A smart electrolytic capacitor circuit that is able to signal its worn-out state online has been studied and realized. As the impedance is almost equal to the equivalent series resistance above 10 kHz, this smart capacitor circuit can be used in most power supplies. For a static converter operating below 10 kHz, active diagnostics consisting of a voltage injection above 10 kHz could be used. This low-price system may be extended to large capacitors and may be subject to circuit integration.