Design for solenoid driving circuit based on regulations of current

Abstract

A solenoid driving circuit contains solenoids, each of which is driven to produce a magnetic field for driving each of keys of a player piano. A NPN transistor is provided to allow or block a flow of current across each solenoid. The solenoid is connected between a DC power source for providing a source voltage and a collector of the NPN transistor whose emitter is grounded. A drive signal, which is subjected to pulse-width modulation, is supplied to a base of the NPN transistor, so that the NPN transistor is switched over between ON and OFF. A diode is introduced to provide prescribed forward voltage for attenuation of the current across the solenoid when the NPN transistor is turned OFF. Herein, an anode of the diode is connected to a connection between the solenoid and NPN transistor, while a cathode of the diode is connected to a cathode of a zener diode having prescribed reverse voltage. An anode of the zener diode is connected to the DC power source. An effective time constant of the solenoid is represented in a mathematical form using the forward voltage, reverse voltage and source voltage as well as a real time constant of the solenoid. So, the solenoid driving circuit designed in such a way that the effective time constant of the solenoid is sufficiently small as compared to a maximum value of an operating frequency of the key of the player piano (i.e., action cutoff frequency of the player piano).