Ultrasonic Liquid Atomizer With an Improved Soft Start Circuit

The present invention relates to an ultrasonic liquid atomizing apparatus useful for humidifying dried spaces and atomizing medicated liquid.

Conventional ultrasonic liquid atomizers include a free-running Colpitts oscillator that generates high frequency energy in the range between 800 kHz and 2000 kHz and drives a piezoelectric vibrator mounted in energy transfer relation with liquid contained in a chamber. Full-wave rectified, nonfiltered sinusoidal source voltage is derived from a mains supply and a bias voltage is derived from the full-wave source voltage. The oscillator includes a transistor which is powered by the full-wave source voltge and biased by the full-wave bias voltage. The vibrator presents an energy radiating surface which is parallel with the surface of the liquid to maximize the energy transfer efficiency. A liquid sensing switch is provided in the liquid to complete a circuit for coupling the bias voltage to the oscillator when the chamber is filled with liquid, but opens the circuit to prevent the oscillator from being overloaded in the event that the liquid is depleted.

One disadvantage of the prior art apparatus resides in that fact that acoustic energy, initially generated in response to application of a mains supply to the apparatus or in response to reapplication of the bias voltage, is totally reflected from the liquid surface and impinges on the vibrator with a high level of energy. The energy reflected back to the vibrator induces a current in the vibrator. The induced current adds to the current generated by the oscillator with the result that the combined current may exceed the ratings of the vibrator, a transistor of the oscillator and other components of the oscillator.

Previous attempts to eliminate this problem involve starting the oscillator with reduced power and gradually increasing the power until continuous oscillation is attained. One approach, which is considered most relevant to the present invention, employs a zero crossing detector that detects when the apparatus is ready for operation and detects a zero crossover point of a full-wave source voltage to drive a thyristor into conduction. Oscillation starts in response to the thyristor being rendered conductive at the zero-volt point of the full-wave source voltage and gradually increases in amplitude to attain a maximum level within the period of a halfwave of the source voltage. The prior art soft start circuit of this type has a memory and logic circuit to initiate conduction of the thyristor. The use of a zero crossing detector, thyristor and memory results in a costly and complex apparatus.

An object of the present invention is to provide a liquid atomizing apparatus having a simple and inexpensive soft start circuit.

Another object of the present invention is to provide a liquid atomizing apparatus having a soft start circuit and a bias voltage stabilizer which prevents the soft start circuit from responding to unwanted voltage fluctuations generated in the oscillator.

In accordance with the invention an apparatus used with an ultrasonic liquid atomizer having a liquid chamber for holding liquid and a piezoelectric vibrator for generating pressure variations in the liquid in response to ultrasonic energy comprises an oscillator having a power supply terminal and a resonant circuit including a piezoelectric vibrator. A power supply circuit supplies gradually increasing and decreasing voltage variations to the power supply terminal. The oscillator is sequentially prevented from conducting and enabled for conduction only during a brief period of initial operation. During the brief initial period the oscillator conducting intervals are progressively increased.

According to a feature of the present invention, a power supply circuit derives from a mains supply a full-wave rectified supply voltage and a bias voltage. An oscillator has a first transistor biased by the bias voltage and a resonant circuit, a part of which is formed by a piezoelectric vibrator. The first transistor and the resonant circuit are connected to receive the full-wave rectified supply voltage to generate ultrasonic energy in the piezoelectric vibrator in the presence of the bias voltage. The soft start circuit comprises a time constant circuit responsive to a bias voltage and a second transistor which is biased by an output from the time constant circuit. The second transistor provides switching action in phase with the full-wave rectified supply voltage so that ultrasonic energy is generated in the form of a series of bursts with durations gradually increasing as a function of time during an initial brief interval from the initial generation time of the bias voltage.

According to a further feature of the invention, a bias stabilizer having a second time constant circuit is connected to the power supply circuit and a third transistor is connected to be responsive to the output of the second time constant circuit. The time constant value of the second time constant circuit is greater than the time constant value of the first time constant circuit. The third transistor establishes a low impedance path from a junction between the first time constant circuit and the second transistor after the termination of the initial brief interval.

