interference of the previous stage; the offset adjustment circuit is added to the original ground terminals of CL and RL to ensure accurate grounding; adjustable resistors are connected to pins 2 and 6 to adjust the output frequency to a suitable point; and a pull-up resistor is connected to the output pin 3 to meet the requirements of TTL and CMOS input.

As shown in Figure 7, the F/V practical circuit has an output voltage of:

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2.5 Advantages of improved circuit

Interference may be coupled at various points before the optical isolation. If an interference signal is input at the V/F circuit, since the input end of the V/F converter is connected to a large filter capacitor, the general pulse can be absorbed completely. In addition, the LM331 works in the integral state, and the peak time of the interference pulse is short (microseconds), and its integral value can be ignored; in addition, the output pulse frequency of the V/F conversion is proportional to the average value of the input level, and the interference pulse with a short duration has no effect on it. The optical isolation is driven by current and consumes a lot of electric energy. The interference pulse cannot form a continuous current and cannot affect the operation of the optical isolation. If an interference is occasionally coupled to the front of the F/V converter, it will only affect the formation of one pulse, with an error of % 1HZ, while the actual operation is between 0~5KHZ for V/F and F/V, and the output value is almost unaffected.

2.6 Synchronous pulse circuit

In order to know the sampling time, the 80196HSI must get a trigger pulse when the AC voltage passes through 0. The circuit is shown in Figure 8. OP -07 is an amplifier .

3 Conclusion

The early products of this kind of dimmer had poor stability and reliability because they did not adopt the above anti-interference measures. When a DC relay with a coil rated current of 40mA inside the dimmer is powered on or off, it sometimes causes the dimmer to malfunction. Other dimmers, flashlights, and fluorescent lamps above 40 watts in the same power supply environment as the dimmer will start or stop working, which will cause the dimmer to malfunction or freeze or program runaway. This circuit can accurately detect the size of positive and negative phase currents, ensuring the safe operation of the power supply circuit. At the same time, the improved circuit basically isolates the interference of the process channel, and together with the separate power supply and shielding measures adopted by the system, the control system host can operate continuously.