Thermoelectric device for measuring electric current characteristics

Abstract

The invention relates to measuring equipment and can be used for measuring the root mean square values of electrical signals in a high-frequency range: current, voltage and power, as well as for galvanic isolation or the like. A thermoelectric device for measuring electric current characteristics includes a heater designed to allow the passage therethrough of electric current to be measured, and a heat flux sensor designed to be capable of registering the heat generated by the heater and converting it into a thermal emf output signal. The heater is a thermoelectric Peltier module, and the heat flux sensor is also a thermoelectric Peltier module. A first side of the thermoelectric module of the heater and a first side of the thermoelectric module of the heat flux sensor are connected to one another to form a thermally conductive contact. A second side of the thermoelectric module of the heat flux sensor is designed to allow thermally conductive connection to a thermally conductive base, and a second side of the thermoelectric module of the heater is also designed to allow thermally conductive connection to the thermally conductive base. The technical result is that of enhancing the measured signal, providing a linear transfer characteristic between the measured input current or voltage and the thermal emf output signal, and reducing the influence of external factors.

Description

[0001] Thermoelectric device for measuring electric current characteristics

[0002] The invention relates to measuring equipment and can be used to measure the effective values ​​of electrical signals in the high-frequency range: current, voltage, power, as well as for galvanic isolation, etc.

[0003] In electrical measuring technology and metrology, semiconductor devices-sensors, called thermoelectric converters, are known and widely used (see, for example, Thermoelements and thermoelectric devices: Handbook / L. I. Anatychuk, - Kyiv: Nauk, Dumka, 1979, 385 p.) and are used as sensors for measuring the effective values ​​(RMS) of electrical signals in the high-frequency range: current, voltage, power, and also as galvanic isolation, etc.

[0004] These devices contain two main components: an electric resistive heater (sometimes multiple heaters) and a thermocouple (usually a thermocouple or thermocouple array). The heater, to which the measured current is applied, is the input circuit of the converter, and the thermocouple, which generates the thermoelectric power, is the output circuit.

[0005] An electric heater is a resistive element that, according to Ohm's law, generates electrical power as a measured signal. A thermoelectric element (a thermocouple or several connected thermocouples) measures the temperature to which the heater is heated. The output signal is the thermoelectric power of the thermoelement.

[0006] The relationship between the current supplied to the heater and the thermoelectric power developed by the measuring thermoelement is quadratic or quasilinear relative to the released thermal power. This allows the converter to be used to measure the effective values ​​(RMS) of alternating voltage, current, power, and other parameters.

[0007] Common disadvantages of existing thermoelectric transducers-sensors are low sensitivity, nonlinearity of the transfer characteristic, and significant dependence of measurement accuracy on design factors: parasitic heat loss, etc.

[0008] The low sensitivity of thermoelectric transducers / sensors is due to the fact that the thermocouples used in many cases are metal thermocouples with a low specific thermoelectric power. Prior art solutions are known for increasing sensitivity (see, for example, WO96 / 31784, published 10.10.1996, IPC G01R 21 / 02) by constructing a sensor from a series of thermocouples, which increases sensitivity. However, this increase in sensitivity is usually insufficient.

[0009] The nonlinearity of the transfer characteristic, namely the dependence of the output signal on the measured current, is due to the fact that the heating value of the heater is proportional to the power of the measured signal, which is in a quadratic relationship with the supplied current. However, such linearity between the measured power and the output signal is not strict. It is affected by parasitic heat losses emitted into the surrounding space and into the structural elements of the device. Various methods for reducing the influence of external factors are known in the prior art, for example, by introducing an active reference channel, disclosed in patent US7372249B2, published 13.05.2008, IPC G01R 3L / 00, or by introducing additional elements into the electrical circuits, as disclosed, for example, in patent SU1364168, published 23.09.1991, IPC NO 1L 35 / 02.

[0010] A prior art solution for addressing the shortcomings of traditional thermocouples used in electrical signal meters is disclosed in patent US7705582B2, published April 7, 2007, IPC G01 K 17 / 00. This solution can be selected as the closest equivalent to the proposed technical solution. The known sensor includes a resistive heater and a measuring section consisting of a series of metal thermocouples connected in series. The entire structure is constructed on a thin membrane to reduce the influence of external factors on measurement accuracy and to minimize useful heat loss.

[0011] By reducing the thermal loss of the measured signal and using a series of thermocouples to increase the measuring signal in the known solution, it is possible to achieve a certain result; however, the sensitivity of the sensor remains at a fairly low level, and there is also no linearity between the measured current and the output signal.