The present invention will be described in further detail with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of a vibrator oscillating circuit according to the present invention;

FIG. 2 includes several waveform diagrams associated with the circuit of FIG. 1; and

FIG. 3 is a diagram of a liquid nebulizing apparatus incorporating the circuit of FIG. 1.

Referring now to FIG. 1, a power supply 3 includes a step-down transformer 31 connected in use to a mains supply 10 through power switch 11 to provide an AC voltage of 40 to 60 volts to a full-wave rectifier formed by bridge diodes 32a, 32b, 32c and 32d. Full-wave rectified, nonfiltered sinusoidal halfwaves are developed across positive and negative power lines P and N between which is connected a capacitor 33 to reduce the impedance of the negative line and thereby suppress line noise and radiation noise.

A bias circuit 4 includes a series circuit of resistors 41a and 41b which is connected across power lines P and N to divide the full-wave voltage to generate a sinusoidal bias voltage which is coupled through a liquid-sensing, float-contact switch 42 and a variable resistor 43 to a bias line B. Liquid-sensing switch 42 is located in the liquid chamber of an atomizer, not shown, and is arranged so contacts thereof close and open when the liquid chamber is respectively filled with liquid and partially filled, as when the atomizer is tilted, or when liquid has been exhausted from the chamber.

The bias voltage is fed through a soft-start circuit 5 to a Colpitts oscillator circuit 2 which drives a piezoelectric transducer or vibrator 1. Soft-start circuit 5 comprises a voltage dropping resistor 51 interposed in the bias line B, a switching transistor 52 having a collector connected to the low voltage terminal of resistor 51; the switching transistor includes an emitter which is connected to a reference point or negative line N. Voltage-dropping resistor 51 serves as an attenuator for attenuating short-term voltage fluctuations of oscillator 5 to decouple the fluctuations from the bias voltage on line B.

A time constant circuit formed by diode 53 and capacitor 54 is connected from line B to the base of transistor 52. The ON-state resistance of diode 53 and the capacitance of capacitor 54 determine a time constant value which defines a "soft" start period T.sub.1 for oscillator 2. A soft start for oscillator 2 implies that the oscillator is smoothly started with gradually increasing periods of oscillation. Since capacitor 54 initially presents a low impedance to the application of a potential to bias line B, current flows through diode 53 and capacitor 54 to the base of transistor 52, causing it to conduct. Transistor 52 is then turned off when bias voltage drops to a near zero-volt point to allow the bias potential to be applied to oscillator 2, but turns on again as the bias voltage increases, cutting off the bias supply to oscillator 2. This process is repeated in step with the sinusoidal bias potential on line 2. The current through diode 53 eventually decreases as the voltage on capacitor 53 approaches an exponential asymptote. Thus, the turn-off period of transistor 52 decreases successively and the bias supply period of oscillator 2 increases successively during the soft-start period T.sub.1 as shown by waveform B in FIG. 2. Soft start circuit 5 further includes a resistor 55 connected in parallel with diode 53 and a diode 56 having an anode coupled to the low voltage terminal of resistor 51 and a cathode to the base of transistor 52. Resistors 51 and 55 and diode 56 form a circuit for discharging capacitor 54 when the bias voltage is removed from bias line B.

Colpitts oscillator circuit 2 includes transistor 21 having collector and emitter electrodes coupled respectively to the positive and negative lines and a base coupled to the low voltage terminal of resistor 51 of soft-start circuit 5. A parallel resonant circuit is formed by a coil 22 connected in negative line N and a capacitor 23 connected between the positive and negative lines. The parameters of this resonant circuit are determined so that the resonant circuit presents a capacitive load to transistor 21 near the resonant frequency of piezoelectric transducer 1. Coil 22 may alternatively be connected to the collector of transistor 21. The piezoelectric transducer 1 forms a feedback circuit with a DC decoupling capacitor 24 between the collector and base of transistor 21 and acts as an inductive element of the oscillator circuit. A capacitor 25, connected between the base and emitter of transistor 21, controls the amount of oscillator feedback to determine the oscillation amplitude.

