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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Attachment to the multimeter for measuring temperature. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur The operation of the attachment is based on the well-known linear dependence of the voltage drop across a silicon diode on temperature. At a current of 100 μA and zero temperature, the voltage drop across low-power silicon diodes is about 600 mV and decreases by 2 mV with an increase in temperature by 1°C. The attachment diagram is shown in fig. 1a.
The prefix is connected with plugs X1 - X3 to three sockets of the M-832 multimeter - to its inputs "VΩmA", "COM" (Common) and to the socket for connecting the emitter of the pnp transistor when measuring the parameters of the "E PNP" transistors. The multimeter is used in DC voltage measurement mode with a scale of 200 mV. Between the "E PNP" and "COM" jacks in the multimeter there is a stabilized voltage of about 3 V, it is used to power the set-top box. A converter is assembled on the DA1 chip and capacitors C1 - C3, which generates a voltage of -3 V relative to the common wire. The operation of such a converter is described in the author's article "Voltage Converters on Switched Capacitors" in Radio, 2001, No. 12, p. 44, 45. To measure the temperature, a diode VD1 was used, the current through which is about 100 μA is set by a stabilizer on a field-effect transistor VT1. So that at zero temperature the voltage supplied to the multimeter is also equal to zero, a resistive divider R1R2R3 is included. Fine tuning of zero readings is carried out by resistor R2. A temperature of +20°C should correspond to an output voltage of 20 mV, in which case the multimeter readings in millivolts will correspond to the temperature in degrees Celsius (taking into account the sign). A change in the voltage across the diode - 2 mV / deg leads to a change in voltage of 1 mV / deg at the output of the set-top box due to the divider formed by the output resistance of the divider R1R2R3 and resistors R5 and R6. The division factor is fine-tuned by resistor R5. Approximate voltages at some points of the structure relative to the common wire are shown in fig. 1,a for zero temperature, in parentheses - for +20°С. The current consumed by the prefix from the multimeter stabilizer does not exceed 140 μA. The prefix is assembled on a printed circuit board (Fig. 2) from one-sided foil fiberglass 1,5 mm thick (1 mm is possible). Fixed MLT resistors were used (C2-23 is also suitable), tuning SPZ-19a, capacitors K50-16 (C1-C3) and an imported analog K50-35 (C4). It is permissible to use any other resistors and capacitors that are suitable in size. Transistor VT1 must have a cut-off voltage of no more than 2,2 V. In addition to that indicated in the diagram, 2P103A, KP103Zh, KP303I are suitable. The inclusion of the KP303I transistor is shown in the diagram of fig. 1b. It is permissible to replace the KR1168EP1 microcircuit with its imported analogue - ICL7660 or ICL7660A.
As VD1, you can use any low-power silicon diode, for example, the KD503, KD522, KD103 series. Preference was given to the KD512A diode because of its small dimensions (diameter 1,2 mm, length 2,8 mm). To connect the set-top box to the multimeter, two split pins with a diameter of 4 mm are fixed on the board with nuts from the plugs ("VΩmA" and "COM" circuits) and a brass pin with a diameter of 0,8 mm is soldered ("E PNP" circuit). The board is covered with a casing soldered from foil fiberglass and fixed on the board at several points by soldering. The casing foil is connected to a common wire and acts as a screen. The design of the temperature sensor depends on the intended areas of use of the thermometer. For example, the option shown in Fig. 3. For its manufacture, a glass tube 1 with a diameter of 4 ... 6 mm is taken, its end, heated on the fire of a gas burner or spirit stove, is pulled to reduce the diameter to approximately 3,5 ... XNUMX mm. Then the thin part of the tube should be broken and soldered on the same flame.
One of the terminals of the diode 6 used as a sensor should be bent to its body, solder wires 2 (MGTF-0,07 0,5 m long) to both terminals, put on each of them two pieces of PVC or fluoroplastic tube 4 and 5. Insert the diode with wires into the glass tube and fix the wires at its open end with a drop of epoxy glue. Such an option is also possible. Wires are soldered to the terminals of the diode, a polyvinyl chloride or fluoroplastic tube about 300 mm long is put on them so that the diode is located with a slight offset relative to its middle, after which the tube is folded in half and the ends are tightly wrapped with a thread, having previously filled them with glue. Such a sensor can be lowered into the aquarium for a part of its length. If you intend to use a thermometer to measure the air temperature in a room, no special design of the sensor is required - it is enough to solder the diode to the board. Setting up a thermometer is easy. First, check the operation of the converter on the DA1 chip. The voltage at its pin 5 should be approximately -3 V relative to the common wire (COM circuit). Then, by including a microammeter in the drain circuit of the transistor VT1, by selecting the resistor R4, the drain current is set within 90 ... 110 μA. Having placed the sensor in melting ice or snow, the trimming resistor R2 should set zero readings on the indicator, if necessary, select the resistor R1. Having lowered the sensor into a vessel with water heated to a temperature of 50 ... 60 ° C (controlled by an accurate thermometer), you need to set the corresponding readings on the indicator with resistor R5, selecting resistor R6 if necessary. When calibrating, constantly stir the water in the vessel. If there is no KR1168EP1 chip (or ICL7660), the set-top box can be powered directly from the multimeter battery. To do this, a small-sized socket should be installed on its body, connecting it to the negative terminal of the battery. Supply the drain output of the transistor VT1 with a flexible conductor with a plug, which is inserted into the additional socket of the multimeter. Author: S. Biryukov
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