Design of Pulse Width Modulation Control Circuit Based on LF347 Four Operational Amplifiers

LF347 is an affordable and reliable integrated circuit. This paper presents a pulse width modulation (PWM) control circuit designed using four operational amplifiers from the LF347 IC, which effectively regulates DC output. The design is divided into three main sections: the basic circuit structure, detailed analysis of each component, and the output control mechanism. The motivation for this project comes from the widespread use of PWM technology in industrial and household applications. Due to its maturity, PWM has significantly reduced the cost of DC/AC inverters and made frequency conversion control of AC motors more accessible. However, the question arises: can PWM also be used to control DC motor speed, and can it be done at a low cost? To address this, I designed a simple and effective PWM controller using the LF347 IC, achieving excellent results with minimal cost. The circuit consists of four key components: a reference voltage section, a triangular wave generator, a voltage comparator, and a power output stage. A block diagram of the system is shown in Figure 1-1, where one op-amp is used for the reference voltage, two for the triangular wave generator, and one for the comparator. Figure 1-2 illustrates the internal structure of the LF347 IC. In the reference voltage section, as shown in Figure 1-2, the circuit includes resistors R1, RP1, R2, and a potentiometer. By adjusting RP1, the voltage at point A can be varied between +4V and -4V. This voltage is then connected to the non-inverting input of the comparator via resistor R3, while the inverting input is directly connected to the output, forming a voltage follower with a gain of approximately 1. The triangular wave generator uses two op-amps, ICD and ICA. As illustrated in Figure 1-3, ICD, along with diodes VD1 and VD2, forms a square wave generator. The output of ICD switches between ±12V depending on the voltage comparison at its inputs. This square wave is used as the input to ICA, which integrates the signal to produce a triangular waveform. By adjusting the potentiometers RP2 and RP3, both the frequency and amplitude of the triangular wave can be controlled. The voltage comparator, ICC, compares the triangular wave at its inverting input with a DC reference voltage at its non-inverting input. When the triangular wave is higher than the reference voltage, the comparator outputs -10V; otherwise, it outputs +10V. This creates a PWM signal whose duty cycle is determined by the level of the reference voltage. Adjusting the reference voltage allows for precise control over the average voltage applied to the load, making it ideal for controlling DC motor speed. The power amplifier stage employs transistors VT1, VT2, and VT3 in an OCL configuration, ensuring that the output waveform matches the input. VT4, a field-effect transistor, acts as a switch controlled by the OCL output. When a positive pulse is applied, VT4 turns on, allowing current to flow through the load. When the pulse is negative, VT4 turns off, stopping the current. This switching action enables efficient control of the motor’s speed. Through testing and adjustment, the triangular wave was set to a frequency of 1 kHz by tuning RP2 and an amplitude of ±3V by adjusting RP3. By varying the reference voltage at point A, the PWM duty cycle could be adjusted from 0% to 100%. The circuit was successfully implemented in a car air conditioner fan, serving as a smooth speed controller. However, a negative power supply (LM7912) was required for full functionality. This project demonstrates that a low-cost IC like the LF347 can be effectively used to build a functional PWM control system, offering a practical solution for DC motor speed regulation.

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