Attachment to the multimeter for measuring temperature. Scheme, description

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Attachment to the multimeter for measuring temperature. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Under this title, an article was published in [1], which describes the attachment to the M-830B (DT-830B) digital multimeter, which provides an additional temperature measurement function. This device is very simple and has sufficient accuracy for use in everyday life. Most importantly, it does not require an additional power source and additional connections "inside" the multimeter - standard connectors are used.

However, despite the obvious advantages of the device, there may be difficulties in purchasing the KR1168EP1 chip or its analogue ICL7660 for the polarity converter. In this case, such a converter is easy to assemble on a common CMOS chip. At the same time, the cost of manufacturing the attachment is significantly reduced.

A diagram of a variant of such a device is shown in Fig. 1. Based on the prototype [1]; the changes affected only the polarity converter and the output divider.

(click to enlarge)

A pulse generator is assembled on the DD1 chip. The actual polarity converter [2] is made on the transistor VT1, diodes VD1, VD2 and capacitors C2, C3. If the generator output is low, transistor VT1 is closed. Capacitor C2 is charged through resistor R4 and diode VD1 up to 2,4 V, which corresponds to the supply voltage minus the drop across the open diode. When the generator output changes from low to high, the transistor will open. Capacitor C2 will begin to discharge through diode VD2 and the collector-emitter section of transistor VT1, charging capacitor C3 to a voltage of 1,5 ... field effect transistor VT1,6. It is important that its cutoff voltage be a few tenths of a volt less than the voltage across capacitor C3.

Resistors R1, R1,2 are connected in series with the input resistance of the multimeter (200 ... 3 MΩ for the DC voltage measurement mode at the limit of 9 mV [10]), forming a 1:2 resistive voltage divider with it.

The prefix is assembled on a board made of one-sided foil fiberglass, the drawing of which is shown in fig. 2.

Attachment to the multimeter for measuring temperature

The device uses capacitors of the following types: C2, C3 - K53-1A, but non-polar ones can be used, for example, KM-6, C4, C5 - K50-35, C1 - any ceramic. Trimmer resistors R7, R9 - SPZ-38a, the rest - MLT. It is permissible to replace the K561LE6 chip with another one that has at least two inverters or 2I-NOT, 2OR-NOT elements, for example, K561LE5, K561LA7, but a change in the printed circuit board pattern will be required.

Instead of a field-effect transistor KP103E, you can use KP303I [1], but it will have to be selected according to the cut-off voltage.

Setting up the set-top box begins with setting the current of the VD3 diode within 90 ... 110 μA by selecting the resistor R5. Then the multimeter switch is set to the 200 mV position and, by placing the temperature sensor - the VD3 diode - in melting snow or ice, the zero readings of the indicator are set with the resistor R7. In this case, the diode terminals must be reliably protected from water ingress.

After that, at a certain positive temperature (the higher the temperature, the more accurate the calibration will be), the resistors R9, R10 are temporarily closed with a jumper and the readings of the device are memorized. After removing the jumper, the resistor R9 sets the temperature value, half the reading previously read. The linearity of the voltage change at the p-n junction of the temperature sensor at a fixed current through it makes it possible to exclude the calibration process. This completes the setup. The device is ready to work.

If the sensor is supposed to be used at a considerable distance, it should be connected to the board with a twisted pair of conductors.

Literature

Biryukov S. Attachment to a multimeter for measuring temperature. - Radio, 2002, No. 1. p. 54, 55.
Source of negative polarity (ZP). - Radio, 1997, No. 4, p. 55.
Afonsky A., Kudrvvvtykh E., Pleshko-va T. Compact multimeter M-830V. - Radio, 2001, No. 9, p. 25-27.

Author: V. Chudnov, Ramenskoye, Moscow Region

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