[0012] The technical problem that the present invention is aimed at solving is to increase the sensitivity and accuracy of measuring the electrical characteristics of high-frequency signals and direct current: current, voltage, power.

[0013] The technical result achieved by solving the technical problem consists of increasing the measured signal and implementing a linear transfer characteristic between the input measured current, voltage and the output thermoEMF signal, reducing the influence of external factors.

[0014] The technical problem is solved and the technical result achieved by a thermoelectric device for measuring electric current characteristics, which includes a heater configured to conduct the measured electric current, and a heat flow sensor configured to record the heat generated by the heater and convert it into a thermoelectric power (TEF) output signal. The heater is a Peltier thermoelectric module, and the heat flow sensor is also a Peltier thermoelectric module. The first side of the heater's thermoelectric module and the first side of the heat flow sensor's thermoelectric module are connected to each other to form a thermally conductive contact.The second side of the thermoelectric module of the heat flow sensor is configured to be heat-conductingly connected to the heat-conducting base, and the second side of the thermoelectric module of the heater is also configured to be heat-conductingly connected to the heat-conducting base.

[0015] The present invention is further explained by the following drawings.

[0016] Fig. 1 shows a schematic representation of the claimed thermoelectric device.

[0017] Fig. 2 shows a diagram of the heat flows of the thermoelectric heater module during its operation.

[0018] Fig. 3 shows experimental graphs of the dependence of the output signal U x thermoelectric module of the heat flow sensor from the input current I of the measured DC signal.

[0019] Fig. 4 shows a comparison of the experimentally measured heat flux Q xthrough the thermoelectric sensor of the heater from the electrical power of the measured electrical signal QR.

[0020] Fig. 5 shows a comparison of the experimental dependencies of the measured thermoEMF Ux in the case of a measured DC signal and the measured thermoEMF Uxl / 2 of a high-frequency signal with a frequency of 100 MHz and a duty cycle of 50%.

[0021] Fig. 6 shows the results of measuring a high-frequency signal with a frequency of 100 MHz at different duty cycles.

[0022] Fig. 1 illustrates a schematic representation of the claimed thermoelectric device. The thermoelectric device includes a thermoelectric heat flux sensor module (1) and a thermoelectric heater module (2). The thermoelectric modules are connected to each other at one of their sides to form a thermal contact. The connection is accomplished, for example, by soldering. The opposite side of the thermoelectric heat flux sensor module (1) is mounted on a heat-conducting base, for example, the housing (3) of the object, and thermal contact of the thermoelectric heater module with the environment (for heat input or heat removal) can be achieved either by means of current-supplying wires or by mounting the opposite side of this module on a heat-conducting base.The direct current of the measured signal or high-frequency current I is applied to the thermoelectric heater module (1), so that it heats the side common with the thermoelectric module of the heat flow sensor (2). As a result, the heat flow sensor generates an output signal U. x due to the resulting temperature difference and the heat flow passing through this thermoelectric module. The output signal is proportional to the effective value of the current I. Fig. 2 illustrates the heat flow diagram of the thermoelectric heater module during its operation. When current I is applied to the thermoelectric heater module, Peltier heat Q is generated on the heated side. p and half of the Joule heat (QR / 2) is supplied by the current I passing through the thermoelectric module of the heater. This total generated heat flow is removed in two directions. The first is the thermoelectric module of the heat flow sensor Q x, due to the resulting temperature difference with the base (Th-T a and the second - Q in the opposite direction from the heated to the cold side of the thermoelectric module of the heater, also due to the created temperature difference (Th-TQ.

[0023] Fig. 3 illustrates experimental graphs of the dependence of the output signal U x thermoelectric module of the heat flow sensor from the input current I of the measured DC signal for the claimed invention, and for the case of a traditional resistive heater U X R. It is shown that the sensitivity (the magnitude of the output signal U x ) the claimed invention has several times more than the sensor manufactured according to the traditional design with a resistive heater U XR. Also shown is the experimental dependence of the output signal {Dc for the case where the side of the heater's thermoelectric module connected to the sensitive side of the heat flux sensor's thermoelectric module is not heated, but cooled. The measured signal is also present, although smaller than U x , however, larger than U X R.

