Method and system for sensing a liquid level

1. A system for sensing liquid level in a receptacle, said system comprising:

a capacitive sensing probe for sensing a capacitance between the capacitive sensing probe and a grounded electrically conductive portion of the receptacle;

a control circuit connected to the capacitive sensing probe for detecting a level of the liquid in the receptacle based on said capacitance;

a non-isolated AC power supply arranged to power the control circuit, the non-isolated AC power supply having an electrical connection between its input and its output; and

at least one filter capacitor (C 1 , C 2 ) connected in series between the capacitive sensing probe and the control circuit, wherein the at least one filter capacitor (i) isolates the capacitive sensing probe from an electrically live part of the non-isolated AC power supply and the control circuit and (ii) prevents earth leakage current from the non-isolated AC power supply with respect to the capacitive sensing probe.

2. The system as claimed in claim 1 , wherein the control circuit is arranged to receive an AC voltage from a mains supply and convert the AC voltage into a zero-crossing signal used to trigger the capacitive sensing.

3. The system as claimed in claim 1 , wherein the control circuit is arranged to perform a capacitive sensing measurement at a rising edge and/or a falling edge of the zero-crossing signal.

4. The system as claimed in claim 1 , wherein the at least one filter capacitor (C 1 , C 2 ) is a Y-capacitor.

5. The system as claimed in claim 1 , wherein the non-isolated AC power supply is at least one of a switched mode power supply, a capacitive power supply, and a resistive power supply.

6. The system as claimed in claim 1 , wherein the control circuit and the non-isolated AC power supply are located on a printed circuit board assembly.

7. The system as claimed in claim 1 , wherein the capacitive sensing probe acts as a first electrode (E 1 ) of a sensing capacitor, and a grounded electrically conductive portion of the receptacle acts a second electrode (E 2 ) of the sensing capacitor.

8. A boiler system comprising a receptacle and a system as claimed in claim 1 , wherein the capacitive sensing probe is located within the receptacle and is electrically insulated from the receptacle.

9. A system for sensing liquid level in a receptacle, said system comprising:

a capacitive sensing probe for sensing a capacitance between the capacitive sensing probe and an electrically conductive portion of the receptacle;

a control circuit connected to the capacitive sensing probe for detecting a level of the liquid in the receptacle based on said capacitance;

a non-isolated AC power supply arranged to power the control circuit, the non-isolated AC power supply having an electrical connection between its input and its output; and

at least one filter capacitor (C 1 , C 2 ) connected in series between the capacitive sensing probe and the control circuit, wherein the control circuit is arranged to receive an AC voltage from a mains supply and convert the AC voltage into a zero-crossing signal used to trigger the capacitive sensing, and wherein the capacitive sensing is triggered with a time delay after a zero-crossing point of zero-crossing signal is detected, the time delay is a pre-determined value or range.

10. A system for sensing liquid level in a receptacle, said system comprising:

a capacitive sensing probe for sensing a capacitance between the capacitive sensing probe and an electrically conductive portion of the receptacle;

a control circuit connected to the capacitive sensing probe for detecting a level of the liquid in the receptacle based on said capacitance;

a non-isolated AC power supply arranged to power the control circuit, the non-isolated AC power supply having an electrical connection between its input and its output; and

at least one filter capacitor (C 1 , C 2 ) connected in series between the capacitive sensing probe and the control circuit, wherein the control circuit is arranged to detect whether an edge of the zero-crossing signal is a rising edge or a falling edge, and the control circuit is arranged to detect the level of the liquid in the receptacle based on the capacitance of the capacitive sensing probe based on results of the detection.

11. A method of detecting a level of liquid with a system comprising a capacitive sensing probe for sensing a capacitance between the capacitive sensing probe and a grounded electrically conductive portion of a receptacle, a control circuit connected to the capacitive sensing probe for detecting a level of the liquid in the receptacle based on said capacitance and arranged to receive an AC voltage from a mains supply, and a non-isolated AC power supply arranged to power the control circuit, wherein the non-isolated AC power supply has an electrical connection between its input and its output, and at least one filter capacitor (C 1 , C 2 ) connected in series between the capacitive sensing probe and the control circuit, wherein the at least one filter capacitor (i) isolates the capacitive sensing probe from an electrically live part of the non-isolated AC power supply and the control circuit and (ii) prevents earth leakage current from the non-isolated AC power supply with respect to the capacitive sensing probe, the method comprising:

using the control circuit to convert AC voltage from the mains supply into a zero-crossing signal, and

performing capacitive sensing using the capacitive sensing probe at a time determined by the zero-crossing signal.