Circuitry and shunt arrangements for temperature-compensated measurements of current
View Patent ↗Shunt arrangements and current measuring circuits are provided for temperature compensated current measurements as well as current measurements that are compensated for changes in the characteristics of the shunt material as a result of manufacturing tolerances. The current measuring circuits may include a temperature sensing element in a negative feedback path of an operational amplifier for providing temperature compensation. The shunt arrangements may include a calibration shunt formed in the same material as a main shunt and circuitry for measuring the temperature drift error of the calibration shunt and applying that error to compensate for temperature drift of the main shunt.
1. A circuit for providing a temperature-compensated measure of a current flowing through a shunt, comprising:
a temperature sensing element mounted on said shunt that measures the temperature of said shunt,
a first amplifier connected across said shunt for measuring a voltage caused by said current, said voltage including a temperature drift error due to changes in temperature of said shunt,
and
a second amplifier connected to the output of the first amplifier and including said temperature sensing element in a negative feedback path, wherein the output of the second amplifier is compensated for said temperature drift error.
2. The circuit of claim 1 , wherein the temperature sensing element is a resistive temperature device.
3. The circuit of claim 1 , wherein the temperature sensing element is mounted in thermal contact to said shunt so that its temperature tracks the temperature of said shunt.
4. The circuit of claim 1 wherein the first amplifier is an instrumentation amplifier.
5. The circuit of claim 1 wherein the second amplifier is an operational amplifier.
6. The circuit of claim 5 wherein the second amplifier is a non-inverting amplifier.
7. A shunt arrangement comprising,
a main shunt formed in a first conductive volume of a material through which a current to be measured flows;
a calibration shunt having higher resistance than said main shunt formed in a second conductive volume of said material through which a calibration current flows, and
a stable shunt having higher resistance than said main shunt connected in series with said calibration shunt.
8. The shunt arrangement of claim 7 , wherein the first conductive volume is at the same temperature as the second conductive volume.
9. The shunt arrangement of claim 7 , wherein the first conductive volume has the same manufacturing tolerances as the second conductive volume.
10. The shunt arrangement of claim 7 wherein the resistance of the stable shunt and calibration shunt are each substantially the same.
11. The shunt arrangement of claim 7 wherein the stable shunt is formed from manganin or similar temperature stable alloy.
12. A method of using the shunt arrangement of claim 7 , comprising the steps of:
measuring the difference between a voltage across said calibration shunt and a voltage across said stable shunt when a current is passed therethrough, said difference corresponding to a temperature drift error in the calibration shunt, and
using said difference to compensate for the temperature drift error of said main shunt.
13. A circuit for providing a temperature-compensated measure of the flowing through the main shunt of the shunt arrangement of claim 7 , comprising:
a first amplifier connected across said main shunt for measuring a voltage generated by current flow through said main shunt,
a second amplifier connected across said calibration shunt for measuring a voltage generated by current flow through said calibration shunt,
a third amplifier connected across said stable shunt for measuring a voltage generated by current flow through said stable shunt, and
a differential amplifier having inputs connected to the outputs of the second and third amplifiers, and an output connected to respective active voltage dividers at the outputs of the first and second amplifiers, such that a temperature drift error is determined by the differential amplifier and compensates the output of the first amplifier for temperature drift of the main shunt.
14. A measuring system for providing a temperature-compensated measure of current flowing through a main shunt, comprising:
a main shunt formed in a first conductive volume of a material;
a first amplifier connected across said main shunt for measuring a voltage due to current flow through said main shunt,
a calibration shunt formed in a second conductive volume of said material;
a second amplifier connected across said calibration shunt for measuring a voltage due to current flow through said calibration shunt,
a stable shunt connected in series with said calibration shunt, said stable shunt having substantially no temperature drift error,
a third amplifier connected across said stable shunt for measuring a voltage due to current flow through said stable shunt, and
a fourth amplifier for receiving the outputs of the second and third amplifiers, the output of said fourth amplifier being connected to respective active voltage dividers at the outputs of the first and second amplifiers, such that a temperature drift error is determined by the fourth amplifier and is applied to compensate the output of the first amplifier for temperature drift of said main shunt.
15. The measuring system of claim 14 , wherein said stable shunt and calibration shunt have resistances substantially greater than the main shunt.
16. The measuring system of claim 15 , wherein said stable shunt and calibration shunt each have resistances that are n times greater than the resistance of the main shunt, where n is in the range of about 50 to 100.
17. The measuring system of claim 14 , wherein the active voltage dividers are implemented by MOSFETs.
18. The measuring system of claim 14 , wherein the stable shunt is formed from manganin, or similar temperature stable alloy.
19. The measuring system of claim 14 , wherein the fourth amplifier is a differential operational amplifier.