[0024] Fig. 4 shows a comparison of the experimentally measured heat flux Q xThe heat flux through the thermoelectric module of the heat flux sensor is determined from the electrical power of the measured electrical signal QR. It is experimentally confirmed that the heat flux measured by the thermoelectric module of the heat flux sensor significantly exceeds the electrical power of the measured signal. A comparison with the calculated value of heat flux through the thermoelectric module of the heat flux sensor Qth is also provided. The comparison also demonstrates full agreement between the experimental data on the manufactured sample and the theoretical calculations.

[0025] Fig. 5 shows a comparison of the experimental dependencies of the measured thermopower U x in the case of a measured DC signal and measured thermoEMF U xl / 2 of a high-frequency signal with a frequency of 100 MHz and a duty cycle of 50%. The effective current of such a high-frequency signal is equal to half the amplitude of the direct current, as a result of which the output signal (thermoEMF) is also two times smaller than with direct current over the entire measured current range.

[0026] Fig. 6 shows the results of measuring a high-frequency signal with a frequency of 100 MHz and a duty cycle of up to 100%. The amplitude current is 10 mA. Experiments have confirmed that the sensor's output signal is directly proportional to the effective current value, which is equal to the amplitude current multiplied by the duty cycle.

[0027] The claimed thermoelectric device operates as follows. Let's first consider the case where a constant current I is applied to the thermoelectric heater module. For simplicity, let's assume both the heater and heat flow sensor thermoelectric modules are identical in design.

[0028] When current is supplied to the electric circuit of the thermoelectric heater module, we will have a voltage consisting of an ohmic component U R and thermoEMF U a (thermoEMF of the heater).

[0029] Formula for ohmic voltage U R corresponds to Ohm's law (1). And the magnitude of the thermoelectric power U a caused by the creation of a current / temperature difference T h - T с on this thermoelectric heater module. where / is the measured current of the electrical signal; R H- the intrinsic resistance of the thermoelectric heater module; a - the Seebeck coefficient (thermoelectric power coefficient) of the thermoelectric heater module; N - the number of thermoelements in the thermoelectric heater module; DT Н = T h - T с - temperature difference on the thermoelectric heating module.

[0030] The ratio of these two components (U R and U a ), as is known, is determined by a simple formula in the case of small currents for the Peltier element, when the change in temperature T can be neglected h - preferably no more than 20% of the maximum current of the Peltier element module. where Z is the thermoelectric figure of merit of the thermoelectric heater module; T h - the temperature on its heated (hot) side.

[0031] Temperature T hThe heated side of the heater's thermoelectric module is simultaneously the temperature of the sensitive side of the heat flow sensor's thermoelectric module. Since the reverse side of the heat flow sensor's thermoelectric module remains at its initial temperature, T а , since there is thermal contact with the environment and corresponding heat dissipation, the resulting temperature difference on the thermoelectric module of the heat flow sensor causes the appearance of thermoEMF U x in this module.

[0032] U x = N x a x DT Х = N x a x (T h - T а ~) (4) where DT Х - temperature difference on the heat flow sensor module (HFS) Х = T h - T а ). ThermoEMF U a and U x on both thermoelectric modules are proportional to the resulting temperature differences on each of them, respectively.

[0033] Since the opposite side of the thermoelectric heater module is simultaneously cooled to temperature T с (i.e. it becomes lower than the ambient temperature T а ), then the temperature difference on it should be approximately twice as large as on the thermoelectric module of the heat flow sensor.

[0034] T ft - T c « 2 x (T ft - T a ) (5)

[0035] Therefore, thermoEMF U x on the thermoelectric module of the heat flow sensor is two times less than the thermoelectric power U a on the thermoelectric module of the heater (we remind you that the thermoelectric modules were chosen to be identical).

[0036] U a ^ 2 x U x (6)

[0037] Measured thermopower U x thermoelectric module of the heat flow sensor taking into account (3) and (6) has the form:

[0038] Or, keeping in mind (1), we finally obtain an expression for the relationship between the obtained thermoEMF and the measured current. where the product A is a constant coefficient (for a small change in temperature in Kelvin).

[0039] Formulas (7) and (8) demonstrate the linear dependence of the output measured signal U x from current / or voltage U R measured electrical signal.

[0040] Let us consider the heat balance on the hot side of the thermoelectric heater module (Fig. 2) where Q x - heat passing towards the thermoelectric module of the heat flow sensor; Q x - Peltier heat released due to the passage of current through the thermoelectric module of the heater; - heat flow of thermal conductivity in the thermoelectric module of the heater due to the occurrence of a temperature difference; Q R - Joule heat generated by the passage of electric current.

