A simple multi-point thermometer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers

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By installing several temperature sensors of the AD22100 series indoors and outdoors and assembling a very simple device from a conventional pointer microammeter and a few more details, you can find out the temperature at points of interest at any time.

Temperature sensors of the AD22100 series are produced in housings of two modifications (Fig. 1).

In addition to the housing design, sensors with different letter indices differ in operating temperature ranges: КТ (KR) - 0...+100 °С, AT (AR) - -40...+85 °С and ST (SR) - -50 ...+150 °С. With a supply voltage of 5 V, the current consumption does not exceed 0,5 mA.

The output voltage Uout (between terminals 2 and 3 or 2 and 4) depends linearly on the temperature of the sensor housing. Its value at a temperature T, given in degrees Celsius, can be found by the formula

which is valid for the supply voltage Un from 4 to 6 V. The deviation from this law does not exceed 1 °C (for sensors with indices ST and SR - 2 °C).

Thus, at Un=5 V and T=0 °C, the voltage at the sensor output is 1,375 V, changing by 0,0225 V with each degree of temperature. The characteristics of the sensors are strictly standardized, therefore, if necessary, they can be connected in turn to the same temperature meter without additional calibration. On fig. Figure 2 shows a schematic of a multi-point thermometer that implements this idea.

The number of VK1-VKn sensors placed in the required places is limited only by the total current consumed from the GB1 battery. Any of them is connected to the measuring node by pressing the corresponding button SB1-SBn. At the same time, the second group of button contacts closes the power supply circuit of the device. The high steepness of the temperature characteristics of the sensors made it possible to do without an amplifier, using the PA1 microammeter as a temperature indicator, included in the diagonal of the measuring bridge formed by the sensor and the resistive voltage divider R1R5R6.

In order for the zero temperature to correspond to the zero reading of the microammeter, the total voltage drop across the resistors R5 and R6 must be equal to 1,375 V, which is achieved using the tuning resistor R6. The sum of the resistances of the resistors R2, R4 and the frame of the microammeter is chosen in such a way that each degree of temperature corresponds to the deviation of the arrow of the microammeter RA1 by 1 μA. This allows, taking a microammeter of the required sensitivity, to use the graduation on its scale to read the temperature.

The integral stabilizer DA1 lowers the voltage of the GB1 battery to the 5 V required to power the sensors. The HL1 LED serves as an indicator not only of turning on the device, but also of the state of the GB1 battery. While its voltage is normal (6,8 ... 9 V), when you press any of the SB1-SBn buttons, a voltage of more than 1 1 V will be applied to the HL8 LED and it will glow. The complete absence of the LED glow indicates the need to replace the battery.

In order not to affect the operation of the DA1 stabilizer, the current in the control circuit was chosen to be small, and a high-brightness red LED was used as HL1. If you install an LED of a different color, the indicator threshold will change.

Installation of the thermometer - hinged. Most of the parts, including one of the sensors (for example, VK1), can be placed on a fiberglass board and mounted on the leads of the PA1 microammeter. The latter is placed in a housing made of insulating material. On the front panel of the device, in addition to the microammeter, buttons and the HL1 LED are installed.

If the sensors are located at a distance of more than 1...2 m from the measuring unit, the connecting wires must be shielded. Sensors installed outdoors or indoors with high humidity, as well as soldering wires to their terminals, must be protected with a moisture-resistant, for example, epoxy compound. When measuring the temperature of water or other liquid, special attention should be paid to protecting the sensors from its influence.

The author used a small-sized M4248 microammeter 50-0-50 μA. To improve the accuracy of reading the temperature, it is desirable to use a device with a larger scale, but with the same values ​​of the current of the total deflection of the arrow in one direction and the other. The fact is that the sensors of the AD22100 series cannot accept a current flowing into pin 2 of more than 80 μA, and it is in this mode that they work in this thermometer at a negative temperature.

By balancing the measuring bridge not at zero, but at a minimum negative temperature, you can use a microammeter with zero at the beginning of the scale and a much higher total deflection current (the current flowing out of the sensor can reach several milliamps). To do this, it is enough to set the voltage at the connection point of the resistors R6, R1 and R2 with the help of a tuning resistor R5 equal to the output voltage of the sensor at the desired temperature. Naturally, the digitization of the microammeter scale in this case will have to be changed.

The thermometer is calibrated by placing one of the sensors alternately in a cold and hot environment, for example, water with a temperature controlled by an accurate laboratory thermometer. At a medium temperature close to zero (or another at which the bridge must be balanced), the pointer of the PA1 microammeter is set to the scale division corresponding to the reading of the reference thermometer using the tuning resistor R6.

Then the sensor is transferred to an environment with a temperature that differs as much as possible from the first one, wait for the readings to stabilize (the needle of the PA1 microammeter should stop "creeping") and set the arrow again to the desired division. This time - with a trimming resistor R4. If the R4 adjustment limits are not enough, you should change the value of the resistor R2. The calibration procedure must be repeated several times.

Author: I. Nechaev, Kursk

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