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Title:
METHOD AND APPARATUS FOR MEASURING OF ELECTRICAL POWER AND USE THEREOF
Document Type and Number:
WIPO Patent Application WO/1997/042511
Kind Code:
A1
Abstract:
The invention relates to a method and an apparatus for measurement of electrical power and electrical energy consumed or produced in an electric application such as a domestic appliance or a generator. This takes place by performing a measurement of an electrical power yielded in a transistor to which a current signal and a voltage signal is applied depending on a voltage (U) across and a current (I) in the electric application. The current signal and the voltage signal are proportional to the electrical energy and the electrical power consumed or produced in the electric application. The apparatus for measurement of an electrical power comprises a first circuit with a transistor and a second circuit with a transistor. The first circuit is used as a measuring circuit and the second circuit is used as a reference circuit. A current signal is applied to the first circuit, respectively the second circuit. A voltage differnce, alternatively a temperature difference, is measured as a function of the electrical power and consequently the temperature change and subsequent voltage change taking place in the transistors in the first circuit, respectively the second circuit. The current signal leads to errors caused by non-linearity and depending on the degree of load. This error is outbalanced as a current signal is applied to both the transistor in the first circuit and the transistor in the second circuit. Furthermore, a temperature impact from the surroundings will be outbalanced in the same manner.

Inventors:
WILLADSEN JOERN BLINKENBERG (DK)
ALVSTEN BENGT (DK)
ANDERSEN HANS MOELLER (DK)
Application Number:
PCT/DK1997/000212
Publication Date:
November 13, 1997
Filing Date:
May 07, 1997
Export Citation:
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Assignee:
FIRMAET ARKITEKT JOERN BLINKEN (DK)
ELEKTRONIK V BENGT ALVSTEN A (DK)
CENTEC TRADE & TECHNOLOGY A S (DK)
WILLADSEN JOERN BLINKENBERG (DK)
ALVSTEN BENGT (DK)
ANDERSEN HANS MOELLER (DK)
International Classes:
G01R21/02; G01R21/06; G01R21/14; (IPC1-7): G01R21/14
Foreign References:
US5465044A1995-11-07
US5404585A1995-04-04
US4764720A1988-08-16
US4970456A1990-11-13
Other References:
DERWENT'S ABSTRACT, No. 91-302148/41, Week 9141; & SU,A,1 626 172, (KIROV POLY), 7 February 1991.
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Claims:
CLAIMS
1. A method for measuring electrical power and electrical energy, said method com¬ prising steps wherein a change of a voltage between a first terminal and a second ter minal, preferably a basisemitter voltage (Ube), in a first transistor in a first circuit is measured as a function of a load of the transistor by application of a current signal and a voltage signal with a first phase difference between current (I) and voltage (U), wherein a voltage between a first terminal and a second terminal, preferably a basis emitter voltage (Ube), is measured in a second transistor in a second circuit and used as a reference in the measurement of the voltage in the transistor in the first circuit, and wherein the voltage measurements in the transistor in the first circuit and the transistor in the second circuit are used for determining an electrical power and an electrical en¬ ergy yielded in the transistor in the first circuit, c h a r a c t e r i s e d in that meas¬ urement of the voltage in the transistor in the second circuit is measured as a function of load of the transistor by application of a current signal and a voltage signal with a second phase difference between current and voltage, and that the phase difference between current and voltage in the current signal and the voltage signal in the second circuit is different from the phase difference between current and voltage in the current signal and the voltage signal in the first circuit.
2. A method for measuring an electrical power, c h a r a c t e r i s e d in that said method comprises steps wherein a temperature change of a transistor in a first circuit is measured as a function of a load of the transistor by application of a current signal with a first phase difference between current (I) and voltage (U), wherein a tempera ture of a transistor in a second circuit is measured and used as a reference for meas¬ urement of a temperature of the transistor in the first circuit, wherein a temperature difference between the transistor in the first circuit and the transistor in the second circuit is used for determining an electrical power and electrical energy yielded in the transistor in the first circuit, that measurement of temperature of the transistor in the second circuit is measured as a function of a load of the transistor by application of a current signal with a second phase difference between current and voltage, and that the phase difference between current and voltage in the current signal in the second circuit is different from the phase difference between current and voltage in the current signal in the first circuit.
3. A method according to claim 1 or claim 2, c h a r a c t e r i s e d in that the current signal in the second circuit is proportional to the current signal in the first circuit, pref¬ erably the same as the current signal in the first circuit.
4. An apparatus for measuring an electrical power by using the method according to any one of claims 1 or 2, c h a r a c t e r i s e d in that the apparatus comprises a first circuit consisting of a first transistor and a first operation amplifier, that the transistor has a first terminal, preferably a basis, that is connected to an output terminal of the first operation amplifier, a second terminal, preferably an emitter, that is connected to a first input terminal on the first operation amplifier, and a third terminal, preferably a collector, that is connected to a first terminal of a voltage supply, that the apparatus comprises a second circuit consisting of a second transistor and a second operation amplifier, that the transistor has a first terminal, preferably a basis, that is connected to an output terminal on the second operation amplifier, a second terminal, preferably an emitter, that is connected to a first input terminal on the second operation amplifier, and a third terminal, preferably a collector, that is connected to the first terminal of the voltage supply, that the third terminal on the transistor in the first circuit is connected to the third terminal on the transistor in the second circuit.
5. An apparatus according to claim 1 for use by the method according to claim 2, c h a r a c t e r i s e d in that encapsulations on the transistors in the first circuit, re¬ spectively the second circuit, are provided with thermosensors.
6. An apparatus for use as a an electric supply, c h a r a c t e r i s e d in that said appa¬ ratus comprises a first transistor and a second transistor, that the third terminal, pref erably a collector, on the first transistor and on the second transistor is connected to a first terminal by a voltage connection, that a second terminal, preferably an emitter, on the first transistor is connected to a first operation amplifier, that a second terminal, preferably an emitter, on the second transistor is connected to a first input terminal on a second operation amplifier, that a first terminal, preferably a basis, on the first tran¬ sistor is connected to an output terminal on a first operation amplifier, that a first ter¬ minal, preferably a basis, on the second transistor is connected to an output terminal on the second operation amplifier, that a second input terminal on the first operation amplifier and a second input terminal on the second operation amplifier are intercon¬ nected.
7. A use of a method according to any one of claims 1 or 2 for measurement of power consumption or power production of an electric application.
Description:
METHOD AND APPARATUS FOR MEASURING OF ELECTRICAL POWER AND USE THEREOF

