Capacitor self diagnosis

Electrolytic capacitors are far from the most reliable electronic components. One of the failures is the gradual loss of capacity, which is barely noticeable before a power failure occurs. Therefore, it would be valuable to monitor the condition of the electronics filter capacitors in real time.

The following simple design can be used to monitor the capacitor voltage during power-down and record the out-of-spec state to the NVRAM/EEPROM memory.

This design requires only a microcontroller, ADC, and some non-volatile memory, which means that for most systems, there is basically no additional cost or component.

When the off (P_OFF) signal is sent to the system, μC disables all interrupts and important (and unpredictable) power consumption in the circuit.

Then, μC estimates the filter capacitance by calculating the time required for the voltage to drop by a certain amount. Alternatively, the program can measure the voltage difference before and after a certain time.

In either case, there must be sufficient margin between the minimum output voltage VOUT of all power supplies and the minimum operating voltage required for μC to allow sufficient time for measurements.

If the measured value is not within the specified range, μC will write the diagnostic data to NVRAM, which can be operated on the next power-up.

The functions of R1 and R2 match the ADC input range and serve as the primary "test load" for capacitor discharge. The power supply current drawn by the active circuit changes too much and cannot be relied upon.