DC motors featuring permanent magnet are extensively used and favored by model builders. One notable trait of these motors is the significant difference between their startup torque and nominal torque. To power a permanent magnet motor from a DC supply and maintain a constant high torque at varying operating speeds, a pulse-width modulator (PWM) like the one described here is essential. We create an astable multivibrator employing a pair of gates from a 40106 hex Schmitt trigger IC.
Adjustable Mark-Space Ratio: Potentiometer Control in Multivibrator Circuit
The multivibrator featured here offers a wide-ranging adjustment of its mark-space ratio, or ‘duty factor,’ independently of its operating frequency. By manipulating the potentiometer, the ratio between Ra and Rb, constituting the total resistance of the potentiometer, can be altered. Capacitor C1 is charged through Rb and discharged via Ra. Determining the corresponding mark-space ratio at the oscillator’s output on pin 4 of the 40106.
Output high time is determined by Ra, while output low time is controlled by Rb. The oscillator maintains a constant frequency around 115 Hz. Transistor T1 provides current amplification; when pin 4 of the 40106 is low. T1 activates, and when the output is high, T1 is deactivated. Adequate current is available at the collector of T1 to drive the base of the 2N3055 power transistor. R4 and C2 offer decoupling, ensuring the oscillator remains isolated from the substantial currents switched by the power output stage.
Battery Monitoring and Practical Applications: Utilizing the Multivibrator Circuit
A moving-coil meter connected through R7 serves the purpose of monitoring the battery’s state, particularly beneficial when rechargeable cells are in use. The circuit’s author has employed it to power a motor salvaged from an old cassette tape recorder, eliminating the need for a heatsink for the 2N3055 in this specific application. Enthusiastic readers can explore a wealth of information on the theory and practical aspects of driving DC motors using pulse-width modulation through a simple online search.