Description of the invention The present invention relates to a method for measuring electrical power and electrical energy, said method comprising steps wherein a change of a voltage between a first terminal and a second terminal, preferably a basis-emitter voltage, across a first tran¬ sistor in a first circuit is measured as a function of a load of the transistor by applica¬ tion of a current signal and a voltage signal with a first phase difference between cur- rent (I) and voltage (U), wherein a voltage, preferably a basis-emitter voltage, is measured across a second transistor in a second circuit and used as a reference in the measurement of the voltage across the transistor in the first circuit, and wherein the voltage measurements across the transistor in the first circuit and across the transistor in the second circuit are used for determining an electrical power and an electrical en- ergy yielded in the transistor in the first circuit.

DK 93 00404 describes an apparatus using such a method. The apparatus comprises a first circuit with a transistor and an operation amplifier and a second circuit with a transistor and an operation amplifier. The first circuit is designed to be supplied with a current signal and a voltage signal. Such a current signal will apply power to the tran¬ sistor causing the transistor temperature to rise. The temperature increase leads to a voltage change between two of the transistor terminals, preferably between basis and emitter. The voltage change is measured by a constant closed circuit in the transistor, and the yielded power may be determined as a function of the voltage change. The second circuit is used as a reference circuit. The voltage between two of the transistor terminals, preferably basis and emitter, of the transistor in the second circuit is meas¬ ured. The voltage in the second circuit is used to determine the temperature of the sur¬ roundings so that when measuring the voltage change in the first circuit one may com¬ pensate for any temperature change in the surroundings.

