LF347 is an economical and reliable integrated circuit. This paper presents a pulse width modulation (PWM) control circuit designed using four operational amplifiers from the LF347, which effectively regulates DC output. The design includes the overall circuit structure, detailed analysis of each component, and the implementation of output control.
First, the problem is introduced. PWM technology has become widely adopted in industrial control systems and household appliances. Thanks to its maturity, the cost of DC/AC inverters has been significantly reduced, and frequency conversion technology makes it easier to control AC motor speeds. However, can PWM also be used to control DC motor speed? And can this approach reduce costs? To address these questions, I implemented a PWM controller using the LF347 (a 4-op-amp IC) to regulate the speed of a DC motor. The results were promising, with good performance and low cost.
Second, the basic structure of the circuit is described. The circuit consists of four main sections: a reference voltage generator, a triangular wave oscillator, a voltage comparator, and a power amplifier. A block diagram is shown in Figure 1-1, where one op-amp is used for the reference voltage section, two op-amps form the triangular wave generator, and one op-amp serves as the voltage comparator. Figure 1-2 illustrates the internal layout of the LF347 IC.
Third, the analysis of each part of the circuit follows. The reference voltage section uses resistors R1, RP1, R2, and the op-amp ICB. By adjusting RP1, the potential at point A can be varied between +4V and -4V. This voltage is then connected to the non-inverting input of ICB through resistor R3, while the inverting input is directly connected to the output, forming a voltage follower. The output of this stage is fed into the non-inverting input of the comparator ICC.
The triangular wave generator consists of two op-amps, ICD and ICA. ICD, along with voltage regulators VD1 and VD2, forms a square wave generator. The output of ICD switches between +12V and -12V based on the comparison between its inputs. The output of ICD is used as the input to ICA, which integrates the signal to produce a triangular wave. The frequency of the triangular wave can be adjusted using potentiometer RP2, while RP3 controls the amplitude.
The voltage comparator, ICC, operates in open-loop mode, resulting in a high gain and rapid switching between ±10V. The inverting input receives the triangular wave, while the non-inverting input is set by a DC reference voltage UA. When the triangular wave exceeds UA, the comparator outputs -10V; otherwise, it outputs +10V. This creates a PWM signal whose duty cycle depends on the magnitude of UA, allowing for precise control of the output voltage.
The power amplifier section uses transistors VT1, VT2, and VT3 to form an OCL (output capacitor-less) amplifier. The output waveform matches the input, producing a rectangular wave with adjustable pulse width. VT4, a power MOSFET, acts as a switch controlled by the gate voltage. When the gate is negative, the transistor turns off; when the gate voltage exceeds the threshold, the current increases proportionally. This allows for efficient control of the load current, which in turn adjusts the motor speed.
Finally, the circuit was tested and verified. The triangular wave frequency was set to 1kHz by adjusting RP2, and the amplitude was set to ±3V using RP3. By fine-tuning RP1, the reference voltage could be adjusted between +4V and -4V, enabling a PWM duty cycle ranging from 0% to 100%. The circuit was installed in a car and used as a variable-speed controller for an air conditioner fan, proving to be convenient and effective. However, a negative power supply (LM7912) was required to complete the circuit.
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