JPS6118635Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a dimming circuit for adjusting the brightness of a lamp.

Conventionally, as a dimming circuit used in optical equipment such as microscopes, the AC voltage from the power supply is temporally controlled by a thyristor such as a triax connected to the primary side of the transformer, and then connected to the secondary side of the transformer. There are known devices that can change the brightness of a lamp. However, in such a conventional dimming circuit, since the inductance of the transformer is used as the load of the thyristor, the high frequency noise associated with the rapidly changing current caused by the switching of the thyristor can adversely affect other measuring instruments and The drawback was that it caused a humming noise from the transformer. Therefore, it was not preferable as a dimming circuit used in optical equipment such as a microscope.

The purpose of this invention is to temporally control AC signals,
An object of the present invention is to provide a dimmer circuit that reduces the influence of high-frequency noise in a dimmer circuit that controls the brightness of a lamp.

The present invention will be explained below based on examples.

FIG. 1 shows a first embodiment of the present invention. The AC voltage from the AC power supply 40 is stepped down by the transformer 38 and then full-wave rectified by the full-wave rectifier circuit 36 .
A full-wave rectified voltage from the full-wave rectifier circuit 36 is applied between terminals a and b such that terminal a is positive. The full-wave rectified voltage waveform applied between terminals a and b is shown in FIG. 2a.

A variable resistor 2, a semi-fixed resistor 4, a resistor 6, and a capacitor 8 constitute a time constant circuit 1. The semi-fixed resistor 4 is a resistor for preventing the time constant from becoming too large when the time constant is changed by the variable resistor 2. When the voltage at the connection point C between the resistor 6 and the capacitor 8 becomes equal to or higher than the sum of the Zener voltage of the Zener diode 10 and the base-emitter voltage of the transistor 12, a base current begins to flow through the transistor 12. Thereby transistor 12
is conductive. The collector current of the transistor 12 flows through the resistor 16 and acts to cause the base current of the transistor 20 to flow through the resistor 18. Therefore, the transistor 20 becomes conductive.
When the transistor 20 becomes conductive, current flows through the resistors 22 and 24, and the base voltage of the transistor 14 connected to the connection point between the resistors 22 and 24 increases, making the transistor 14 conductive. Due to conduction of transistor 14, the base current of transistor 20 increases and transistor 20 approaches saturation.
In this way, the interaction between transistors 12, 14, and 20 causes transistor 20 to rapidly turn on (ie, saturated). FIG. 2b shows changes in the collector voltage of the transistor 20 with respect to the full-wave rectified voltage waveform applied between terminals a and b. The collector voltage of the transistor 20 is approximately equal to the voltage at the terminal a from when the transistor 20 is turned on until it is turned off (until the voltage at the terminal a approaches zero volts).
When it is in the OFF state, it is zero volts. The above transistors 12, 14, 20, Zener diode 1
0, the high-speed switch circuit 3 consisting of resistors 16, 18, 22, and 24 instantly switches from the ON state to
Since the transition to the OFF state occurs, it can be assumed that the transistor operates regardless of variations in the characteristics of each transistor or temperature changes. The low-pass filter 5, which is composed of a resistor 26 and a capacitor 28, is a circuit for smoothing out the sudden change in voltage (in this case, a change due to a rise) in FIG. 2b as shown in FIG. 2c. Above time constant circuit 1 and high speed switching circuit 3
and a low-pass filter 5 constitute a control circuit. The switching circuit 30 consisting of two Darlington-connected transistors is activated by the voltage shown in FIG. 2c, which is the output of the low-pass filter 5, from time t ₀ to t ₁ shown in FIG. , and is in a saturated state from time t ₁ until time t ₂ . Therefore, the current flowing through the lamp 32 does not change abruptly, but becomes a current proportional to the voltage change as shown in FIG. 2c. Then, the lamp 32 lights up with a brightness that corresponds to the integral amount of current flowing through it.

The transistor 34 is a transistor for discharging the charge accumulated in the capacitor 8, and the collector current of the transistor 20 begins to flow, and the resistor 22
When the voltage at the connection point between and 24 exceeds a certain voltage, conduction occurs and the accumulated charge in the capacitor 8 is discharged. The transistor 12 becomes non-conductive due to the discharge of the capacitor 8, but since the transistor 14 is conductive, the transistor 20 maintains the ON state.

Note that the low-pass filter 5 is not essential to the present invention, but is added in order to obtain better performance.

FIG. 3 shows a second embodiment of the present invention.

The difference between the first embodiment and the second embodiment is that the configuration of the control circuit is different, and the way the switching circuit is controlled is reversed due to the difference in the configuration of the control circuit.

A time constant circuit 7 is configured by a variable resistor 42 and a capacitor 44 connected between terminals a and b. The time constant of the time constant circuit 7 is selected by changing the value of the variable resistor 42. Unijunction transistor 46 becomes conductive when the voltage at its emitter connected to the connection point between variable resistor 42 and capacitor 44 exceeds a certain level, and discharges the accumulated charge in capacitor 44 . Conduction of unijunction transistor 46 causes transistor 48 to conduct. The switching circuit 30 is turned on by the conduction of the transistor 48.
The base voltage of the transistor 31 constituting the lamp is approximately zero volts, and no lamp current flows. When the voltage at the connection point between the variable resistor 42 and the capacitor 44 is below a certain level and the unijunction transistor 46 is non-conductive, the transistor 48 is also non-conductive. Therefore, the base of the transistor 31 constituting the switching circuit 30 is forward biased by the voltage at the terminal a via the resistor 50. Therefore, the transistor 31 quickly becomes saturated and a lamp current flows. FIG. 4 shows the change a in voltage at terminal a and the change b in lamp current with respect to terminal b.

Note that, as shown by the dotted line in FIG. 3, a capacitor 52 is connected between the collector of the transistor 48 and terminal b.
When connected, this capacitor 52 and the resistor 50 play exactly the same role as the low-pass filter 5 of the first embodiment.

Further, the present invention is particularly effective for large-capacity lamp loads.

As mentioned above, in the present invention, the switching circuit is provided on the secondary side of the transformer, and the resistance of the lamp is used as the load of the switching circuit instead of the inductance of the lance of the conventional circuit. The effect of high frequency noise generated during switching can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a first embodiment of the dimmer circuit of the present invention, Fig. 2 is a waveform diagram showing the waveforms of the main parts at each point of the dimmer circuit of Fig. 1, and Fig. 3 is a circuit diagram showing the first embodiment of the dimmer circuit of the present invention. FIG. 4 is a waveform diagram showing waveforms of main parts at each point of the dimming circuit of FIG. 3. FIG. Explanation of the main parts

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Claims (1)

[Claims for Utility Model Registration] A rectifier circuit for rectifying the step-down voltage output from the transformer is connected to the secondary side of a step-down transformer whose primary side is connected to an AC power source, and the output terminal of the rectifier circuit In between, there is provided a series connection circuit of the lamp and the transistor switching circuit, and a time constant circuit for changing the time interval between conduction and non-conduction of the transistor switching circuit. , and a control circuit for inputting a control signal to the transistor switching circuit to control conduction or non-conduction of the circuit, and connected in parallel.