Patent Publication Number: US-4318062-A

Title: Ultrasonic wave nebulizer driving circuit

Description:
BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION 
     This invention relates to an ultrasonic nebulizer driving circuit capable of stabilizing the driving power of an ultrasonic vibrator to maintain a constant nebulizing rate. 
     In an ultrasonic liquid nebulizer of conventional type, it is usual to drive a piezo-electric vibrator provided at the bottom of a vessel containing a liquid to be nebulized with a driving circuit so as to generate ultrasonic waves, and the energy thereof nebulizes the liquid, such as water. Generally, no output stabilization is provided, and variation of the input AC voltage (e.g. 48 V) causes a sharp variation of the nebulizing rate. Accordingly, a conventional circuit cannot be used for the atomization of kerosene in burners or the nebulization of medicine in medical apparatus, e.g., where a constant nebulizing rate is required. 
     Accordingly, an object of the present invention is to provide an ultrasonic wave nebulizer driving circuit capable of stabilizing the piezo-electric vibrator driving power to maintain a stabilized nebulizing rate. This object is obtained by utilizing negative feedback to stabilize oscillation, derived from sensing voltage or current or power supplied to the piezo-electric vibrator. 
     Explanation will be made hereinafter of presently preferred forms of ultrasonic wave nebulizer driving circuits embodying the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a circuit diagram of an ultrasonic nebulizer driving circuit embodying the present invention. 
     FIG. 2 shows a transfer characteristic of the FET included in the circuit of FIG. 1. 
     FIG. 3 is a graphical representation of the relationship between the DC voltage V2 and the base bias current of the transistor 3 in the circuit of FIG. 1. 
     FIG. 4 is a graphical representation of the relationship between base bias current of the transistor 3 and nebulizing rate in the circuit of FIG. 1. 
     FIG. 5 is a circuit diagram of an alternative driving circuit embodying the invention. 
    
    
     DETAILED DESCRIPTION 
     In the Colpitts oscillator of FIG. 1, an AC voltage applied to power source terminals A and B is rectified by a rectifier 1, then smoothed by a smoothing condenser 2. A DC voltage thus produced is supplied to a positive line P and a negative line N. The collector of a transistor 3 is connected to the positive line P through a coil 4, and the emitter of the transistor 3 is connected to the negative line N through a coil 5. A condenser 6, constituting a parallel resonance circuit together with the coil 4, is connected between the collector of the transistor 3 and the negative line N. A piezo-electric vibrator 8 having a diameter of 20 mm and a resonance frequency of 1.65 MHz, e.g., is driven by a circuit between the collector and the base of the transistor 3 that includes a condenser 7 that blocks the flow of DC current. The condenser 7 may be omitted without affecting the oscillation. A condenser 10 is connected between the base of the transistor 3 and the negative line N. The primary coil 20A of a transformer 20 is connected between the collector and the base of transistor 3 through condenser 7, and the piezo-electric vibrator 8 is connected to secondary coil 20B of the transformer. 
     An output stabilizing circuit 30 in accordance with the invention is provided on the base side of the transistor 3. In the output stabilizing circuit 30, a diode 31 is connected to a tertiary coil 20C of the transformer 20 to pick up the negative half of a cycle of a high frequency voltage V1 proportional to the input power, voltage or current of the piezo-electric vibrator 8. The output, rectified by the diode 31, is applied to a variable resistor 33 after being smoothed by a smoothing condenser 32. A DC voltage V2 provided by the variable resistor 33 is impressed upon the gate of an FET 34. The drain of the FET 34 is connected to the positive line P through a resistor 35, while the source or other FET electrode is connected to the negative line N through a resistor 36. The source voltage is impressed upon the base of a transistor 37 controlling the current therein; the transistor 37 supplies a base bias current to the transistor 3 through a resistor 38. 
     In the circuit of FIG. 1, the driving power of the piezo-electric vibrator or oscillator 8, i.e., the nebulizing rate, depends on the DC voltage V2 (the voltage between the gate and the source of the FET 34) set by the variable resistor 33 provided that there is no variation in the external conditions, such as the AC input voltage. That is because the DC voltage V2 determines the drain current of the FET 34, which in turn controls the base current of the transistor 37, which in turn controls the base bias current of the transistor 3. 
     When the nebulizing rate increases due to variation of the AC input voltage, the high frequency voltage V1 becomes high, the DC voltage V2 increases in the negative sense, and the gate of the FET 34 is biased more in the negative sense. Since the transfer characteristic of the FET 34 is as shown in FIG. 2, the drain current decreases, the base voltage of the transistor 37 is lowered and the base bias current of the transistor 3 decreases. Consequently, the base bias current varies corresponding to the variation of the DC voltage V2 as shown in FIG. 3. Since the relation between the base bias current and the nebulizing rate is as shown in FIG. 4, the nebulizing rate varies in the negative sense according to a decrease of the bias current to cancel the increase of the nebulizing rate caused by the variation of the external condition such as increased AC input voltage. 
     On the other hand, when the nebulizing rate decreases, caused by variation of the AC input voltage, the high frequency voltage V1 becomes lower and the absolute value of the DC voltage V2 also becomes lower. Consequently, the gate of the FET 34 is biased in a positive direction, and the base bias current of the transistor 3 increases, causing the nebulizing rate to increase to cancel the decrease of the nebulizing rate caused by the variation of the external condition. 
     According to this embodiment of the invention, it is possible to maintain the nebulizing rate approximately fixed by providing an output stabilizing circuit 30 as shown in FIG. 1 to an otherwise conventional Colpitts oscillator. 
     In the embodiment of FIG. 1, the output stabilizing circuit 30 is constituted of a combination of an FET and transistors. However, an operational amplifier may be used instead of an FET and transistors, and the current and voltage of the piezo-electric vibrator 8 may be produced by connecting a current transformer in series with the piezo-electric vibrator instead of the 3-coil transformer 20 of FIG. 1. FIG. 5 shows such a modified circuit. The current and voltage from the piezo-electric vibrator 8 is taken out by current transformer 40 including primary winding 40A and secondary winding 40B. The circuit also includes RF diode 51, condenser 52, variable resistor 53 for input adjustment, operational amplifier 54, resistors 55 and 57 for adjustment of the DC amplitude of the operational amplifier, variable resistor 53 for feedback adjustment, and transistor 59 for control of the base current flowing in the base of transistor 3. 
     The present invention provides an ultrasonic nebulizer driving circuit capable of stabilizing the piezo-electric vibrator driving power to stabilize the nebulizing rate. Modifications to the preferred embodiments may be made. The invention thus should be defined by the following claims.