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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Pasechny electroknife. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Home, household, hobby The article describes a beekeeping electric knife, free from the shortcomings of similar products manufactured by the industry. The simplicity of the design, the use of the available elemental and material base make it possible to repeat it at home. The most popular way to open honeycombs when pumping honey is to cut off the wax caps that seal the cells with a special bee knife. The traditional technology is as follows: a set of beekeeping knives is immersed in a container of boiling water - usually at least four or five pieces. They take out a knife, shake off water from it (getting it into honey leads to its sourness) and quickly, until the knife has cooled down, sawing movements open the honeycombs. The opening is carried out by melting the wax. In this case, mechanical forces should be minimal in order to avoid crushing of the cells. After a few seconds, the knife cools down (its temperature becomes lower than the melting point of the wax), so it is again immersed in a container of boiling water, the next knife is taken out, and then the process is repeated. In order to avoid the raid of bees, the honeycomb is opened and the honey is pumped out indoors: in the field it is a tent, in stationary conditions it is any adapted room. Working conditions are quite difficult: high temperature due to the continuously working stove, high humidity due to the presence of a container of boiling water. The presence of water on the knife and high air humidity due to the high hygroscopicity of honey worsen its quality. Nevertheless, despite the disadvantages associated with the use of bee knives heated in boiling water, most beekeepers prefer the traditional method of opening combs, refusing to use the seemingly more convenient electric bee knives produced by the industry. Unfortunately, industrial electric knives are indeed not devoid of "congenital" shortcomings. Thus, the heating of the knife blade during the inevitable breaks in work exceeds the boiling point of water, which is why, with the resumption of work, caramelization of honey sugars and burning occurs. The insufficient power of the heater and the relatively low thermal conductivity of the material of the knife blade lead to the fact that during operation the temperature of the cutting edge drops below the melting point of the wax. And a nichrome wire heater is not the most reliable part of an electric knife. As a result of many years of experiments, I developed the design of an electric bee knife, which I have been using for over 10 years. In developing it, I proceeded from the following considerations. 1. The opening of the honeycomb is carried out by thermal action on the seal of the honeycomb. Mechanical forces should be kept to a minimum to avoid creasing of the cells. The temperature of the cutting edge of the knife blade must be above the melting point of the wax (64 оC) taking into account the intensity of heat dissipation by at least 10 °C, i.e. approximately 75 °C. The maximum temperature of the knife blade in any mode must be below the boiling point of water, i.e. it must not exceed 95 °C. You should not be afraid of overheating of honey - due to the low thermal conductivity of honey and wax to a temperature above 45 ° C, at which honey enzymes are destroyed, only a thin layer of honey that is in direct contact with the knife blade is heated, and these are hundredths and thousandths of a percent of the total volume of honey , which practically does not affect its quality. 2. To increase productivity, the heater power should be large enough, and the thermal resistance of the heater-knife cutting edge circuit should be minimal. The knife must be able to be powered from both AC and DC sources (batteries, vehicle electrical systems, etc.). It should consume the greatest power from the network; from autonomous sources, the power consumption should be reduced. The knife operates in a wide range of heating temperatures, so its power can be adjusted discretely. The requirement for minimum thermal resistance can be met by making a knife blade from a material with high thermal conductivity. The highest coefficient of thermal conductivity - 418,7 W / m-K - for silver, but a knife made of this metal will be too expensive. Copper has a slightly worse named parameter - 389,6 W / m-K. Steel is completely unsuitable for an electric knife, in which the coefficient of thermal conductivity is 8,5 times less than that of copper. Having made a knife from sheet copper 3 mm thick, we get a thermal resistance comparable to that of a silver knife with a blade 2 mm thick. You should not be afraid of chemical reactions of copper with organic acids of honey - even our great-grandmothers considered copper basins to be the best utensils for making jam. 3. The heater temperature should not exceed 100 оC, and to improve the transfer of heat from the heater to the knife blade, the maximum contact area of the heater and the knife blade should be provided. Based on the foregoing, an apiary electric knife was developed, the description of which is offered to the attention of readers. The diagram of the electronic part of the knife is shown in fig. 1, and the design - in Fig. 2. The device contains a heater made of high-power transistors VT1-VT5 operating in the active mode, a sealed thermal relay KK1 that turns off the heater when the temperature reaches 95 ° C, and an assembly of Schottky diodes VD1, which is used to rectify alternating current when operating in stationary conditions.
