Capacitor measurement

A circuit for measuring an equivalent series resistor value of a capacitor, the circuit comprising:

• a discharge circuit couplable to a first terminal of a resistor, the discharge circuit configured to supply a discharge path for the capacitor during a discharge routine from a first time to a second time, a second terminal of the resistor couplable to a terminal of the capacitor;
• and
• circuitry configured to:
• measure a first change in voltage at the terminal of the capacitor at the first time during the discharge routine,
• measure a first change in voltage across terminals of the resistor at the first time,
• measure a second change in voltage across the terminals of the resistor at the second time,
• measure a second change in voltage at the terminal of the capacitor at the second time,
• compute the equivalent series resistor value as a function of a sum of the first change in voltage and the second change in voltage at the terminal of the capacitor and inversely related to a sum of voltage differences between the voltage at the terminal of the capacitor and the voltage at a second terminal of the resistor coupled to the discharge circuit at a first time and the voltage at the terminal of the capacitor and the voltage at the second terminal of the resistor at the second time, and
• determine a health of the capacitor based on a difference between the computed equivalent series resistor value and a threshold value.

A system for measuring an equivalent series resistor value of a reserve capacitor in a supplemental restraint system (SRS), the system comprising:

• a resistor having a first terminal and a second terminal, the first terminal of the resistor coupled to a terminal of the reserve capacitor;
• a discharge circuit coupled to the second terminal of the resistor, the discharge circuit configured to supply a discharge path for the reserve capacitor during a discharge routine from a first time to a second time;
• and
• circuitry configured to:
• measure a first change in voltage at the first terminal at the first time during the discharge routine,
• measure a first change in voltage across terminals of the resistor at the first time,
• measure a second change in voltage across the terminals of the resistor at the second time,
• measure a second change in voltage at the first terminal at the second time,
• compute the equivalent series resistor value as a function of a sum of the first change in voltage and the second change in voltage at the terminal of the capacitor and inversely related to a sum of voltage differences between the voltage at the terminal of the capacitor and the voltage at a second terminal of the resistor coupled to the discharge circuit at a first time and the voltage at the terminal of the capacitor and the voltage at the second terminal of the resistor at the second time, and
• determine a health of the reserve capacitor based on a difference between the computed equivalent series resistor value and a threshold value.