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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Ratiometric thermometer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology In this thermometer, built on a standard temperature sensor of the TSM series, widely used in the industry, and a double-integration ADC chip KR572PV2, specially designed for measuring instruments, all measures are taken to compensate for the influence of error sources and increase the accuracy of temperature readings. The ratiometric method of measuring the resistance of a resistive temperature sensor (ratio method) allows a simple way to eliminate the influence of the instability of the current flowing through the sensor on the conversion accuracy. The principle of this method is illustrated in Fig. 1. Current I creates a voltage drop Ud=I·Rd on the sensor resistance Rd. An exemplary resistance R is connected in series with the sensor0, on which the voltage drops U0. Measurement result N=Ud/U0=Rd / Ro does not depend on the current, since Ud and Uo change proportionally to it. The measurement accuracy depends only on the temperature stability of the reference resistance R0.
The KR572PV2 microcircuit (similar to the imported ICL7107) is designed specifically for such measurements. It has differential mutually isolated inputs of the measured (input) Uin and exemplary Uobr voltage, and the measurement result is the ratio of Uin to Uobr.
When measuring temperature on the Celsius scale, it is also required to display the sign of the temperature. To do this, it is necessary to enter into the measuring circuit, as shown in Fig. 2, bias resistor Rcm, the resistance of which should be equal to the resistance of the sensor at a temperature of 0 оC. The measurement result will be N \uXNUMXd (Ud - Ucm) / Uo \uXNUMXd (Rd - Rcm) / Ro. The measurement accuracy in this case depends on the temperature stability not only of Ro, but also of Rcm. However, the KR572PV2 microcircuit does not have inputs for supplying voltage Ucm. In the proposed version of the thermometer, not only this, but also other problems are solved. It is insensitive to the stability of the current flowing through the sensor, the drift of zero and the drift of the gain of the operational amplifier included in the instrument, the resistance of the wires connecting the sensor and the thermometer, the transient resistance of the sensor connector contacts, and in the case of using several switched sensors, to the transient resistance of the contacts switch. The thermometer measures temperature in the range from -50 to 180 оC with a resolution of 0,1 оC. The sensor is a standard copper resistance thermometer (TCM) with a characteristic of 23 [1] and a resistance of 53 ohms at 0 оC. The linearity of the scale of the device depends only on the sensor and is maintained throughout the entire range of the measured temperature. The thermometer circuit is shown in fig. 3. The voltages supplied to the inputs of the DD5 microcircuit are formed on the capacitors C11-C14, which are connected in turn to the output of the op-amp DA1 by the selector-multiplexer DD4 (K561KP2), capable of switching analog signals. Synchronously with DD4, the selector-multiplexer DD1 (K561KP1) connects the voltage from the measuring circuit resistors to the input of the op-amp.
The selectors-multiplexers are controlled by the counter DD3.1, to the input of which pulses with a frequency of 50 kHz are applied from the generator on the Schmitt trigger DD2.1. The frequency is set by selecting the resistor R8. Resistor R1 sets the current flowing through the sensor RK1, and voltages Ucm and Uobr are formed on resistors R2-R7. Op-amp DA1 (KR140UD1408A) serves as a voltage follower with a high input, low output resistance and a transfer coefficient equal to one. However, it shifts the levels of the signals passing through the repeater by the value of the OA zero drift Udn. To highlight the drift of zero, the selector-multiplexer DD1 with code 11 on the address inputs connects the input of the repeater to a common wire. Then the selector-multiplexer DD4 connects the capacitor C11 to the output of the repeater, which is charged to the voltage Udn. This voltage is applied to the -Uobr input of the DD5 microcircuit. It can be shown that the influence of the zero drift of the OA on the temperature measurement result is completely eliminated by this. Elements DD2.2-DD2.4, resistors R11-R13, diode VD2, transistors VT2-VT4 are used to extinguish an insignificant zero on the indicator HG1.2 (discharge of tens of degrees). Diode VD1 blocks zero damping at temperatures above 99,9 оC, when the display HG1.1 displays one. Transistors VT1, VT2 and VT4 amplify the outputs of the DD5 chip, providing their levels acceptable for the DD2 chip.
