Leakage current noise reduction for ionization chamber based alarms

1 . An apparatus comprising:

an ionization chamber;

a voltage source to drive the ionization chamber;

a voltage sensor to measure an ionization chamber output voltage;

a control sense driver to drive a floating control sense line electrically isolated from the ionization chamber;

a current sensor to measure output current through the control sense driver;

a calibration circuit to compensate the ionization chamber output voltage based on a correction factor; and

a monitoring circuit to trigger an alarm if the compensated output voltage meets a predetermined condition;

wherein the calibration circuit is operable to determine the correction factor to compensate for any leakage current affecting the ionization chamber output voltage, wherein the correction factor is based at least in part on the measured output current through the control sense driver.

2 . An apparatus according to claim 1 , comprising a memory storing a factory baseline signal corresponding to a factory original condition of the ionization chamber prior to deployment.

3 . An apparatus according to claim 1 , wherein the calibration circuit is operable to:

drive a guard lead to a second voltage different from the ionization chamber output voltage;

measure a second output voltage from the ionization chamber in response; and

determine the correction factor based at least in part on a difference between the ionization chamber output voltage and the second output voltage.

4 . An apparatus according to claim 1 , wherein the calibration circuit is operable to:

drive a guard lead to a second voltage different from the ionization chamber output voltage;

measure an output current from the guard driver; and

determine the correction factor based at least in part on the output current.

5 . An apparatus according to claim 1 , wherein the calibration circuit is operable to:

drive a guard lead to a second voltage different from the ionization chamber output voltage, then return the guard lead to the ionization chamber output voltage;

measure a duration of time required for the ionization chamber output voltage to return to an unbiased guard level; and

determine the correction factor based at least in part on a change in the duration of time compared to a factory baseline.

6 . An apparatus according to claim 1 , wherein the calibration circuit is operable to:

drive a guard lead to a second voltage different from the ionization chamber output voltage, then returns the guard lead to the ionization chamber output voltage;

measure a duration of time required for a current from the voltage driver to return to an unbiased guard level; and

determine the correction factor based at least in part on a change in the duration of time compared to a factory baseline.

7 . An apparatus according to claim 1 , wherein the calibration circuit is operable to:

determine a current leakage; and

activate an alert if the current leakage meets a predetermined threshold.

8 . A smoke detector comprising:

an ionization chamber;

an inlet to provide a sample to the ionization chamber;

a voltage source to drive the ionization chamber;

a voltage sensor to measure an ionization chamber output voltage;

a control sense driver to drive a floating control sense line electrically insulated from the ionization chamber;

a current sensor to measure output current through the control sense driver;

a calibration circuit to compensate the ionization chamber output voltage based on a correction factor; and

a monitoring circuit to trigger an alarm if the compensated output voltage meets a predetermined condition;

wherein the calibration circuit is operable to determine the correction factor to compensate for any leakage current affecting the ionization chamber output voltage, wherein the correction factor is based at least in part on the measured output current through the control sense driver.

9 . A smoke detector according to claim 8 , comprising a memory storing a factory baseline signal corresponding to a factory original condition of the ionization chamber prior to deployment.

10 . A smoke detector according to claim 8 , wherein the calibration circuit is operable to:

drive a guard lead to a second voltage different from the ionization chamber output voltage;

measure a second output voltage from the ionization chamber; and

determine the correction factor based at least in part on a difference between the ionization chamber output voltage and the second output voltage.

11 . A smoke detector according to claim 8 , wherein the calibration circuit is operable to:

drive a guard lead to a second voltage different from the ionization chamber output voltage;

measure an output current from the guard driver; and

determine the correction factor based at least in part on the output current.

12 . A smoke detector according to claim 8 , wherein the calibration circuit is operable to:

drive a guard lead to a second voltage different from the ionization chamber output voltage, then return the guard lead to the ionization chamber output voltage;

measure a duration of time required for the ionization chamber output voltage to return to an unbiased guard level; and

determine the correction factor based at least in part on a change in the duration of time compared to a factory baseline.

13 . A smoke detector according to claim 8 , wherein the calibration circuit is operable to:

drive a guard lead to a second voltage different from the ionization chamber output voltage, then return the guard lead to the ionization chamber output voltage;

measure a duration of time required for a current from the voltage driver to return to an unbiased guard level; and

determine the correction factor based at least in part on a change in the duration of time compared to a factory baseline.

14 . A smoke detector according to claim 8 , wherein the calibration circuit is operable to:

determine a current leakage; and

activate an alert if the current leakage meets a predetermined threshold.

15 . A smoke detector according to claim 8 , comprising a memory storing a factory baseline signal corresponding to a factory original condition of the ionization chamber prior to deployment;

wherein the predetermined criterion is based at least in part on the factory baseline signal.

16 . A method comprising:

driving an ionization chamber with a voltage source;

measuring an ionization chamber output voltage with a voltage sensor;

driving a lead to a second voltage different from the ionization chamber output voltage, the lead electrically insulated from the ionization chamber;

measuring a reaction to the second voltage;

analyzing the reaction and based on the reaction, determining a correction factor to compensate for any leakage current affecting the ionization chamber output voltage, wherein the leakage current includes current across a printed circuit board (PCB) from the voltage source to the voltage sensor;

applying the correction factor to the signal with a calibration circuit;

comparing the compensated signal to a predetermined criterion, wherein the predetermined criterion includes an alarm condition; and

responding to the alarm condition if the compensated signal meets the predetermined criterion.

17 . A method according to claim 16 , wherein:

the lead comprises a guard lead;

the reaction includes a second output voltage from the ionization chamber; and

analyzing the reaction includes determining a difference between the output voltage and the second output voltage; and

the compensation factor depends at least in part on the determined difference.

18 . A method according to claim 16 , wherein:

the lead comprises a guard lead;

the reaction includes an output current from the guard sensor;

analyzing the reaction includes determining a value for the output current; and

the compensation factor depends at least in part on the value of the output current.

19 . A method according to claim 16 , wherein:

the lead comprises a guard lead;

the method further comprises, after driving the guard lead to the drive voltage, driving the guard lead back to the ionization chamber output voltage;

analyzing the reaction includes determining a duration of time required for the ionization chamber output voltage to return to an unbiased guard level; and

the compensation factor depends at least in part on a change in the determined duration against a baseline duration.

20 . A method according to claim 16 , wherein:

the lead comprises a guard lead;

the method further comprises, after driving the guard lead to the drive voltage, driving the guard lead back to the ionization chamber output voltage;

analyzing the reaction includes determining a duration of time required for an output current from the voltage source to return to an unbiased guard level; and

the compensation factor depends at least in part on a change in the determined duration against a baseline duration.

21 . A method according to claim 16 , wherein:

the lead comprises current sense leads;

analyzing the reaction includes determining an output current from a control sense driver; and

the compensation factor depends at least in part on the output current.

22 . A method according to claim 16 , wherein determining the compensation factor includes measuring a new baseline signal corresponding to an active leakage current.

23 . A method according to claim 16 , comprising:

measuring a signal corresponding to an active leakage current; and

activating an alert if the signal meets a predetermined threshold.

24 . An apparatus according to claim 1 , wherein the control sense driver alternately drives the control sense line at a normal voltage and at a measurement voltage different from the normal voltage.

25 . An apparatus according to claim 8 , wherein the control sense driver alternately drives the control sense line at a normal voltage and at a measurement voltage different from the normal voltage.