Introduction of an intrinsically safe power supply for mines

introduction

The underground environment of coal mines is special, with the presence of flammable and explosive substances such as gas, dust, and carbon monoxide. Therefore, the power supply of various communication, monitoring, instrumentation, automation and other system equipment in the mine must meet the explosion-proof requirements. Intrinsic safety is the best form of explosion-proof. The so-called intrinsically safe circuit refers to a circuit in which any electric sparks or effects generated under normal operation or specified fault conditions cannot ignite the specified explosive gas environment.

The power supply introduced in this article is designed for the complex environment requirements of coal mines. It is a two-level protection intrinsically safe DC power supply for mines. The maximum output current and voltage of this power supply meet the intrinsic safety requirements in normal working and fault conditions. In normal working conditions, the power supply can stably provide 12VDC/1.2A driving capacity, which can meet the power supply needs of most low-voltage electrical equipment in mines.

Power supply overall design

The intrinsically safe power supply consists of step-down, rectification, filtering, DC voltage regulation and protection circuits, as shown in Figure 1.

Introduction of each unit circuit

Buck, rectifier, filter circuit

The step-down circuit is mainly composed of a power transformer, and its function is to reduce the AC high voltage to the required voltage value. Since AC127V or AC36V is often used to power low-voltage equipment in mines, this power supply uses an AC127V/AC36V dual-input, AC20V output industrial frequency transformer to achieve voltage reduction.

The function of the rectifier circuit is to convert the AC power output by the step-down circuit into DC power with unidirectional pulsation. This power supply uses 4 1N4007 diodes to form a bridge rectifier to achieve AC to DC conversion.

The main function of the filter circuit is to smooth the unidirectional pulsating DC power output after rectification to obtain a DC voltage. The filtered DC power can directly power the electronic circuit. The filter circuit uses a 220μf aluminum electrolytic capacitor and a 103 ceramic capacitor in parallel for filtering.

Voltage stabilization circuit

The voltage stabilization part uses a three-terminal adjustable integrated chip LM317. When the output voltage range of LM317 is 1.25V~37V, it can provide a current of more than 1.5A. The reference voltage of 1.25V is fixed between the output end and the adjustment end. The voltage stabilization value can be easily adjusted by changing the external resistance of LM317. As shown in Figure 3 below: The output voltage is often ignored because it is controlled to be less than 100. After the AC power is rectified and filtered to become DC, it is input to the three-terminal adjustable voltage stabilization module LM317, and the output obtains a stable voltage, which provides the working voltage for the subsequent overcurrent and overvoltage circuit modules, and at the same time obtains the comparison voltage, which provides the reference comparison voltage for overvoltage and overcurrent control.

In the power supply circuit introduced in this article, a 2-level LM317 dual voltage regulator is used. Under normal circumstances, the voltage difference between the input and output of U2 is greater than 0.7V, the transistor Q1 is turned on, R3 and R5 are connected in parallel, and the values ​​of resistors R3, R4, R5, R7, and R8 are reasonably set to meet

In this way, when U1 or U2 fails and shorts (as long as one LM317 can still work normally), the power supply can continue to output a stable 12V voltage, achieving two-level voltage regulation protection.

Overcurrent protection circuit introduction

Through resistor voltage sampling, a high-sensitivity voltage comparator LM393 is used to form a two-stage overcurrent protection circuit, as shown in Figure 5, which is the second-stage overcurrent protection circuit. When the intrinsically safe output terminal is short-circuited or the output current is greater than the predetermined value, on the one hand, R9 and R10 are connected in series to divide the voltage, so that the input voltage of the positive input terminal of U3 is reduced. On the other hand, the current passes through R11 to form a loop to produce a voltage drop, so that the input voltage of the reverse input terminal of U3 is reduced, so that the voltage comparator U3 outputs a low level. In this circuit, the output terminal of U3 is connected to the adjustment pin (first pin) of U2. When the intrinsically safe output is overcurrent, U3 outputs a low level, so that the resistor R8 is short-circuited to the ground. According to the output voltage characteristics of LM317, it can be seen that the output voltage of the voltage regulator chip U2 is 1.25V, so the circuit is protected at this time.

Similarly, the same overcurrent protection circuit is used to protect the voltage regulator chip U1, so that the circuit forms a two-level overcurrent protection. At the same time, two identical 12V voltage regulator tubes D1 and D2 are used at the intrinsically safe output end for double voltage protection to prevent the output voltage from increasing unexpectedly.

test

A power supply was randomly selected for testing, and the output voltage was measured to be 11.9V, the overcurrent protection action value was 1.2A, and the short-circuit current was stable at 23mA, which met the intrinsic safety requirements of GB3836.4-2010. After the power supply was connected to a load and worked continuously for 72 hours, the power supply parameters still met the requirements. In actual design, the overcurrent protection action value of the power supply can be changed by adjusting the value of the voltage divider resistors R9 and R10. The intrinsic safety parameters of the power supply forensics are: UO=12V, IO=1200, CO=3μf, LO=0.1mH, and the short-circuit stable current value is ≤40mA.

Conclusion

This article introduces a mine intrinsically safe power supply, which has obtained the coal mine safety standard and is widely used in the power supply of various equipment in coal mines. After long-term use, the power supply has reliable performance and stable operation. In fact, this power supply can not only be used in coal mines, but also in environments containing flammable and explosive gases such as petroleum and chemical industries.