The basis of the design of the electric knife is blade 6, made of sheet copper 3 mm thick. A copper tube 3 bent in the form of the Latin letter Z with a diameter of 8 ... 10 mm is soldered to it with hard solder. Handle 2, made of insulating material, is fixed on the tube. A plug of connector 1 (XP1) is fixed at its end. She and her corresponding socket XS1 (on a connecting cable) are made of a detachable connector ШР with pins with a diameter of 0,8 ... 1 mm, for which the necessary fragments are cut off from the pin and socket parts of the connector with a hacksaw. Transistors 7 (VT1-VT5) are fixed on the blade with 6 countersunk M3 screws screwed into the threaded holes in the blade. The same screw was also used to fasten (through a mica spacer) diode assembly 9 (VD1). Thermal relay 8 (KK1) is pressed against the blade with a bracket and M2 screws (not shown in Fig. 2). To improve heat transfer, the contacting surfaces of the blade and the named parts are lubricated with KPT-8 heat-conducting paste. Since the static current transfer coefficient of the base of the composite transistors of the KT829 series has a significant spread (only its minimum value is normalized, equal to 750), the resistors R1-R5 are selected for each transistor separately until a collector current of 1,5 A is obtained (Fig. 3a). Heat dissipation from these resistors is difficult, so it is desirable that they dissipate as little power as possible. Obviously, this is possible with a large resistance of the resistors, i.e., when using transistors with the highest possible static current transfer coefficient.
All electrical connections are made by tin-lead soldering. For connection with plug 1, a wire with a cross section of 1 mm is used2 in heat resistant insulation. Upon completion of the installation, the parts and wires are covered with an excess of auto-sealant 10 and closed with a cover 4. It is made of sheet copper 0,6 ... In the place of conjugation with the cover 0,8, the tube 4 is flattened, and the cover is sawn to ensure smooth mating. When repeating the design, it is recommended to temporarily attach the cover to the knife blade with M2 screws and nuts. A day later, after the sealant has cured, the screws can be replaced with countersunk head copper rivets (they cannot be installed immediately - the sealant will leak out due to the vibrations created during riveting). Excess sealant is removed, the blade is sharpened, as shown in fig. 2, and grind. A knife made in this way ensures the working temperature on the cutting edge, regardless of the intensity of work, and eliminates the burning of honey. Covering the mounting with a layer of sealant and using a tight-fitting lid eliminates the contact of honey with solder containing heavy metals. As noted, the collector current of transistors VT1-VT5 is chosen to be 1,5 A. Thus, at a supply voltage of 12V, each transistor consumes (and dissipates) power 121 5 = 18 watts. In stationary conditions, the knife is powered by a step-down transformer T1 with a power of 150 watts. Its secondary windings II.1 and II.2 with a voltage of 12 V each are connected according to-series. The XS1 socket is connected to the secondary windings of the transformer with a PVA 3x1,5 wire. When docking the XS1 socket with the XP1 plug, a full-wave rectifier is formed on the VD1 diode assembly. A jumper between pins 2 and 3 of the socket ensures that all transistors are turned on simultaneously, the power consumed by the knife is 5 18 = 90 watts. When operating from an autonomous source, power is supplied to pins 1 and 2 (power consumption is 3 18 = 54 W) or 3 and 4 (power consumption is 2 18 = 36 W). The diode assembly VD1 in this case serves to protect against the supply voltage in the wrong polarity. In the absence of transistors of the KT829 series, composite transistors from two ordinary ones can be used (Fig. 3, b). As VT1, it is convenient to use transistors of the KT315 series, since they have a flat case and are most convenient in installation, as VT2 - low- or medium-frequency npn transistors in the TO-220 package with a maximum power dissipated on the collector of at least 25 W, an allowable voltage between collector and emitter of at least 25 V and a maximum collector current of at least 3 A (KT805AM, KT805BM, KT819 with any letter index, etc.). The resistance of the resistor in the base circuit must be at least 5 kOhm, otherwise the transistor VT1 should be replaced with another one with a high current transfer coefficient. Assembly diodes VD1 must allow a forward current of at least 10 A and a reverse voltage of at least 25 V. These can be domestic assemblies KD271AS, KD271VS, KD272AS, KD272VS, KD273AS, KD273VS or individual diodes with the same parameters in the TO-220 package. Sealed thermal switch TLRS-9700 with a response temperature of +85...95 оWith and a switching current of 10 A, you can replace two YC9700 or KSD9700 thermal relays with a switching current of 5 A by including them in a break in the wires connecting the emitters of transistor groups with contacts 2 and 3 of the XP1 plug. The knife is available for making at home. Hard soldering can be difficult, but this operation can be performed in a metal repair shop or "Rembyttechnika", in extreme cases, the handle tube can be attached to the knife blade with rivets. The knife is easy to use, reliable and durable. Author: K. Moroz
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