This article explains some basic parameters of capacitors – insulation resistance, DCL leakage current and breakdown voltage / withstanding voltage.
Important feature of capacitor apart its capacitance is:
- its ability to keep the charge for some time without self-discharging due to its internal leakage (conductivity) mechanisms. This is characterized by either IR Insulation Resistance or DCL leakage current electrical parameters. These are reciprocal parameters describing the same capacitor stage, so it does not matter which of those parameters is used. Electrostatic capacitors such as paper, organic film or ceramic capacitors are usually characterized by IR values, while electrolytic capacitors (aluminum, tantalum) with low IR values are using DCL leakage current specification instead.
- withstand a voltage before it breakdown. This is defined by its maximum Operating Rated Voltage and Breakdown Voltage. Rated voltage is a common parameter that you can find in catalogues and it practically means that manufacture guarantee the specified electrical parameters and reliability are maintained up to this voltage. The maximum rated voltage can be categorized by AC/DC operation or maximum allowed operating temperature (so called Category Voltage – will discuss this in next lectures)
The dielectric of a capacitor has a large area and a short length. Even if the material is a good isolator there always flows a certain current between the charged electrodes (the current increases exponentially with the temperature). This leakage can be described as a parallel resistance with a high value, an Insulation Resistance (Figure 1.). We use the abbreviation IR in the following.
Measurement of the IR and Leakage Current
At an IR determination one measures the DC leakage current through the capacitor. The measuring circuit, however, always contains a certain series resistance.
Hence we need take into consideration the charging time. The circuit diagram and charging curve for a capacitor are shown in Figure 2.
The charging current to the capacitor is shown in Figure 3. (circuit diagram as in Figure 2.). If the capacitor is ideal the current would rapidly attain the limiting value corresponding to the IR. The ideal current curve is designated IC-ideal. But because the polarization in the dielectric requires a finite time for dipoles to reorient the real charging current follows the curve IC-polarization.
In order to attain the actual IR we would need to wait for a very long time. In practice we content ourselves with a specified IR value corresponding to a measuring current at the time tmeasure in Figure 4. Here we have marked a specified current value which on the measuring devices are graded in the corresponding IR value. A common time for IR readings is in IEC specifications 1 minute. The MIL specifications often call for 2 minutes or more. Considerably shorter times are applied at incoming and production inspections. Information in this book is based on the 1 minute value if nothing else is stated. Additionally the IR concerns the ”room temperature condition” (RT), approximately 23 °C. The IR decreases with an increase in part temperature and may, at the maximum temperature, be several 10 powers lower than at RT.
The IR of capacitors of a specific type and voltage rating decreases proportionally to the growth in capacitance (i.e., the increasing area). And vice versa. A reduced capacitance by a correspondingly lessened area will increase the IR. However, up to a specific maximum value of capacitance the IR actually is so high that it’s actually the outer construction and the molding or conformal coating that determines measured values. Up to this point one specifies the IR in M. Above this breakpoint the specifications call for the constant product of IR x C (in seconds). This product also is designated the time constant (see next section).
For electrolytic capacitors with their relatively low IR rather the leakage current is specified.
See also follow up article with more details on Leakage Current Characteristics of Capacitors
The time constant
If we leave a charged capacitor with open connections the charge successively will leak from one electrode to the other through the internal insulation resistance. Eventually the voltage will drop to zero. Because of the very high IR of the electrostatic capacitor (non electrolytic) a complete discharging will take an extremely long time. A more comprehensible measure of the discharge speed is the time constant. It is defined as the time for the initial voltage E to drop to the value 1/e by E (Figure 5.). With reference to Figure 1. and figure 2. we can define as the product IR x C. This quantity is deduced as Ω x As/V = Vs/V = s(econds). Periodically one sees the expression ohm-farad (ΩF) or the somewhat awkward megohm-microfarads (MΩF). Instead of using the expression IR x C it’s customary to mention only the RC product of the capacitor. Then R is understood as IR, i.e. IR x C = RC = τ .
τ = RC (s) or (ΩF) ………….[1]