However, this apparatus possesses certain disadvantages. The leak current, and thus the zero voltage and power, at the resting position of the first circuit and the second circuit is identical. Due to the temperature dependency to which the leak current and consequently the resting position is subject, the voltage across the transistor will change as a function of the temperature. Heat applied to the transistor in the first cir¬ cuit stems both from the surroundings and from yielded electrical power when apply¬ ing a closed-circuit signal as well as from yielded electrical power when applying the current signal. Heat applied to the transistor in the second circuit, however, only stems from the surroundings and from yielded electrical power when applying a closed- current signal. Thus, the transistors in the first circuit, respectively the second circuit, are both supplied with a closed-circuit voltage. When loading the transistor in the first circuit, an error contribution arises that is superposed on the closed-circuit voltage. Measurement of the electrical power when using a first "active" circuit and a second "passive" circuit as a reference measure causes the error arising in the first circuit when applying a current signal to contribute to an error in the measurement of the electrical power yielded in the transistor in the first circuit. It is not possible to deter¬ mine the size of this error in the measurement in a fast and unambiguous manner. Eliminating the error contribution requires a correction.

US 5 465 044 describes a circuit forming a wattmeter and capable of measuring a phase shift and power consumption in a circuit by measuring a temperature difference across transistors. The circuit comprises a first transistor circuit in which a first transis¬ tor and a second transistor are supplied with a voltage and a current from an electrical power unit while a second transistor unit is only supplied with the current from the electrical power unit. Thus, a voltage difference is established between the first tran¬ sistor circuit and the second transistor circuit, which voltage difference is used for measuring a power consumption from the electrical power unit. The first transistor circuit and the second transistor circuit must be subjected to identical electrical and thermal characteristics.

This circuit possesses further disadvantages. It is not desirable to use two transistor circuits for measurement since different characteristics of the components in the two

circuits may exist or arise. This means that it is constantly necessary to pay attention to making repeated calibrations, if any, of the entire circuit and of the two circuits. It is further disadvantageous that the two transistor circuits must be subjected to identical electrical and thermal characteristics: This may be difficult since the two transistor circuits are exposed to both external thermal influences and internal thermal influences when a current is sent through the transistors.

Thus, it is the object to eliminate the errors that may arise in the known apparatuses and circuits and to disclose a method of measurement that is capable of performing power measurement in the first circuit wherein thermal error contributions arising in the first circuit are outbalanced when a current signal is applied.

This is obtained by a method that is characterised in that measurement of the voltage across the transistor in the second circuit is measured as a function of load of the tran- sistor by application of a current signal and a voltage signal with a second phase dif¬ ference between current (I) and voltage (U), and that the phase difference between current and voltage in the current signal and the voltage signal in the second circuit is different from the phase difference between current and voltage in the current signal and the voltage signal in the first circuit.

A method of measurement in which the second circuit, forming the reference circuit, is used more "actively" by application of a current signal means that the contribution that errors lead to in both the first circuit and the second circuit when applying a cur¬ rent signal across the first circuit will not as previously contribute to errors in the measurements but be outbalanced instead. This means that the error contribution from both circuits has no influence on the measurement of electrical power yielded in the transistor in the first circuit.

The method may be used for measurement of electrical power both on the input side and on the output side. This means that the method may be used in combination with both power-consuming and power-producing applications. Power-consuming applica¬ tions may be e.g. electrical appliances in household. Power-producing apparatuses

may be e.g. a windmill generator. The method may also be used in combination with applications that are both power-consuming and power-producing such as a genera¬ tor/motor in an electric car or an accumulator.

