Capacitors play a critical role in power supplies, primarily used to smooth out the output voltage and filter out electrical noise. By storing electrical energy temporarily and releasing it during demand spikes, capacitors help maintain a stable and clean power output. This function is essential in reducing the impact of voltage fluctuations and noise, which can interfere with the performance and longevity of electronic devices.
Additionally, capacitors in power supplies help to manage sudden changes in load current. When a device draws more power, the capacitor provides the necessary current without a significant drop in voltage, ensuring the power supply remains consistent. This capability is particularly important in applications where a steady voltage is crucial, such as in sensitive audio equipment or precise digital circuits, protecting them from potential damage due to power irregularities.
Moreover, in switching power supplies, capacitors contribute significantly to the management of switching frequencies and assist in the energy conversion process. Their role here is twofold: first, they minimize the energy lost during the switch transitions by temporarily storing charge, and second, they smooth the output of the power supply to prevent disruptive interference in the circuit. This dual functionality not only improves the operational efficiency of the power supply but also enhances the overall performance of the device it powers, ensuring that energy is used effectively and efficiently.
Failing aluminum electrolytic capacitors can have significantly adverse effects on electronic circuits. Most technicians have seen the tale-tell signs – bulging, chemical leaks, and even tops that have blown off. When they fail, the circuits that contain them no longer perform as designed – most often affecting power supplies. For example, a failing capacitor can affect the DC output level of a DC power supply because it can’t effectively filter the pulsating rectified voltage as intended. This results in a lower average DC voltage and causes a corresponding erratic behavior due to unwanted ripple – as opposed to the expected clean DC voltage at the load. For example, below shows a healthy linear power supply. As you can see, the output (Green Line) is a relatively clean DC voltage with very low ripple. Ripple is the unwanted AC component that the capacitor is intended to filter or (smooth) out. On the rising edge of the rectified waveform (in purple), the capacitor charges. On the falling edge, the energy stored in the capacitor supplies enough voltage to the load to tie it over until the next rising edge.
The next example shows the same power supply with a failing output filter capacitor. Because the ESR (Equivalent Series Resistance) of the capacitor has increased, the circuit no longer performs as designed. This causes two things to happen. It is as if an extra resistor was placed in series with the capacitor. Also, the surface area of the capacitor plates has effectively decreased – reducing capacitance. So instead of filtering out the unwanted AC ripple, that ripple appears across both the newly introduced resistive component within the physical capacitor as well as the effectively reduced capacitance. This results in an unclean output voltage (Green Line) with a lower than required average DC level to the load. So when the rectified voltage (in purple) rises, the capacitor is unable to store enough of that energy – so that on the falling edge, the output voltage (in green) just drops off to a reduced level.
Replacing the capacitor usually resolves this issue. The circuit can once again function as designed – filtering out the unwanted ripple voltage and delivering a clean DC voltage to the load. But why do these caps fail? What can be done to prevent this? How do you prevent this from recurring? For one, electrolytic capacitors have a limited life. Most aluminum electrolytic capacitors are guaranteed to last 1000 – 10,000 hours at their rated temperature, depending on the capacitance and voltage. For power supplies that run 24/7 (such as those in appliances that supply power to the “on” button), this translates to 42 days to 1 1/2 years. The overall life also depends on the load the power supply is under, the ambient temperature around the capacitor (they can last exponentially longer hours as operating temperature decreases), and the duty cycle of use (how any hours/day the supply is energized). High operating temperature is one reason that electrolytic capacitors are one of the most commonly failing components in electronics.
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Post time: Dec-26-2025