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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Heater for boxes of television cameras. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers Cameras of specialized television systems usually operate outdoors, and therefore require protection from climatic influences. For this reason, most often they are placed inside sealed boxes. Most television cameras (TK) have an operating temperature range of -20...+55°C, so the boxes have to be equipped with heaters that turn on when the ambient temperature drops below 0°C. Unfortunately, certified boxes with fairly reliable heating and control devices are expensive. Cheap ones are very unreliable. As a result, the task of creating inexpensive and reliable heaters remains very relevant. A description of one of these devices is given below. The device is designed to work inside hermetic boxes with a volume of 2...10 dm3, which do not have special thermal insulation, in the climate of the middle latitudes of Russia. It is a heater that turns on when the temperature in the box drops and ensures its maintenance at a certain level with an error (taking into account its non-uniform distribution inside the controlled volume) no more than 1...3°C. The heater works on the principle of threshold temperature control. Its electrical circuit is shown in fig. 1. The primary unstabilized source with a voltage of Upit = 20 V is used to power only the heater and the stabilizer on the DA1 chip. The TC control device is powered by a stabilized voltage Upit.stab = 12 V, which is generated at the output DA1.
It should be noted that the temperature instability of the output voltage of three-terminal integral stabilizers is greater than that of other types of stabilizers. This instability also manifests itself during self-heating of the KR142EN8D microcircuit by the current flowing through it. TCs of different types consume a current of 0,1 ... 0,2 A, so the DA1 stabilizer had to be equipped with a hinged heat sink with an area of \u30b\u2babout XNUMX cmXNUMX. The presence of temperature instability of the voltage Upit.stab must be taken into account when choosing the threshold circuit of the heater control device. The temperature-voltage converter is made in the form of a voltage divider on resistors R1, R2 and thermistor R4. The divider is loaded on the input resistance of the logic element DD1.1, which is about 1012 ohms, so the operating current of the thermistor R4, which is approximately 0,5 mA, does not depend on the load of the divider. The functions of the threshold device are performed by the element DD1.1 of the DD1 microcircuit, which compares the voltage drop across the thermistor R4 with the input voltage level Upor2, at which DD1.1 itself is triggered. For two types of logic elements, the Uth values can be determined from the static transfer characteristics shown in fig. 2a. The voltages Upor are located in the sections of the characteristics that are between the levels of the minimum voltage of the logical unit U1min and the maximum voltage of the logical zero U0max. The intervals of the input threshold voltages of the logic elements corresponding to these sections are relatively small, so it can be approximately assumed that Uthr corresponds to the middle of this interval, i.e. Uthr=0,5Upit.stab. This approximation makes it possible to determine Uthr with an error of the order of tens of millivolts.
Due to the temperature instability of the voltage Upit.stab. in the range of operating temperatures of the TC, it is important that the ratio of the threshold element Uthr to the voltage drop across R4, equal to R4Up.stable/(R1+R2+R4), remains unchanged. Logic elements of the CMOS series meet this requirement well, as shown in Fig. 2b. The dependences shown on it show that the ratio Upor / Upit.stab. = 0,5 is maintained throughout the entire range of supply voltages allowed for logic elements of K176 series microcircuits. Since the DD1.1 inputs are affected by a slowly changing voltage drop across the thermistor R4 following temperature changes, the DD1.1 element stays in active mode for a long time, amplifying both the useful signal and the noise. To suppress interference at the input and output of DD1.1, a low-pass filter is included - R1R2R4C1 and R3C2, respectively. Elements DD1.2, DD1.3 and DD1.4 additionally amplify and form a useful signal coming to them from the output of the R3C2 filter. The output signal of the DD1.2 element controls the reference voltage source, which is a parametric stabilizer made on the VD1 zener diode and the HL1 LED. A distinctive feature of such a source is the absence of a ballast resistor and its power supply directly from the output of the DD1.2 element. This is possible due to the relatively large output resistances of CMOS transistors in the elements of the K176 series microcircuits. The parametric stabilizer is powered through a transistor with a p-type channel. The output current-voltage characteristics of this transistor for logic elements from the composition of the K176LA7 microcircuit are shown in Fig. 3. The working area of these characteristics is limited by the hyperbola of the permissible power dissipation of the K176LA7 microcircuit (Pmax). On the characteristics: |U| is the voltage drop across the p-channel transistor, and In is the current flowing through it. Since the voltage drop across the zener diode VD1 and the HL1 LED is approximately 7 V, for Upit.stab=12 V, the position of the operating point of the transistor corresponds to |U|=5 V and In=10 mA. In this case, the output resistance of the logic element will be approximately 1 kOhm, and the p-channel transistor will be a current limiter for the VD1 and HL1 diodes. The reference voltage itself is formed on the engine of the variable resistor R5.