An alternative method is characterised in that said method comprises steps wherein a temperature change of a transistor in a first circuit is measured as a function of a load of the transistor by application of a current signal with a first phase difference between current (I) and voltage (U), wherein a temperature of a transistor in a second circuit is measured and used as a reference for measurement of a temperature of the transistor in the first circuit, wherein a temperature difference between the transistor in the first circuit and the transistor in the second circuit is used for determining an electrical power and electrical energy yielded in the transistor in the first circuit, that measure¬ ment of temperature of the transistor in the second circuit is measured as a function of a load of the transistor by application of a current signal with a second phase differ- ence between current and voltage, and that the phase difference between current (I) and voltage (U) in the current signal in the second circuit is different from the phase difference between current and voltage in the current signal in the first circuit.

An apparatus for use by the method according to the invention is characterised in that the apparatus comprises a first circuit consisting of a first transistor and a first opera¬ tion amplifier, that the transistor has a first terminal, preferably a basis, that is con¬ nected to an output terminal of the first operation amplifier, a second terminal, pref¬ erably an emitter, that is connected to a first input terminal on the first operation am¬ plifier, and a third terminal, preferably a collector, that is connected to a first terminal of a voltage supply, that the apparatus comprises a second circuit consisting of a sec¬ ond transistor and a second operation amplifier, that the transistor has a first terminal, preferably a basis, that is connected to an output terminal on the second operation amplifier, a second terminal, preferably an emitter, that is connected to a first input terminal on the second operation amplifier, and a third terminal, preferably a collector, that is connected to the first terminal of the voltage supply, that the third terminal on the transistor in the first circuit is connected to the third terminal on the transistor in the second circuit.

An apparatus with these characteristics makes it possible for the physical dimensions of the apparatus to be considerably reduced compared to known apparatuses for meas¬ urement of electrical power. Thus, the apparatus may be incoφorated into power- consuming or power-producing applications as an integral part thereof. The apparatus may also be used separately in conjunction with power-consuming or power- producing applications.

An alternative apparatus is characterised in that encapsulations on the transistors in the first circuit, respectively the second circuit, are provided with thermosensors.

By combining the apparatus according to the invention with a timer it is possible to measure a given energy consumption by multiplying the power measured by the appa¬ ratus by the time during which the power in question was measured.

In an embodiment of the apparatus according to the invention it is provided with a power supply such as stated in claim 6.

DESCRIPTION OF THE DRAWING The invention will now be described in further detail with reference to the accompany¬ ing drawing, wherein

Fig. 1 is a graphic view of a possible measurement of power both with a known apparatus and with an apparatus according to the invention, Fig. 2 is a schematic view of a first embodiment of an apparatus according to the invention, Fig. 3 is a schematic view of a second embodiment of an apparatus according to the invention, Figs. 4A-4C are graphic views of discrete values of basis-emitter voltage in transis- tors in the first circuit, respectively the second circuit,

Fig. 5 is a graphic view of discrete values of basis-emitter voltage and basis- emitter voltage in the transistors in the first circuit, respectively the second circuit,

Fig. 6 is a graphic view of an average voltage across the transistors in the first circuit, respectively the second circuit, in a situation of full load on the transistors,

Fig. 7 is a graphic view of an addition of voltage measurements across the transistors in the first circuit, respectively the second circuit, in a situa¬ tion without load on the transistors, Fig. 8 is a graphic view of discrete and added current measurements across the transistors in the first circuit, respectively the second circuit, and

Fig. 9 is a schematic view of a second embodiment of a power supply accord¬ ing to the invention,

Fig. 10 is a schematic view of a third and preferred embodiment of an appara- tus according to the invention,

Fig. 1 1 is a graphic view of a first measurement with the third embodiment of an apparatus according to the invention.

Fig. 1 shows a graphic view of a measurement of voltage across a transistor in a first circuit, respectively a transistor in a second circuit, by a method according to the prior art as previously mentioned and a method according to the invention. When measuring the voltage, a contribution u,, u 2 arises that constitutes errors. Measurement U k accord¬ ing to the known method means that only the error contribution u, in the first circuit contributes to the measurement. Measurement U n according to the method of the in- vention means that both the error contribution u, in the first circuit and the error con¬ tribution u 2 in the second circuit contribute to the measurement, but since the error contributions ιi | and u 2 are of equal size, the two contributions outbalance each other. Measurement according to the method of the invention means, therefore, that ΔU, which is the voltage difference to be determined, is equal to one half of U n .

