Pulse-Width Modulation (PWM) Explained
Pulse-width modulation (PWM), also known as pulse-duration modulation (PDM). It is a technique employed to reduce the average power delivered by an electrical signal. This method involves dividing the signal into discrete parts by rapidly switching the connection between the supply and the load on and off. By controlling the duration of the on and off states, the average voltage (and current) sent to the load is effectively managed. The longer the switch remains on compared to the off periods, the higher the total power supplied to the load. This method, in conjunction with maximum power point tracking (MPPT), offers versatile control over electrical devices.
PWM Control for Electric PCB Drills
Pulse-width modulation (PWM) proves highly effective in regulating small electric PCB drills with currents up to 2 A. With appropriate cooling for T2 and an increased value of C2, larger currents can be managed. The design leverages the inherent nature of the drill, which operates using a small direct current (d.c.) motor. The rotary speed of this motor is directly influenced by the voltage applied across it, allowing precise control and modulation of the drill’s performance. This approach ensures efficient and tailored power management for electric devices.
Design Overview (Fig. 1)
The design illustrated in Figure 1 utilizes the astable multivibrator IC1a. Where the output remains low for a duration determined by R1 and high for a period set by R and P1.
Operation of the Circuit
Upon discharge of C1, the input level of IC1a drops below the lower threshold, causing the output of this stage (pin 2) to go high. Subsequently, the capacitor charges rapidly through D1 and R1, reaching the upper threshold in approximately 1.5 ms. At this point, the output of IC1a switches low, leading to the discharge of C1 through D2, R2, and P1. In the prototype, the discharge time could be adjusted between 0.2 ms and 25 ms, allowing the duty factor of the output signal to range from 5% to 90%.
Motor Control and Speed Adjustment
The inverted signal is then directed to the base of T1. Transistors T1 and T2 are activated, energizing the motor during the negative period of the output pulse from IC1b (pin 4). Notably, when the resistance of P1 is at its minimum, the rotary speed of the drill motor reaches its maximum. This setup ensures precise control over the motor’s speed and operation.