Fast temperature meter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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This device, operating on the principle of a measuring bridge, allows you to determine the temperature of a working microcircuit, resistor or other part of a radio engineering device in 3 ... 4 seconds with an accuracy of no worse than 0,1 C. In many cases, it can significantly speed up the repair of complex equipment assembled even on microcircuits of the CMOS structure. The search for a faulty microcircuit is based on determining the increased heat dissipation of the radio component, which, as a rule, is associated with a violation of its electrical mode of operation.
(click to enlarge)
There are two measurement ranges set by switch SA3: 0...+40°С (I) and +10...+30°С (II). The range limits can be shifted by resistor R9 "Initial setting" when calibrating the device before measurement or extended by understand resistors R6 and R7 in the process of adjusting the constructed device.
The high measurement speed is achieved due to the use of a thermistor ST3-19 (R10) as a temperature sensor, which has a low mass. Structurally, the thermistor is placed at the end of the body of a ballpoint pen and connected to the device with a piece of twisted two-wire cable 0,6 ... 1 m long. X3 connector - any.
The device can be powered both from an internal source with a voltage of 4,5 V (GB1), and from an external source with the same voltage connected to sockets X1, X2.
Microammeter RA1 - M1691 for a full deflection current of the arrow 10 μA, with zero in the middle of the scale, or similar, preferably with a large scale. Trimmer resistors R6, R7 and R9 - multi-turn SP5-2 or similar.
To calibrate the device, it is desirable to use a heat chamber with automatic maintenance of the set temperature.
First, the temperature sensor disconnected from the device is placed in a heat chamber (the ends of the cable are left outside) and its resistance is measured as accurately as possible at a temperature of 20 ° C. Then, using a digital ohmmeter, a resistor R4 is selected, and if necessary, then a resistor R3 of such a value that their total resistance is equal to the measured resistance of the thermistor, and solder them in place.
By connecting the sensor to the bridge and setting the switch SA2 to the "Calibration" position, and SA3 - to the "I" position, turn on the power source and resistor R9 bring the microammeter needle to the middle - zero - division of the scale. The sensor remains in the thermal chamber. For the first ("I") measurement range, the zero division of the scale will correspond to a temperature of +20°C.
Next, the SA2 switch is set to the "Measurement" position and at a temperature in the thermal chamber of +40 ° C, the resistor R7 is used to achieve the deflection of the microammeter needle to the final right division of the scale.
To obtain greater accuracy, calibration at a sensor temperature of +20 °C and measurement at a temperature of +40 °C should be repeated two or three times.
And finally, the switch SA3 is moved to the "II" position and at the temperature of the sensor +10 and +30°C, the resistor R6 sets the initial and final limits of the second measurement range, respectively.
The described device has some sensitivity margin. But a significant increase in sensitivity will lead to an increase in measurement error due to the influence of even weak air circulation.
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