Fig. 2 schematically shows a possible embodiment of an apparatus according to the invention. The apparatus comprises a first circuit 1 and a second circuit 2. The first

circuit 1 consists of a first transistor 3 and a first operation amplifier 4. The first tran¬ sistor 3 has a basis 5 constituting a first terminal, an emitter 6 constituting a second terminal, and a collector 7 constituting a third terminal. The second circuit 2 consists of a second transistor 8 and a second operation amplifier 9. The second transistor 8 also has a basis 10 constituting a first terminal, an emitter 1 1 constituting a second terminal, and a collector 12 constituting a third terminal.

The collector 7 on the first transistor 3 in the first circuit 1 is connected to a first ter¬ minal 13 of a power supply grid. The emitter 6 is connected to a first input terminal 14 on the first operation amplifier 4. Basis 5 is connected to an output terminal 15 on the first operation amplifier 4.

The collector 12 on the second transistor 8 in the second circuit 2 is also connected to the first terminal 13 of the power supply grid. The emitter 1 1 is connected to a first input terminal 17 on the second operation amplifier 9. Basis 10 is connected to an out¬ put terminal 18 on the second operation amplifier 9.

A second input terminal 16, 19 on the operation amplifier 4, 9 in the first circuit 1, respectively the second circuit 2, is connected to the ground 20. Resistors 21 are in- serted in the first circuit 1 , respectively the second circuit 2, between the output termi¬ nal 15, 18 on the operation amplifier 4, 9 in the first circuit 1, respectively the second circuit 2, and ground 20.

The voltage ΔU to be determined is the voltage between the output terminal 15 on the operation amplifier 4 in the first circuit 1 and the output terminal 18 on the operation amplifier 9 in the second circuit 2. This takes place by measurement of the voltages between basis 5, 10 and emitter 6, 11, the basis-emitter voltage U be in the first transis¬ tor 3 and the second transistor 8.

Fig. 3 schematically shows a preferred embodiment of an apparatus according to the invention. A power-consuming or power-producing electric application 22 is con¬ nected to a power supply grid 23. The apparatus in Fig. 3 differs from the apparatus in

Fig. 2 by being supplied with a first unit 24 constituting power supply and regulating the voltage from the power supply grid, and a second unit 25 generating the current signal to the first circuit 1 and the second circuit 2. The power supply 24 may be con¬ nected serially or parallel with the first circuit 1, respectively the second circuit 2. Moreover, the apparatus in Fig. 3 is supplied with a third circuit 26 constituting tem¬ perature regulation, and a fourth circuit 27 converting the voltage ΔU measured be¬ tween the output terminals 15, 18 on the operation amplifiers 4, 9 for display in a dis¬ play unit 28.

Figs. 4A-4C graphically show discrete measurements of the voltage across the transis¬ tor in the first circuit, respectively the second circuit. Fig. 4A shows the voltage across the transistor in the first circuit, Fig. 4B the voltage across the transistor in the second circuit, and Fig. 4C a combination of the graphs in Fig. 4A and Fig. 4B. The meas¬ urements of voltage difference are performed on transistors that are fully loaded.

The voltage difference across the transistor in the first circuit oscillates between ap¬ proximately 650 mV and approximately 720 mV, i.e. an amplitude of 70 mV. The frequency is 50 Hz. The voltage difference across the transistor in the second circuit oscillates between approximately 640 mV and 730 mV, i.e. an amplitude of 90 mV. The frequency is also 50 Hz.

The combination of the voltage difference across the transistor in the first circuit, re¬ spectively the second circuit, as shown in Fig. 4C, shows that the voltage between the third terminal and the second terminal, the basis-emitter voltage, in the transistor in the first circuit has a maximum when the voltage across the transistor in the second circuit has minimum and vice versa. This is the case when the phase shift between voltage and current in the first circuit is in phase and the second circuit is in opposi¬ tion, i.e. has a phase shift of 180°.