[0041] Let's rewrite From (9) it follows that the total heat flow released on the hot side of the thermoelectric heater module due to Joule heating (— ) and heat

[0042] Peltier (Q p ) is divided into two heat conduction flows: the useful flow - through the thermoelectric module of the heat flow sensor Q x , the second flow is in the thermoelectric module of the heater itself in the opposite cold side Q.

[0043] Let's uncover the formula where to х , k - thermal conductivity of thermoelectric modules of the heat flow sensor and heater, respectively.

[0044] Since the temperature difference T h - T с on the thermoelectric module of the heater is approximately twice as large as in the thermoelectric module of the heat flow sensor T h - T а (5), and the thermoelectric modules are the same (k^ = / s х), then the ratio of heat flows of thermal conductivity is similar - two times.

[0045] And then, the heat flux measured by the thermoelectric module of the heat flux sensor will be approximately 1 / 3 of the total generated heat flux.

[0046] Heat flux Q x the heat flow entering the thermoelectric module of the heat flow sensor is measured by it, resulting in an output signal U x . where S a - integral sensitivity of the thermoelectric heat flow sensor.

[0047] From formula (12) it follows and is experimentally confirmed (Fig. 4) that the heat flow Q x , measured by the thermoelectric module of the heat flow sensor is several times greater than the electrical power of the measured signal Q R = I 2 R. This is especially evident at low measured currents, when the generated Peltier heat (NaIT h) significantly exceeds the electrical power (I 2 R is the Joule heat). This explains the significantly higher sensitivity (several times greater output signal) of the thermoelectric module of the heat flow sensor than in the case where the heater is a resistive heater (Fig. 3).

[0048] Taking into account (8), from (13) we can obtain a formula that is useful for further consideration.

[0049] The claimed thermoelectric device will also operate if the side of the thermoelectric heater module shared with the thermoelectric heat flow sensor module is not heated, but cooled (by reversing the direction of the supplied current). However, the heat balance equation must consider the cold side of the thermoelectric heater module (Fig. 1), which is connected to the sensing side of the heat flow sensor module.

[0050] From here, by analogy with (10)-(15), we obtain the value of the thermoEMF.

[0051] From the comparison of (18) and (15) it follows that the cooling of the common side of the thermoelectric module of the heater and the thermoelectric module of the heat flow sensor, the thermoEMF (18) on the thermoelectric module of the heat flow sensor will be less by the value of the Joule I 2 R heat compared to the heating of this side (15). This is confirmed by experimental data (Fig. 3). Although the thermoelectric heater module can operate with both directions of the measured current, the current direction in which the common side of the thermoelectric heater module and the thermoelectric module of the heat flux sensor is heated by the measured current is preferable for a larger measuring signal.

[0052] Let us consider the supply of a high-frequency signal with a period Z, pulse duration t and pulse current amplitude I to the thermoelectric heater module.

[0053] In the case of a high-frequency signal, the thermal balance will be somewhat different than with direct current (10). Namely, in the thermoelectric module of the heater on the heated side, the thermal power and Peltier Q heat p will be allocated only in during the time t of the action of the current pulse I, and the thermal flows of thermal conductivity in both directions will take place during the entire period t of the frequency signal.

[0054] Then, for the full period t of the frequency signal, we have the total heat balance. It should be noted that equation (19) is valid when the times t and t are significantly shorter than the thermal time constant of the thermoelectric heater module and the heat flux sensor module. Then the change in temperature T can be neglected. hduring the period t. For thermoelectric modules, the minimum thermal time constant is at the level of 1.0-0.1 s. Accordingly, it is preferable that the frequency of the alternating signal be more than 1 - 10 kHz (the duration of the period t is no more than 1.0-0.1 ms).

[0055] Taking into account (11), we have

[0056] Heat flow into the heat flow sensor module per unit of time

[0057] Let's substitute the expression for the amplitude current / (15) into (18)

[0058] And by analogy with (13) the output thermoEMF U x in case of a frequency signal

[0059] The expression / X - in formula (24) is nothing more than the effective value of the frequency signal current.

[0060] Expression (24) shows that in the claimed technical solution, in the case of measuring a high-frequency electrical signal with such a thermoelectric device, the output voltage (thermoEMF) has a linear dependence on the effective value of the measured current.

[0061] The thermal signal sensor's thermoelectric power output signal is proportional, through the same constant coefficient A, to the effective (average) value of the measured current, as for direct current. The measured input signal, in the form of direct current, is a special case of the general linear characteristic (21).