The heater is a current source assembled on transistors VT1, VT2, resistor R7 and ballast resistors R8, R9 connected according to the Shiklai scheme. When adjusting the reference voltage, the collector current of the transistor VT2 can vary from zero to 1 A, and the power dissipated by it can reach 18 watts. To ensure reliable operation of the heater under such conditions, it is important to stabilize the collector current of the transistor VT2 to a temperature of approximately + 80 ° C. This is achieved with the help of the following circuit and design solutions. To reduce the instability of the collector current due to changes in the voltage drop at the base-emitter junction when the transistor is heated, it is equipped with a heat sink, the surface area of \u1b\u2bwhich is chosen so that when operating in this box at a collector current of 80 A, the transistor VTXNUMX does not overheat above + XNUMX ° С. Now let's talk about the operation of the heater. Let in the initial state the temperature in the box be higher than the ambient temperature and the threshold temperature set by the adjusting resistor R2. In this case, the resistance of the thermistor R4 is small, and the voltage drop across it is less than Upor. In this case, there is a low logic level at the output of the DD1.2 element and no current flows through the heater. Over time, the temperature in the box due to its cooling will begin to decrease. The resistance of the thermistor R4 and the voltage drop across it will begin to grow and, when the voltage reaches the level Upor, a low-level low voltage edge will form at the output of DD1.1. During the formation of this front, the states of the outputs of the logic elements DD1.2, DD1.3, DD1.4 will change, as a result of which the heater control device will switch. At the output of the element DD1.2, a voltage will be established corresponding to the stabilization voltage VD1 and the voltage drop across the HL1 LED, and a given current will flow through the transistor VT2. The VT2 heat sink will warm up the air in the box. The temperature of the thermistor R4 will begin to rise, and the voltage across it will decrease. When the approximate equality of the voltage drop across the thermistor R4 and the voltage Upor is reached again, the control device will switch to its original state, and the current through the transistor VT2 will stop again. These switchings are repeated at intervals, the duration of which is determined by the characteristics of the heat exchange of the box. In this case, the air temperature in the box will change near the value set by the position of the resistor R2 slider. The main functional units of the described device are located on the printed circuit board (Fig. 4). Outside the board is a transistor VT2. To ensure the heating of the entire volume of the box, the transistor VT2 and thermistor R4 should be spaced as far as possible. The heater involves the use of the following elements: transistors VT1, VT2 in plastic cases, microcircuits K176LE5 or K176LA7 (DD1) and KR142EN8D in a plastic case (DA1), resistors R1, R3, R6 - R9 - MLT, S2-33, MT or their analogues, R2, R5 - SP5-2, R4 - MMT with a nominal value of 8 ... 12 kOhm, capacitors C1-C3 - KM of any group.
The location of the heater inside the TK box is shown in fig. 5. The VT2 transistor is mounted on an aluminum alloy heat sink with dimensions of 120x70x3 mm. It is fixed to the mica spacer with a PTFE bushing that insulates the fixing screw, and therefore has no electrical contact with the heat sink. In turn, the heat sink does not have metal fasteners that directly attach it to the box body. On the edge of the heat sink, facing the box window, there are two rows of holes that improve air circulation. In order for the heat-generating elements DA1, R8, R9 to influence the thermistor R4 as little as possible, it is raised above the board to a height of 10 ... 15 mm.
Adjustment of the operating mode consists in keeping the open box at a temperature equal to the desired switch-on threshold, in the absence of current in the heater for 20...30 minutes. Avoid getting moisture inside the box. Having set the desired temperature in it, with the adjusting resistor R2 you need to make the HL1 LED glow, stopping the regulation when the voltage across the thermistor R4 is equal to the voltage Upor. Author: G.Pilko, Kiev, Ukraine
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