Fig. 5 shows an average voltage across the transistors in the first circuit, respectively the second circuit. The measurements of the voltage difference across the transistors

extend over half a second and have been performed across transistors that are fully loaded.

The average voltage difference converges towards a constant value of approximately 695 mV. The convergence is not due to the apparatus according to the invention but is an expression of the calculation technique in the measuring instrument used to deter¬ mine values of the basis-emitter voltage in the transistors.

Fig. 6 also shows the average voltage across the transistors in the first circuit, respec- tively the second circuit. However, the measurements of the voltage difference across the transistors extend over a second so that the convergence towards a constant value is more outspoken. Measurements of the voltage difference have been performed on transistors that are fully loaded.

Fig. 7 also shows the average voltage across transistors in the first circuit, respectively the second circuit. The measurements of the voltage difference across the transistors also extend over a second. However, the measurements have been performed across transistors that are not loaded, i.e. the graph illustrates the resting position of the tran¬ sistors. The voltage across the transistors in the resting position is 705.649 mV.

Fig. 8 shows discrete and added measurements of a difference in amperage across the transistors in the first circuit, respectively the second circuit. The currents in the tran¬ sistors in the first circuit, respectively the second circuit, are numerically of equal size and extend between 1 mA and 21 mA. However, the currents in the transistor in the first circuit, respectively the transistor in the second circuit, are oppositely directed.

Thus, the added amperage across the transistors is constant with an amperage of 1 1 mA.

Fig. 9 shows a possible power supply for use in combination with the first circuit, re- spectively the second circuit. The power supply is arranged in series with the first and second circuits. The power supply performs a voltage load so that the current through the first and the second circuit is constant.

Fig. 10 schematically shows a preferred embodiment of an apparatus according to the invention. The apparatus comprises a first circuit 1 and a second circuit 2. The first circuit 1 consists of a first transistor 3, which is preferably a MOS-FET transistor such as shown. Alternatively, other transistors may be used such as bipolar transistors. The first transistor has a gate 5 constituting a first terminal, an emitter 6, which in a MOS- FET transistor is called a source, constituting a second terminal, and a collector 7, which in a MOS-FET transistor is called a drain and which constitutes a third termi¬ nal. The second circuit 2 consists of a second transistor 8, which is also preferably a MOS-FET transistor such as shown. The second transistor also has a gate 10 constitut¬ ing a first terminal, a source 1 1 constituting a second terminal, and a drain 12 consti¬ tuting a third terminal.

The drain 7 on the first transistor 3 in the first circuit 1 is connected to a first terminal 32 of a power supply unit 13. Alternatively, the gate 5 may be connected to the first terminal 32. The source 6 is connected to a second terminal 33 of the power supply unit 13. The drain 12 on the second transistor 8 in the second circuit 2 is connected to a first terminal 32 of the power supply unit 13. Alternatively, the gate 10 may be con¬ nected to the first terminal 32. The source 1 1 is connected to the second terminal 33 of the power supply unit 13. Resistors 21 are inserted in the first circuit 1 , respectively the second circuit 2, between the source 6, 1 1 and the second terminal 33 on the power supply 13 and, likewise, between the gate 5 on the first transistor 3, respectively the second terminal 33 on the power supply, and between the gate 10 on the second tran¬ sistor 8 and the first terminal 32 on the power supply 13. Condensers 29 are connected between the source 6, 1 1 on the first transistor 3, respectively the second transistor 8, and a measuring unit 30. A generator 31 is connected to the source 6, 1 1 by the tran¬ sistors 3, 8. A current signal from the generator 31 performs a load of the transistors 3, 8 to generate a measurement signal depending on the temperature of the transistors 3, 8.