If you measure the temperature above 99,9 оC is not assumed, resistor R10, diodes VD1, VD2 and transistor VT1 can be removed, and the remaining free terminals of element DD2.4 and resistor R13 can be connected to each other. In the power supply (Fig. 4), a negative voltage of -4,7 V is formed in the manner described in [2], which makes it possible to use the T1 transformer with a smaller number of secondary windings. The resistors used in the thermometer can be any. For critical measurements, it is recommended to use resistors R2-R5 with a low temperature coefficient of resistance - C2-29V, C2-36, C2-14. Trimmer resistors R6 and R7 are better to use non-wire multi-turn, for example, SP3-24, SP3-36, SP3-37, SP3-39, SP3-40, RP1-48, RP1-53, RP1-62a. Their denominations may differ from those indicated in the diagram and reach several tens of kilo-ohms. Capacitors C9-C14 - K72-9, K71-4, K71-5, K73-16, K73-17. Oxide capacitors can be anything. The remaining capacitors are any small-sized ceramic ones. Capacitors C1 and C2 are located as close as possible to the power terminals of the op amp DA1, and capacitors C23-C25 are located near the microcircuits DD1-DD5. The integral stabilizer DA3 is mounted on an aluminum plate with an area of at least 16 cm2. Transformer T1 - TP132-19 or other overall power of at least 3 VA with two secondary windings with a voltage of 9 V. To establish a thermometer, a resistance store is required, which is connected instead of the RK1 sensor. Before starting the adjustment, turn all the store switches several times from lock to lock to remove the oxide film formed on their contact surfaces. Set the trimmer resistors R6 and R7 to approximately the middle position, and the resistance store switches to the 53 Ohm position. Having done this, set the trimmer resistor R6 to 0,0 on the thermometer indicator оC. Next, switch the switches either to the position of 77,61 Ohm, which corresponds to a temperature of 99,0 оC, or to the position of 93,64 ohms (temperature 180,0 оWITH). Adjust the trimmer resistor R7 to set the desired temperature on the indicator. To control the switches, move to the position of 41,71 ohms. The indicator should show -50,0 оC. A description of such an operation is available in [3]. In the absence of a resistance box, the adjustment can be made in a well-known way. Fasten the sensor and the reference thermometer together and place in a vessel with melting ice, where the amount of unmelted ice should prevail over the amount of melt water. The thermometer and sensor must not touch the ice and the walls of the vessel. After diving, wait some time for the thermometer to settle down. When they stabilize, set the trimmer R6 on the indicator to 0,0 оC. Then place the sensor and reference thermometer in thoroughly mixed heated water. The higher its temperature, the more accurate the adjustment will be. After stabilizing the readings with a trimming resistor R7, bring them to the readings of a reference thermometer. It is recommended to repeat the adjustment several times. When making the sensor yourself, measure for it a piece of copper wire of any diameter of such a length that its resistance at the actual ambient temperature corresponds to that indicated in Table. 1. Estimated wire length at 20 оWith depending on its diameter is given in table. 2. The resistivity of copper at this temperature is assumed to be 0,0175 Ohm mm2/ m. Table 1
Table 2
The easiest option is to measure the wire with a margin, and then shorten it, achieving the desired resistance. But it is especially accurate to adjust the resistance of the sensor to those indicated in Table. 1 value is not worth it. Indeed, in the process of establishing, you still have to use trimming resistors R6 and R7. Wind the sensor wire on the coil in a bifilar way, having previously folded it in half. Such a sensor does not have inductance, and all electromagnetic pickups on each half of its wire are mutually neutralized. When setting up a device with a self-made sensor using a resistance box, it is necessary to take into account the deviations of the actual resistance of the sensor from the standard one [1]. The voltage source 5 V (d) supplying the sensor circuit must be galvanically isolated from other circuits. To refuse such a source will allow the use of an instrumental amplifier AD623. Such an amplifier is also desirable because it has a large coefficient of attenuation of common mode noise that inevitably occurs on the connecting wires of the sensor. The circuit for connecting the amplifier to the thermometer is shown in fig. 5. Other types of instrumentation amplifier can be used, such as AD8221, LT1168, MAX4194.
On fig. 6 shows a circuit of an instrumental amplifier in which any op-amp can be used. The recommended values for all resistors are 51 kOhm, but they may be different. It is only necessary to fulfill with the greatest possible accuracy (with an error of fractions of a percent) the conditions R1=R2 and R3=R4=R5=R6.
The gain of the instrumental amplifier depends on the resistance of the external resistor Rg: K = 1 + (R1 + R2)/Rg. In its absence, it is equal to one, and the resistors R1 and R2 can be replaced with jumpers. The current passing through the sensor heats it up, which leads to an error in temperature measurement. Resistor R1 (see Fig. 3) is calculated so that a current of about 4,43 mA flows in the sensor circuit, at which a change in temperature by one degree causes a change in voltage Ud by 1 mV. You can reduce the current by increasing the resistance R1. However, how many times the current was reduced, by the same amount it is necessary to increase the stage gain at the op-amp DA1, for which it is necessary to change the thermometer circuit, as shown in Fig. 7. In this case, the gain is K = 1 + R2`/R1`. But you should not get carried away with reducing the current, since when the useful signal is amplified, the interference will also increase. The temperature drift of the gain will not affect the measurement results, since all the signals involved in the measurement pass one by one through the same amplifier and change proportionally. Their relationship remains unchanged.
Application of the filter, the scheme of which is shown in Fig. 8 will significantly reduce common-mode interference, as well as protect the inputs of the DD1 chip from overvoltages that can form on the wires connecting the sensor to the thermometer in any emergency situations. The two-winding choke L1 can be found in the mains supply circuits of many electronic devices, such as computer monitors. The filter is included in the breaks in the circuits connecting pins 2 and 4 of the X1 connector with the pins of the DD1 microcircuit. The places of breaks are shown in fig. 3 crosses.
If you intend to use several sensors, then all five wires connecting the sensor to the thermometer, including the common wire, should be switched. The switch can be anything. Literature
Author: V. Prokoshin
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