Thermometer attachment for digital multimeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In the article by V. Ratnovsky "Prefix for measuring temperature with a digital multimeter"("Radio", 1999, No. 3, p. 31) a device was described that allows you to measure the ambient temperature. The sensor is a thermistor, which, unfortunately, has a non-linear characteristic. In addition, the measurement accuracy depends on the supply voltage, and it not stabilized.

The proposed prefix (Fig. 1) differs from the one mentioned by the presence of a simple voltage regulator, made on a field-effect transistor VT1. The output voltage is removed from the extreme terminals of the tuning resistor R6. From its engine, part of the voltage is applied to the gate of the field-effect transistor. By changing the position of the slider, the desired stabilized voltage is set.

The function of the temperature sensor is performed by a silicon diode VD1. Together with resistors R1 - R5, it forms a measuring bridge, the voltage from the diagonal of which is fed to the input of the multimeter.

The voltage temperature coefficient across the diode is close to -2 mV/°C. However, thanks to the divider R1R2 at the output of the measuring bridge, its value will be half as much, i.e. 1 mV / ° С. and the sign changes due to the appropriate connection of the outputs of the bridge to the multimeter.

With the ratings of the elements indicated in the diagram, the transistor operates at a thermally stable point, so the dependence of its characteristics on temperature is small. Of course, in the case of repetition of the device, it is desirable to determine the thermally stable parameters of a particular transistor. To do this, it is necessary to measure its cutoff voltage and the sad drain current (how to do this is described in detail in the article by V Andreev "Economical stabilizers" in Radio. 1998, No. 6. p. 57 - 60) and then calculate the current of the thermostable points: . Typically, for low-power field-effect transistors, it is in the range of 100 ... 600 μA.

If the value obtained differs from that calculated for this device (about 370 μA at a voltage at the source of the transistor 1 V), it is set by selecting the resistor R6 or, in extreme cases, by changing the voltage at the source of the transistor. to increase the current. you should put a tuning resistor with a lower resistance or increase the stabilized voltage, and vice versa.

Resistors are chosen such resistance that most of the total transistor current flows through the resistor R6. Due to this, the stability of the supply voltage of the measuring bridge is achieved and its very weak dependence on the voltage change across the diode due to temperature changes.

The temperature dependence of the diode is linear only when it is supplied with a stable current. In the case of supplying the measuring bridge with a stable voltage, it differs from a linear function, but it is quite acceptable for practical use. In the temperature range of ±25 С from the set value, the error at the boundaries of the interval does not exceed 2.5°С. Therefore, the proposed thermometer is convenient to use for measuring "street" temperature. In this case, the diode is placed outside the window and connected to the attachment with wires, preferably with a "twisted pair" to reduce interference or, in extreme cases, shunted with an oxide capacitor with a capacity of at least 10 microfarads.

The power source of the set-top box is a galvanic cell of size R6 ("AA"). The current consumption of the device is small, so there is no power switch. The service life of the source reaches several months. If an alkaline cell (LR6) is used for power supply, the service life will be increased.

Instead of what is indicated on the diagram, it is permissible to use a field-effect transistor KP303B, KP303Zh, KP303I. We will replace the diode with any silicon from the KD503, KD519, KD521, KD522 series. Adjusted resistors - SP5-2, SP5-3, constant - MLT-0,125. Switch SA1 - MT3.

The appearance of the set-top box connected to the multimeter is shown in Figure 2.

The case of the device is made of foil fiberglass (the technology of its manufacture has been repeatedly described in the magazine). Mounting is carried out in a hinged way. The switch is mounted on the side wall of the case. Attached to the base are two pins for connecting to multimeter sockets.

To set up the set-top box, it is connected to a multimeter, the switch is switched to the "Voltage Control" position and the voltage at which the temperature will be measured is set with resistor R6, for example, in this device 1 V. It should be remembered that the current flowing through the transistor depends on this voltage, therefore its value cannot be set arbitrarily. In the future, during measurements, the voltage should be periodically monitored.

Then, resistor R4, according to the readings of the control thermometer, sets the readings of the initial temperature.

Using a prefix with a digital multimeter Mastech M-830V. the latter should be switched on in the DC voltage measurement mode at the limit of 2000 mV (or 200 mV). In this case, the temperature readings will be displayed on the LCD of the multimeter in degrees Celsius.

Author: V. Chudnov

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