[0062] Thus, the claimed technical solution measures a significantly larger thermal signal (significantly greater Joule heat), which ensures correspondingly greater measurement sensitivity and implements a linear relationship between the output signal (thermoEMF) and the effective value of the measured input current. A DC signal is an extreme example of this general pattern. Example implementation.

[0063] As an example, a thermoelectric device was manufactured according to the circuit shown in Fig. 1. Identical thermoelectric modules - Peltier elements, including 14 pairs of n- and p-type semiconductor thermoelements, each with a cross-section of 0.2x0.2 mm, were used as the thermoelectric module of the heater (2) and the thermoelectric module of the heat flow sensor (1). 2 and a height of 0.8 mm. The electrical resistance of the module is 3.5 ohms. The maximum current of such a Peltier element is 0.25 A.

[0064] The thermoelectric device can be used to measure the electrical characteristics of high-frequency signals, preferably in the range of effective currents of 0...50 mA.

[0065] The heat flow sensor's thermoelectric module is mounted on a heat-conducting base—a heat sink (3)—for example, on a TO5 housing. A thermoelectric heater module (2) is soldered to the upper sensitive side of the heat flow sensor's thermoelectric module, with the side that will be heated by the measured electrical signal.

[0066] For a comparison of characteristics, a device with a traditional resistive heater was assembled. The heat flux sensor module utilized the same 14-pair thermoelectric module, mounted on a similar heat sink base. The resistive heater was formed on its upper sensitive side by sputtering a resistive heating element film onto it. For a fair comparison, the resistive heater's resistance was set to 3.5 ohms, equal to the resistance of the thermoelectric module used in the heater according to the claimed invention.

[0067] A current in the range of 0...50 mA is applied to the heater. The thermoelectric module of the heat flow sensor measures the output thermoelectric power signal.

[0068] The measurement results are shown in Figs. 3-6. The results are also presented in comparison with the calculated results obtained from the formulas given above.

[0069] As follows from the experimental data (Fig. 3), the device according to the present invention has higher sensitivity characteristics due to the greater heat flux supplied to the heat flux sensor. Due to this, its output signal U x several times (up to four times) larger than the signal U X R of a traditional transducer with a resistive heater at a similar current. Furthermore, as can be seen from Fig. 3, the claimed device has a linear output voltage (thermoelectric power) characteristic as a function of the measured current, in contrast to the pseudo-quadratic current dependence of a traditional sensor.

[0070] Also in Fig. 3 the experimental dependence of the output signal U is shown. x c for the case where the side of the heater's thermoelectric module connected to the sensitive side of the thermal signal sensor's thermoelectric module is not heated, but cooled. A measured signal is also present, although smaller than Ux , however, larger than U X R. WHICH is FULLY explained by comparing the calculation formulas (15) and (18) and confirms that the invention has maximum measurement parameters when heating the common side of the thermoelectric module of the heater and the thermoelectric module of the heat flow sensor.

[0071] The experimental result shown in Fig. 4 demonstrates full agreement with the calculated data and demonstrates the nature of the higher performance of the claimed device - due to the greater heat detected by the thermoelectric heat flux sensor. The greater amount of heat Q x , than simply the Joule heat QR generated by the passing current, is due to the operating features of the thermoelectric module of the heater - the Peltier element, due to the Peltier effect. The experimental dependence of Q x is completely consistent with the calculated value of the heat flow Qth following from relation (12).

[0072] The presented results of measurements of the high-frequency signal (Fig. 6 and Fig. 7), as well as for experiments with direct current, show a linear dependence of the measured thermoEMF on the effective value of the current.

[0073] Thus, the claimed invention solves the problem of significantly increasing the sensitivity of measuring the electrical characteristics of high-frequency signals and direct current signals and provides a linear dependence of the output signal on the effective value of the measured current.

Claims

Invention formula 1. A thermoelectric device for measuring the characteristics of an electric current, including a heater configured to allow a measured electric current to pass through it, and a heat flow sensor configured to record the heat generated by the heater and convert it into a thermoEMF output signal, characterized in that the heater is a Peltier thermoelectric module, and the heat flow sensor is also a Peltier thermoelectric module, wherein the first side of the thermoelectric module of the heater and the first side of the thermoelectric module of the heat flow sensor are connected to each other to form a heat-conducting contact, the second side of the thermoelectric module of the heat flow sensor is configured to be heat-conductively connected to a heat-conducting base, and the second side of the thermoelectric module of the heater is also configured to be heat-conductively connected to a heat-conducting base.

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