The Simpson Gtpacohmeter Model 383A is a combination of three basic instruments into a single cabinet. These instruments each perform a different type of test on capacitors. All three work together to provide the most conclusive series of capacitor evaluations available. TYPES OF TESTS MEASURING CAPACITANCE One of the measurements which is essential for determining the quality of a capacitor is its capacitance. The Simpson Capacohmetcr Model 383A indicates capacitance on a direct-reading meter scale· There are five ranges for capacitance measurements from 10 micro-microfarads (MMFD) to 10 microfarad« (MFD). This wide range includes most ceramic, mica, paper, oil-filled, and air dielectric types of capacitors. measuring leakage resistance A second* measurement which is of importance for evaluating capacitors is the amount of leakage resistance. Ordinarily, a Capacitor is assumed to have almost infinite resistance between its two plates, or electrodes, Actually, there i&;always some high resistance DC path between the plates .because there is no such thing as a "perfect” insulate The resistance of the DC path in a good capacitor is measurable in megohms, so the Simpson Capacohmerer has a megohms measuring circuit. Deterioration and damages, both physical and electrical, will reduce the original high leakage resistance to a lower value during the life of the capacitor. The lower limit of leakage resistance value at which the capacitor is still usable depends on a number of factors. There is no definite numerical value of leakage resistance that can be assigned, wherein all capacitors having a higher value of leakage resistance are GOOD, and those having lower leakage resistance are BAD. Rather we must consider the leakage resistance in relationship to the circuit containing the capacitor in question. Nevertheless, as a relative measurement, the megohms indications are a wonderful aid in determining faulty components. The megohmmeter portion of the Simpson Capacohmeter may be used separately to measure high resistance for many applications other than capacitors. See Section IV for some suggested applications. PULSETESTING Many capacitors will respond normally to the first two tests which can be made with the Simpson Capacohmeter, and still will not perform satisfactorily when they are placed in service. This is true because the nature of the circuit into which they are connected is very different from the testing circuit. For the capacity measurement circuit, there is a sine wave AC voltage which is applied to the capacitor. For the megohms measurement, a DC voltage is applied to the capacitor. To complete the types of voltages which are applied for testing, a narrow pulse voltage is applied from a relaxation oscillator in the Simpson Capacohmeter, The amount of peak voltage in the pulse is variable with a front panel control, and the amount applied for each test is related to the amount of voltage stress which the capacitor is rated to withstand. If the capacitor is in good condition, the application of a safe value of pulse voltage will not affect it in any way. But if the capacitor is weak, or is about to become defective, the pulse voltage will cause an internal change in the capacitor which will change the meter indication, IN-CIRCUIT CHECKS Most installed capacitors can be checked without removal from their circuity. This is a distinct advantage over previous testing procedures which require that the capacitor be isolated from i lie circuit by unsoldering a lead. The limitation on this use is the case where a capacitor is in parallel with a low resistance or low reactance component. However, with the Model 383A, most capacitors can be tested without any unsoldering or clipping of leads. This facility is of particular value in checking printed circuits and plated circuits. For convenience, there is a special circuit, requiring the third lead on the Capacohmeter, for testing coupling capacitors without removal from the circuit. The controlled voltage is injected on the input side of the coupling capacitor circuit, and the results are measured on the output side of the capacitor.