The characteristics of a capacitor can be found in the data sheets provided by the manufacturer. Let's look at a few of the most important characteristics:
1) Working Voltage, (WV)
This is an essential capacitor characteristic that gives definition to the maximum continuous voltage (AC or DC) that can be applied to the capacitor without the capacitor failing. In most cases, you can find the working voltage printed onto the side of the body of the capacitor, displaying its DC working voltage.
Because the AC voltage of a capacitor value makes reference to the r.m.svalue and not the peak or maximum value (which happens to be 1.414 greater), AC and DC voltage values are generally not the same for any type of capacitor.
Failure will likely occur if any DC voltage exceeds its working voltage. Failure can also occur if an excessive AC ripple current takes place. Because this is the case, it stands to reason that a capacitor will have an extended working life if it operates within its rated voltage located in a cool environment.
Common working DC voltages include:
You can find each of these voltages printed directly onto the body of the capacitor.
2) Leakage Current
Dielectrics used within capacitors that serve to separate conductive plates aren't perfect insulators. Because of this, a small current or "leak" flows through the dielectric, influenced by the powerful electric fields that build up due to the charge of the plates when a constant supply voltage is applied.
This small DC current flow is called a Leakage Current. Essentially, leakage current occurs when electrons make their way through the dielectric medium (typically around the edges). Eventually, the leakage current will completely discharge the capacitor if the supply voltage is removed from the equation.
In the event of low leakage, common with foil or film type capacitors, the leakage current is referred to as "insulation resistance" ( Rp ) which is expressed as a high-value resistance. The term "Leakage Current" is typically only used when the flow of electrons is very high.
The capacitor leakage current is one of the most important parameters for power supply and amplifier coupling circuits. With that being said, the best choices for storage applications are Teflon, polystyrene, polypropylene, and other types of plastic capacitors.
On the other hand aluminium, tantalum and other types of electrolytic-type capacitors can handle very high capacitances. However, they're prone to high leakage currents. Because of this, they are not well suited for coupling applications or storage. On a final note, leakage current flow for aluminium electrolytics will increase as the temperature rises.
3) Tolerance, ( ±% )
The tolerance rating of a capacitor is expressed with a plus-or-minus value. These represent picofarad's (±pF) that indicate capacitors with low values (typically less than 100pF) or as a percentage (±%) for capacitors with a higher value (typically higher than 100pF).
Essentially, the tolerance value is the full extent to which the capacitance varies from its nominal value. In most cases, the tolerance level can range from -20% to +80%. Capacitor ratings are determined by how close to the actual values they are when compared to the rated nominal capacitance. Letters and colored bands are used to indicate actual tolerance. Common tolerance levels for capacitors sit around 5% - 10%. However, some capacitors made of plastic have been rated as low as ±1%.
4) Working Temperature, (T)
Due to changes in dielectric properties, fluctuations in temperature will have a direct effect on the value of the capacitance. If the surrounding temperature becomes too hot or too cold the capacitance value of the circuit may not operate properly. Generally, most capacitors work well between -30oC to +125oC. Nominal voltage ratings for a working temperature for plastic capacitor types are no more than +70oC.
Electrolytic capacitors and aluminium electrolytic capacitors are susceptible to deformation at high temperatures because of leaking and internal pressure. Furthermore, electrolytic capacitors can't be used at temperatures below -10oC because the electrolyte jelly will freeze.
5) Temperature Coefficient, (TC)
The temperature coefficient of a capacitor is determined by the maximum change in its capacitance over a specific temperature range. Generally, the temperature coefficient of a capacitor is determined in a linear fashion as parts per million per degree centigrade (PPM/oC). It can also be determined as a percentage change over a specific range of temperatures.
Class 2 capacitors are non-linear in nature. As a result, their values increase as the temperature increases thus giving them a temperature coefficient that's expressed as a positive "P." In contrast to Class 2 capacitors, some capacitors actually decrease their value when the temperature rises. As a result, the temperature coefficient in this instance will be expressed as a negative "N."
Some capacitors do not experience a change in value and will remain constant over a specific range of temperatures. These capacitors have a zero temperature coefficient and are expressed with "NPO." These capacitor types are considered to be Class 1.
While the vast majority of capacitors lose their capacitance when they get too hot, an exemption exists with temperature compensating capacitors. These capacitor types can handle temperatures ranging from P1000 through to N5000 (+1000 ppm/oC through to -5000 ppm/oC).
It's very well possible to connect a positive temperature coefficient with a capacitor parallel to a capacitor with a negative temperature coefficient. When this occurs, the two opposite effects will eventually cancel each other out. Application temperature coefficient capacitors can also be used to negate the effect of other components located within a circuit, such as a resistor or an inductor.
6) Nominal Capacitance, (C)
When it comes to importance, the nominal value of the Capacitance, C of a capacitor will always rank at the top of capacitor characteristics. This value can be measured in three ways:
- Micro-Farads (μF)
- Pico-Farads (μF)
- Nano-Farads (μF)
These values are printed directly onto the body of the capacitor in letters, numbers, and colored bands.
7) Equivalent Series Resistance, (ESR)
The Equivalent Series Resistance AKA ESR is the AC impedance of a capacitor when it's used at higher frequencies. It includes the DC resistance of the terminal leads, the resistance of the dielectric material, the capacitor plate resistance, and the DC resistance of the connections to the dielectric; all of which are measured at a specific temperature and frequency.
Equivalent series resistance defines the energy loss of the "equivalent" series resistance of a capacitor. Thus, it must determine the overall I2R heating losses of a capacitor. This is especially the case when power and switching circuits are involved.
Capacitors that have a high ESR are less capable of passing current to and from its plates to an external circuit. This is because of their longer charging/discharging RC time constant. The ESR of electrolytic capacitors will gradually increase with time as the electrolyte within begins to dry out. When using it as a filter, a capacitor with a low ESR rating is recommended.
Capacitor Polarization makes reference to electrolytic type capacitors (mostly aluminium electrolytic capacitors) in regard to their electrical connection. The vast majority of electrolytic capacitors are polarized, meaning that the voltage in the capacitor terminals must have the right polarity (positive to positive, negative to negative).
An incorrect polarization can lead to a breakdown of the oxide layer inside of the capacitor which will ultimately result in large currents flowing through the device. As a result, the capacitor will likely be destroyed.
Most electrolytic capacitors have a negative terminal that's marked with either an arrow, band, black stripe, or chevrons. These are in place to prevent any possible incorrect connections to the DC supply.
Some of the larger electrolytic capacitors with a metal body are connected to the negative terminal. This can be done because the metal body is insulated with electrodes. Keep in mind that when using aluminium electrolytics in power supply smoothing circuits, be careful when preventing the AC ripple voltage and the sum of the peak DC voltage from becoming a "reverse voltage."
Keep in mind that capacitors with a small capacitance (less than 0.01μF) aren't usually a danger to people. However, if a capacitor exceeds 0.01μF, you'll be in for a shock! All capacitors are capable of storing electrical charges which takes the form of voltage even when there is no circuit current flowing.
Generally, you should never touch the leads of capacitors with large values if the power sully has been removed. Some capacitors can store lethal charges of voltage. If you're not certain about the condition of a large capacitor you're attempting to handle, always call upon the assistance of an expert.
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