BASIC AND ADVANCED TEST EQUIPMENT

(BASIC) Digital Meter Capacitance / OHM Meter

(ADVANCED) LCR Meter/AC Source

TEST PROCEDURE – TAKEN OUT OF THE CIRCUIT OR QUALIFICATION TESTING

Disconnect the capacitor from the circuit if required and discharge it by shorting the terminals or wires together. Check the capacitor with an ohmmeter set to the highest resistance scale If the meter indicates a very low resistance then gradually increases, the capacitor may be good. If the meter indicates a very high initial resistance that does not change, it is open and should be replaced. If the meter indicates a very low resistance that does not increase, the capacitor is shorted and should be replaced. The ohmmeter method of testing capacitors will only determine open or shorted capacitors. The capacitance value can be tested by many available portable TRUE RMS meters having that capability, though a test using a dedicated capacitance meter is more conclusive. A capacitor may look good visually, but should be tested for capacitance value or replaced. This is where the LCR meter would be employed. An LCR meter (Inductance (L), Capacitance (C), and Resistance (R)) is a piece of electronic test equipment used to measure the inductance, capacitance and, resistance of a component. In the usual versions of this instrument these quantities are not measured directly, but determined from a measurement of impedance. The necessary calculations are, however, incorporated in the instrument's circuitry; the meter reads L, C and R directly with no human calculation required. Usually the device under test (DUT) is subjected to an AC voltage source. The meter detects the voltage over, and the current through the DUT. From the ratio of these the meter can determine the magnitude of the impedance.

FAILURE MODES AND MECHANISMS

Normal capacitor end-of-life failure. Overheating due to conducted or ambient thermal exposure. Capacitor mounted too close to hot components. Incorrect voltage or capacitor value for application. Mechanical damage due to storage or assembly mishandling, shock, or vibration stresses Design related failure where no de-rating margins were considered at worst case operating conditions Polarity reversed during install or placement Factory process error resulting in faulty product – delaminating or micro cracks in ceramic chips Incorrect material or capacitor type selected for the application

DIELECTRIC TABLES FOR VARIOUS MATERIALS:

Various dielectrics are chosen to manufacture different types of capacitors. Each dielectric has a characteristic behavior that determines how and where it will be used in electrical circuits. Shown below are the various materials and their respective capacitance value ranges.