The power supply 13 here consists of a transformer, which in the embodiment shown is supplied with voltage from a primary coil 34 from the electricity grid 35. Other

types of power supplies may also be used. The voltage at the primary coil 34 generates a voltage from secondary coils 36, 37 between the first terminal 32 and the second terminal 33. The voltage generated in the secondary coil 36, 37 is proportional to the voltage in the electricity grid 35. A load current extending in a cable loop 38 through a power-consuming or power-producing apparatus (not shown) extends through a pri¬ mary coil 39 in the loop 38. A secondary coil 40 is connected to the gate 5, 10 of the transistors 3, 8. The load current in the loop 38 and through the primary coil 39 gen¬ erates a voltage signal in the secondary coil 40, which voltage signal is conducted to the gate 5, 10 by the transistors 3, 8.

As an alternative to connecting the secondary coil 40 to the gate 5, 10, the secondary coil 40 may be connected to the source 6, 1 1 while the generator is connected to the gate 5, 1 1 instead. In that case the secondary coil generates a current signal that is conducted to the source 6, 1 1 while the generator in that case generates a voltage sig- nal that is conducted to the gate 5, 10 and performs a load of the transistors 3, 8.

In the circuit shown in Fig. 10 it is desired to determine a voltage difference. As an alternative to a voltage difference, however, a amperage difference may be deter¬ mined. The voltage difference to be determined is the difference between the voltage at the source 6 on the first transistor 3 relative to the voltage at the source 1 1 on the second transistor 8. Therefore, a measuring unit 30 is connected to the source 6, 1 1. Measurement takes place by measuring a voltage change ΔU in the circuit 1 with the transistor 3 relative to a voltage change ΔU in the circuit 2 with the transistor 8.

Fig. 1 1 is a graph showing the amplitude of the voltage change ΔU, or alternatively an amperage change ΔI, between the transistor 3 and the transistor 8 as a function of a temperature change ΔT of the transistors 3, 8. It appears that there is substantially linearity between a temperature change of the transistors 3, 8 and the voltage change ΔU, or alternatively the amperage change ΔI, between the transistors. This means that the circuit 1 and the circuit 2 may perform precise measurements with different values of the voltage change ΔU, or alternatively the amperage change ΔI, between the tran¬ sistors without any need for a temperature calibration. I.e. there is no need for specific

knowledge of any dependence between a given signal from the transistors and a given temperature of the transistors since both transistors 3, respectively 8, show substan¬ tially linearity towards the voltage change ΔU, or alternatively the amperage change ΔI, between the transistors 3, 8 and the respective temperature change ΔT of the tran- sistors.

The values shown on the ordinate of the graph correspond to voltage changes ΔU of a specific embodiment of an apparatus according to the invention. It will be possible to provide other embodiments of apparatuses according to the invention, in which values of the voltage change ΔU will occur for respective temperature changes ΔT of the transistors 3, 8. Alternative values of the amperage change ΔI between the transistors 3, 8 will show other values, and values of the amperage change ΔI between the transis¬ tors 3, 8 will also vary for respective values of the temperature change ΔT depending on the specific embodiment of the apparatus according to the invention.

The invention has been described above with reference to specific embodiments of apparatuses according to the invention and with reference to determination of voltage across and current in the transistors in the first circuit, respectively the second circuit. However, other phase shifts may also occur. In such cases the method and the appara- tus according to the invention may still be used by applying the formulas P = U • I cosφ for the power yielded in the transistor in the first circuit and P = U I • (cosφ + φ) for the power yielded in the transistor in the second circuit, φ is the phase shift be¬ tween voltage (U) and current (I) applied to the application to be measured on, and φ is the phase shift between the current signal applied to the transistor in the first circuit and the current signal applied to the transistor in the second circuit.

In principle, it is indirectly a temperature increase of the transistor in the first circuit that is measured by measuring the voltage or amperage difference since the voltage or amperage difference across the transistor is changed exactly as a function of the tem- perature of the transistor, and the temperature of the transistor depends on the power yielded in the transistor. In a further alternative embodiment it will, therefore, be pos¬ sible to measure a surface temperature on a capsule on a transistor.