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CHILDREN'S SCIENTIFIC LABORATORY
Homemade photocopier. Children's Science Lab
Directory / Children's Science Lab Xerography appeared several decades ago at the interface between photography and electrostatics. It turned out that when an electrified sheet of paper is exposed, the charge configuration exactly repeats the contours of the image, and its density depends on the intensity of the halftones. Spray paint on such a sheet, it will stick to the electrified paper, and in front of you is an exact copy of the image. We propose to make an installation operating on this principle. You will need a high DC voltage source to electrify the photosensitive material, photosensitive paper, and a developer with ink. Let's start with the preparation of photosensitive material. This is paper coated with a photo-semiconductor emulsion. We'll make it. Mix two parts by weight of dry zinc white and one part of BF-2 glue. Thoroughly mix the mass, periodically adding alcohol. It should turn out to be similar in thickness to sour cream. Now heat the emulsion a little. It will become less viscous and will lay down well on the sheet. It is better to take coated paper and apply an emulsion on it with a thin layer using a brush or roller. After letting the sheets dry for 20 minutes, cover them with a second layer. After the final drying at room temperature (about 2 hours), the paper is ready for photocopying. But she is not yet receptive to light. The surface layer of the photoconductor must be given an electric charge. This is done in the dark, using an electrifying electrode in the form of a metal (copper, brass) disk with a dielectric handle.
The appearance of the electrizer and installation is shown in the figure. It consists of a dielectric base made of getinaks, textolite or vinyl plastic with a thickness of 5 - 6 mm. A copper or brass plate 0,5 mm thick, the size of a typewritten sheet of paper, is fixed on it. To avoid accidental contact and short circuits, we will lay a 0,5 mm thick dielectric layer between the paper and the plate from a fluoroplastic film, a ton of getinaks or textolite. For convenience, it can be fixed with glue. It is better not to use metal screws for assembly, as there is a risk of high voltage breakdown. The electrifying electrode is connected to the negative, and the plate on the base is connected to the positive conversion pole. Pay special attention to the electrical insulation of the current-carrying tires and the places of their attachment to the electrodes. The MGSHV wire is not suitable for this purpose, a VMMA, MPI type wire is required, or you can use a PK75 type cable, having previously removed the shielding braid from it. Turning on the high voltage source, run the electrode over the entire surface of the sheet. The paper is now ready to be exposed. You can't bring it out into the open. After preparing several sheets, start copying, otherwise the charge may "drain" and the material will lose its photosensitivity. To get a print on paper, you need to project the image. This is easy to do with a photo enlarger, and even better with an epidiascope. But remember, in xerography, the process corresponds to the formula: "positive - positive." Exposure time with a light source with a power of 100 ... 150 W from a distance of 0,5 m from 0,5 to 5 seconds. The best results are given by mercury-quartz lamps, but conventional photo lamps can also be used. Now it's time for manifestation. You can use dry or wet methods. But, of course, at home the second one is more convenient. Dilute thick printing ink for rotary presses in refined aviation or motor gasoline at a ratio of 5 grams of ink to 1 liter of gasoline. It is enough to lower the exposed sheet of paper into this solution, and in a minute the print will appear. It does not require fixing, gasoline will quickly evaporate, and the paint will firmly adhere to the surface. Tang can be imaged not only on paper, but also on glass and even ceramics. The most difficult part of a homemade copier is a high voltage source. Let's dwell on it in more detail.
The design of the converter should provide a voltage of the order of 10 - 15 kV at a very low current. The simplest diagram of such a device is shown in the figure. The high voltage source shown in the diagram consists of a step-down network transformer with a diode bridge, a converter and a voltage multiplier. The rectified voltage from the mains transformer Tr1 is fed to the input of a self-excited converter assembled according to a push-pull circuit on transistors of the P210 type. Their collector terminals are connected to winding I, and the base terminals are connected to winding II. Resistor R1 and capacitor C2 provide the required mode of operation.