With the arrangement described above, transistor 21 turns on and off at an ultrasonic frequency when switching transistor 52 is in the OFF state. During the soft start period, oscillator 21 starts whenever the power line voltage drops to a near zero-volt point; oscillation of the oscillator is interrupted as it reaches successively increasing levels as shown by waveform C, FIG. 2. As described above, the duration of the oscillations gradually increases as a function of time, whereby the oscillator is "soft" started to overcome the derivation of surge currents which otherwise would occur. A preferred length of the soft-start period determined by the time constant of diode 53 and capacitor 54 is equal to or greater than the total period of three full-wave rectified sinusoidal halfwaves. After the soft-start period, transistor 52 remains in the OFF-state to allow oscillator 2 to sustain steady-state oscillation.

The disadvantage of the oscillator is that unwanted voltage fluctuations are generated during steady-state oscillation. Unwanted current passes through diode 56 or through diode 53 and capacitor 54 to the base of transistor 52, causing the transistor to conduct. As a result, undesirable bias potential variation occurs at the base of transistor 21.

In a preferred embodiment of the present invention, a bias stablizer 6 is provided. This stabilizer comprises a resistor 61 and a capacitor 62 connected in series between the bias line B and negative power line N. The voltage developed across capacitor 62 is coupled by a resistor 63 to the base of a transistor 64 having an collector-to-emitter path connected from the base of transistor 52 to the negative power line. Resistor 61 and capacitor 62 establish a time constant which is larger than the time constant of the soft start circuit 5 so that transistor 64 conducts after transistor 52 switches to a steady OFF state. When this occurs, transistor 64 establishes a short-circuit path across the base and emitter of switching transistor 52, whereby the spurious signal passes through the short-circuit, to bypass the base-to-emitter path of transistor 52 and maintain transistor 52 in the OFF state.

This bias stabilizer also protects from surge current which might occur as result of a fault in the circuit of piezoelectric transducer 1 when the liquid is filled or a fault in the circuit of liquid sensing switch 42 when the liquid is depleted. The surge current would otherwise cause breakdown of transistor 21. The stabilizer enables dampening of the surge current the circuit comprising diode 56 and transistor 64.

An ultrasonic liquid nebulizer 20 embodying the present invention is represented in FIG. 3. The nebulizer includes reservoir 22 in communication with liquid chamber 21 to supply liquid thereto. In a typical example, chamber 21 is supplied with medicated liquid having a horizontal surface maintained at a predetermined distance above the bottom of liquid chamber 21 where piezoelectric vibrator 1 is mounted in energy transmitting relationship with the liquid. Short-duration pressure changes occur in vibrator 1 in response to ultrasonic energy generated by the oscillator 2. In response to the pressure changes, the liquid rises to form column 23 and is atomized into minute droplets ejected through a nozzle 24 to the outside.

The liquid sensing switch 42 is located in the liquid chamber 21. Switch 42 includes magnetically sensitive reed contacts 42a mounted in a sealed glass tube, in turn located in a cylinder. Coil 42c which is connected to a suitable current source, not shown, is vertically slidably mounted on cylinder 42b so that it moves with the varying surface level of the liquid. With the chamber 21 filled with liquid, coil 42c is in an upper position and switch 42a is closed to couple the bias voltage to the circuit including oscillator 2, soft start circuit 5 and bias stabilizer 6. When the liquid has been exhausted from chamber 21 and reservoir 22 or the nebulizer 20 is tilted so that the liquid surface level drops to a point near the bottom of chamber 22, coil 42c triggers switch 42a into an open condition to cut off the bias supply.

Without soft start circuit 5 piezoelectric vibrator 1 would initially create high amplitude acoustic waves which are reflected from the liquid surface to be incident on vibrator 1. Vibrator 1 responds to the reflected acoustic energy incident thereon to generate an overvoltage which damages the oscillator and associated circuits. Such overvoltages occur when power switch 11 or liquid sensing switch 42 is closed. Soft start circuit 5 softens the initially generated energy by gradually increasing it to prevent the overvoltage condition. Bias stabilizer 6 prevents the soft start circuit from responding to unwanted voltage fluctuations generated by oscillator 2 when the bias voltage supply to the transistor 21 of oscillator 2 is interrupted.