Has long been noted that the viewing of slides, movies, TV shows, as well as any other visual activity (reading, drawing, and examined under a microscope), in unfavourable lighting conditions causes a large overvoltage of view. Our vision has the ability of adaptation, i.e. adaptation to different lighting conditions, but this ability is not unlimited. Frequent change of illumination on the screen, the flicker and insufficient brightness, and off the General lighting in the demonstration, a series of slides or parts of the training film lead, as shown by observation, frequent change adaptation, causing eye fatigue. This is why classrooms and classrooms should be provided with “dimmers”, is gradually changing the brightness of the light.
The principle of operation
Feature of the device that we propose to the reader that the shift control on the basis of the analog odnoimennogo transistor V9, V10 is not using the variable resistor, and automatically, due to the charge (discharge) of the capacitor C2 through the resistor R6 (R11). This changes the drain current of the field effect transistor V11, and therefore the voltage drop at the resistor R12, which in turn causes the variation of the current through the transistor V12. This happens due to increasing or decreasing its internal resistance Rвн. The ratio of the shoulders of the divider R7, Rвн disturbed and thereby changes the bias on the base of the counterpart transistor odnoimennogo. Twice during the period of the comparison voltage on the capacitor C1 and to the base analogue. When they are equal, analogue open, and the capacitor C1 is discharged through the control electrode of the thyristor V6, which, in turn, controls the powerful thyristor V5. Consequently, the current V5 and thereby the voltage of the bulbs will depend on the potential across the capacitor C2.
Control device with two switches S12 and S13: one installed in the remote, the other near the camera, allowing you to independently control the “dimmer” with two seats.
Details and design
The device is designed for continuous operation within the lesson (45 min), and then disconnect it for 10-15 minutes. If you need more time, increase the power dissipation of the resistors R1 and R2 to 4-5 watts.
КТ209К transistor can be replaced by МП105, matching it with possible small initial collector current. The same applies to V9. Instead of the transistor КТ315Б permissible to use КТ315А with a gain of At least 80.
C1 is composed of two capacitors MBM-0,1-160V connected in parallel. The capacitor C2 is electrolytic K50-3 with a small leakage current. R10 is a variable resistor SP-0,4 (it is used to set the level of illumination of working places). Relay RES-10 (RS4 passport.524.302).
Diode D243 interchangeable on Д215. They must be installed on the radiators with a dispersion area of 100 cm3 (fit plate size 9×9 mm from the selenium rectifier). For thyristor V5 need a radiator area of 300-400 cm2 (three plates 9×9 mm). The thyristor V6 running without the radiator.
If the load is less than 1.2 kW can be limited to only one thyristor V6, installed on the radiator area of 200 cm2 (elements V5, R3 and R4 when not needed).
Fig. 1. The concept of “dimmer”.
The installation of this appliance is performed by printing on the Board size 128×55 mm (Fig. 2).
Fig. 2. PCB “dimmer” with the location details.
The establishment
Instead of the contact plates of the relay temporarily turn on the switch. The emitter of the transistor V12 desolder from the Board and connect to the “dimmer” load — incandescent lamp 220V x 500W. Rotating the engine variable resistor R10, see changing the brightness of the lamp. Parallel to it, turn on the voltmeter. In the upper scheme. the position of the slider R10 the tool should display 5-7 V. If the lamp switches off in series with a variable resistor R10 to turn on the DC resistance 1-4,7 com.
Then the engine R10 put in the lower position according to the diagram and solder the emitter output of the V12 in its place. Install the switch so that the capacitor C2 is discharged through resistor R11. The lamp should not glow. Otherwise, increase the resistance of the resistor R12 to 6.8 kω. Now, if toggle switch, the lamp is within 10-12 s to gradually increase the brightness. In this state, leave the device for 15 minutes. If the lamp goes out in series with the emitter V12 install a resistor with a value of 1-3 kω.
The modes of the transistors current is indicated on the schematic diagram (in parentheses are the values of currents when C2 is discharged through R11).
How to connect the”dimmer”?
Usually in class 11 set of fixtures. They are included by using two double-wall switches. Ask the school electrician to measure the voltage between the common terminals of the switches. If there is no voltage, connect them with each other (isolating master switch). Otherwise, the wires from the common terminals of the switches must be translated into one phase (in the panel with thermal circuit breakers-machine guns).
Next to the switches install wall outlet at 6A, 250V (X1 in the diagram). All connections run wire APPV. In the socket you can enable “dimmer” or a special plug, in which the current collectors are internally connected by a thick wire. In the second case, the lamps work as usual.
“Dimmer” is set in the Cabinet of physics of school № 29 in the city of Ulyanovsk.
G. GRISHIN, g, Ulyanovsk
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