An alternating voltage with a frequency of 3000 - 4000 Hz appears on the terminals of the winding I. It is increased hundreds of times by the secondary winding III and fed to a multiplication circuit consisting of high-voltage diodes KTs106G and filter capacitors of the K73-13 type with a capacity of 2200 pF and designed for a voltage of at least 12 kV. The rectified voltage is supplied to the photocopier. The step-up transformer Tr2 is wound on a ferrite core from a television line transformer (TVS-A, TVS-B, TVS-110), consisting of two halves (see Fig.). The high-voltage winding III is wound on a frame glued from fiberglass, electric cardboard or plexiglass 1 mm thick. The width of the frame must be at least 30 mm. The winding contains 8000 turns of PELSHO wire with a diameter of 0,08 - 0,1 mm. You can use the wire PEL, PEV. Winding make a turn to turn, isolating each layer with a ton of fluoroplastic film or varnished cloth. Make sure that the turns of the upper layers do not come into contact with the lower ones. For primary windings, make a bushing from thick cardboard. Winding I contains 14 turns of wire PEL 0,8; and winding II - 6 turns of the same wire with a tap from the middle. It is desirable to cover the windings with paraffin and wrap with electrical tape. Insert the halves of the core into the frames and pull them together with a mount from the horizontal transformer. The step-up transformer can also be wound on a conventional iron core of the Sh-20 type with a set thickness of 30 mm. The overall frame is made of thick cardboard, getinax or textolite. First, windings I and II are wound (respectively 20 turns of PEL 1,2 and 16 turns of PEL 0,5 - both with a tap from the middle) and covered with paraffin, and covered with a layer of insulating tape 1 mm thick from above. Then winding III is wound - 7000-8000 turns of PELSHO 0,1. It is also impregnated with paraffin. It is desirable to install transistors on heat sinks with an area of 60-100 sq. cm made of sheet duralumin or brass with a thickness of 1,5 - 2,0 mm. The parts of the transducer are placed on a getinance board 3 - 5 mm thick and a Plexiglas body insert. When mounting, pay special attention to the connections between diodes and capacitors. Connecting wires should be short, and soldering should be even and smooth. Carefully round off all sharp solder edges and protruding ends of the conductors with a needle file - otherwise they will become a source of corona discharges. The rectifier is assembled according to the bridge circuit on diodes of the D303 - D305 type or similar. The rectified voltage is filtered by capacitor C1. Alternating voltage is removed from the secondary winding of the step-down transformer Tr1. The primary winding is designed for a voltage of 220 V. The rectifier is turned on with the B1 toggle switch. Wind the network transformer Tr1 on a Sh-20 iron core with a set thickness of 30 mm. Winding I for a 220 V network contains 2200 turns of wire PEV-2 0,25; winding II is wound with PEV-2 wire with a diameter of 1,2 mm and contains 120 turns. For a more accurate selection of voltage, it is advisable to make taps from 90, 100 and 110 turns. Any ready-made transformer that provides a voltage of 10-12 V at a current of 2 A is also suitable. Place the rectifier and converter in a common housing. Pay special attention to the insulation of high-voltage terminals from which high voltage is removed. Checking and adjusting are reduced to several operations. First check the rectifier. Instead of a converter, connect a 10 Ohm resistor with a power of 20-25 W of the PEV or Ts5-35 type to it. Connect to the network and measure the voltage across the resistor - it should be at least 10 V. Then connect the converter to the rectifier. If his circuit is assembled correctly, and the parts are in good order, a thin squeak of a high-voltage transformer will be heard. No - swap the extreme terminals of winding I or II. If a sharp squeak or breakdown clicks appear, reduce the supply voltage by soldering the rectifier to the taps of the secondary winding of the Tr1 transformer with a lower voltage. Make sure there are no corona discharges. Turn off the lights in the room and look at the high-voltage part of the device. If purple flashes appear on the pins of the parts, this is recognized as corona. Its indispensable companion is the smell of ozone. Turn off machine, inspect solder points, round or re-solder. Working on a photocopier requires special attention. High voltage in itself is not dangerous. The strength of the current is decisive. As you know, a life-threatening current in excess of 30 μA. But this does not mean that you can even accidentally touch current-carrying conductors. You will receive a tangible prick of a discharge spark! Therefore, our advice is that with any soldering in the circuit, be sure to short the high-voltage wire of the multiplier to the grounded terminal of winding III. All soldering during adjustment should be carried out with the rectifier disconnected from the network. Author: V.Konovalov
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