System and method for contact measurement circuit

1. A contact measurement circuit configured to be coupled between a first contact and a second contact, the contact measurement circuit comprising:

a first transistor comprising a first conduction terminal configured to be coupled to the first contact, a second conduction terminal, and a first control terminal;

a control capacitor disposed in series with a conduction path of the first transistor and between the second conduction terminal and the second contact, the control capacitor comprising a first capacitor terminal coupled to the second conduction terminal and a second capacitor terminal coupled directly to the first control terminal to form a circuit path directly from the second capacitor terminal to the first control terminal such that a voltage at the second capacitor terminal is provided directly to the first control terminal, wherein the second capacitor terminal is configured to be coupled to the second contact; and

a voltage measurement unit coupled to the first capacitor terminal and the second capacitor terminal.

2. The contact measurement circuit of claim 1 , further comprising a first impedance device coupled to the first conduction terminal and configured to be coupled between the first conduction terminal and the first contact.

3. The contact measurement circuit of claim 1 , further comprising a voltage limiting device coupled between the first capacitor terminal and the second capacitor terminal, the voltage limiting device configured to limit the voltage between the first capacitor terminal and the second capacitor terminal to a first threshold voltage.

4. The contact measurement circuit of claim 1 , wherein the voltage measurement unit comprises a sigma-delta analog to digital converter (ADC).

5. The contact measurement circuit of claim 1 , wherein the first transistor comprises a normally-on transistor.

6. The contact measurement circuit of claim 5 , wherein the first transistor comprises a high voltage depletion-mode transistor.

7. The contact measurement circuit of claim 1 , further comprising a second transistor comprising a third conduction terminal coupled to the second capacitor terminal, a fourth conduction terminal, and a second control terminal coupled to the first capacitor terminal, wherein the fourth conduction terminal is configured to be coupled to the second contact.

8. The contact measurement circuit of claim 7 , further comprising:

a first impedance device coupled to the first conduction terminal and configured to be coupled between the first conduction terminal and the first contact, and

a second impedance device coupled to the fourth conduction terminal and configured to be coupled between the fourth conduction terminal and the second contact.

9. The contact measurement circuit of claim 7 , further comprising a voltage limiting device coupled between the first capacitor terminal and the second capacitor terminal, the voltage limiting device configured to limit the voltage between the first capacitor terminal and the second capacitor terminal to a first threshold voltage.

10. The contact measurement circuit of claim 9 , wherein the voltage limiting device comprises:

a first plurality of series connected diodes configured to conduct a current from the first capacitor terminal to the second capacitor terminal when the voltage from the first capacitor terminal to the second capacitor terminal is above the first threshold voltage, and

a second plurality of series connected diodes configured to conduct a current from the second capacitor terminal to the first capacitor terminal when the voltage from the second capacitor terminal to the first capacitor terminal is above the first threshold voltage.

11. A method of measuring contact resistance of contacts using series connected elements between the contacts, the series connected elements comprising a transistor having a conduction path and a capacitor coupled in series with the conduction path of the transistor, the method comprising:

automatically biasing the transistor into an on-state using a control voltage provided directly to the transistor by the capacitor when a voltage across the contacts is below a first threshold;

automatically biasing the transistor into an off-state using the control voltage provided directly to the transistor by the capacitor when a voltage across the contacts is above the first threshold;

measuring a voltage across the capacitor when the transistor is biased in the on-stat, wherein the voltage is created by current flow through the conduction path to the capacitor; and

determining a contact resistance of the contacts based on measuring the voltage across the capacitor.

12. The method of claim 11 , wherein automatically biasing the transistor comprises biasing a control terminal of the transistor using the capacitor without receiving any control signals from an additional control circuit.

13. The method of claim 11 , wherein the transistor comprises a high voltage depletion device.

14. The method of claim 13 , wherein the first threshold is equal to a gate-source threshold voltage of the high voltage depletion device.

15. The method of claim 11 , further comprising limiting a voltage across the capacitor to a second threshold using a voltage limiting element.

16. A contact measurement circuit comprising:

a first terminal;

a second terminal, wherein the first terminal and the second terminal are configured to be coupled across contacts of a conduction device;

a isolation circuit coupled to the first terminal and the second terminal, the isolation circuit comprising a first measurement terminal and a second measurement terminal, wherein the isolation circuit is configured to:

monitor a terminal voltage between the first terminal and the second terminal,

block a signal path between the first terminal and the second terminal based on monitoring the terminal voltage, wherein the signal path is blocked when the terminal voltage is above a first threshold, and

limit a measurement voltage between the first measurement terminal and the second measurement terminal to the first threshold when the signal path is blocked; and

a measurement unit coupled to the first measurement terminal and the second measurement terminal.

17. The contact measurement circuit of claim 16 , wherein the isolation circuit comprises:

a first transistor comprising a first control terminal and having a first conduction path between the first terminal and the second terminal; and

an automatic control circuit coupled to the second terminal and the first control terminal.

18. The contact measurement circuit of claim 17 , wherein the automatic control circuit consists of non-switching impedance elements.

19. The contact measurement circuit of claim 17 , wherein the automatic control circuit is configured to automatically control the first transistor without receiving control signals.

20. The contact measurement circuit of claim 17 , wherein the automatic control circuit comprises a capacitor coupled in series with the first conduction path,

wherein the capacitor comprises a first capacitor terminal and a second capacitor terminal,

wherein the first capacitor terminal is coupled to the first measurement terminal, and

wherein the second capacitor terminal is coupled to the first control terminal and the second measurement terminal.

21. The contact measurement circuit of claim 20 , wherein the isolation circuit further comprises a second transistor comprising a second control terminal coupled to the first capacitor terminal and having a second conduction path coupled between the second capacitor terminal and the second terminal.

22. The contact measurement circuit of claim 21 , wherein the automatic control circuit further comprises:

a first diode coupled between the first capacitor terminal and the second capacitor terminal, the first diode having a first conduction direction; and

a second diode coupled between the first capacitor terminal and the second capacitor terminal, the second diode having a second conduction direction, opposite the first conduction direction.

23. The contact measurement circuit of claim 22 , wherein the first diode comprises a plurality of series connected diodes and the second diode comprises a plurality of series connected diodes.

24. The contact measurement circuit of claim 20 , wherein the automatic control circuit further comprises a voltage limiting element coupled between the first capacitor terminal and the second capacitor terminal, the voltage limiting element configured to conduct current in a first conduction direction when a voltage applied to the voltage limiting element goes above a conduction threshold.

25. The contact measurement circuit of claim 17 , wherein the first transistor comprises a high voltage depletion transistor.

26. The contact measurement circuit of claim 17 , wherein the first threshold is equal to a threshold voltage of the first transistor.

27. The contact measurement circuit of claim 16 , further comprising the conduction device and wherein the conduction device comprises a relay.

28. The contact measurement circuit of claim 16 , further comprising the conduction device and wherein the conduction device comprises a fuse, an electric switch, or a plug contact.

29. The contact measurement circuit of claim 1 , wherein the first capacitor terminal is coupled directly to the second conduction terminal.

30. The contact measurement circuit of claim 1 , wherein the contact measurement circuit is free of voltage limiting elements in parallel with the control capacitor.