Abstract: An Equivalent Series Resistance (ESR) measurement device for testing electrolytic capacitors without removing the test capacitor from the circuit in which it is being used and without first discharging the test capacitor prior to testing. A low voltage AC ohmmeter, which operates at a frequency of approximately 100 KHz, is used as the measurement device for the in-circuit testing of the ESR and thus as an indication of the status or usability of the electrolytic capacitor being tested. A current-blocking capacitor is placed in series with the electrolytic capacitor being tested to allow in-circuit testing without damaging the meter since it blocks any DC charge remaining on the test capacitor.