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Tantalum capacitors enable very high levels of capacitance to be provided within small packages.
Although they do not have the current capacity and they are not as electrically robust as electrolytic capacitors, their size and performance mean that they are widely used in many applications.
Tantalum capacitors are also widely used in their surface mount formats because they are much cheaper than their aluminium electrolytic relations and they can withstand the soldering process better.
Instead of using a film of oxide on aluminium they us a film of oxide on tantalum. They do not normally have high working voltages, 35V is normally the maximum, and some even have values of only a volt or so.
Like electrolytic capacitors, tantalums are also polarised and they are very intolerant of being reverse biased, often exploding when placed under stress. However their small size makes them very attractive for many applications.
Tantalum capacitors are a specific form of electrolytic capacitor. Unlike the more familiar aluminium electrolytic capacitor, tantalum ones are much smaller and offer a very high level of capacitance for a given volume and weight. They also posses a low ESR (equivalent series resistance) than aluminium electrolytics along with a higher operating temperature capability and lower leakage
The tantalum capacitor consists of a small pellet of tantalum which acts as the anode for the capacitor. This is covered by a layer of oxide which acts as the dielectric for the capacitor and in turn this is surrounded by a conductive cathode. The use of tantalum in the capacitor allows for a very thin oxide layer to be used.
The thin oxide layer means that much higher capacitance levels can be achieved than if some other type of dielectric was used., and it also offers excellent stability over time.
Tantalum capacitor failure modes
One of the disadvantages of having a very thin oxide layer as the dielectric is that it is not particularly robust. As a result, care has to be taken when using tantalum capacitors.
Tantalum capacitors are reliable provided they are operated within their specification limits. Many reliability standards recommend operating them at a maximum of 50% or 60% of their rated working voltage to give a good margin. If this is done then they operate reliably and provide good service.
Tantalum capacitors are not tolerant of abuse. If they are reverse biased or their working voltage is exceeded ten they can fail in a dramatic way. At best they can emit a little smoke, but they can also fail explosively as well.
Care must be taken to ensure this does not happen as it can lead to equipment failure or even fires in some instances.
Leaded tantalum capacitors
Leaded tantalum capacitors typically come in a small and encapsulated in epoxy package to prevent damage.
In view of their shape they are sometimes referred to as tantalum bead capacitors.
The capacitor markings are normally written directly onto the encapsulation as figures, although a colour coding system was popular at one time and some capacitors may still be seen using this system.
SMD tantalum capacitorsSurface mount tantalum capacitors are widely used in modern electronics equipment. When designed with sufficient margins they provide reliable service and enable high values of capacitance to be obtained within the small package sizes needed for modern equipment.
Aluminium electrolytics were not initially available in surface mount packages as they were not able to withstand the temperatures needed in soldering. As a result tantalum capacitors which were able to withstand the soldering process were almost the only choice for high value capacitors in assemblies using surface mount technology. Now that SMD electrolytics are available, tantalum is still the capacitor of choice for SMD as they offer an excellent cost, size and performance parameters.
SMD tantalum capacitors come in a variety of sizes. Typically they conform to standard sizes defined by the EIA, Electronic Industries Alliance.
|Surface Mount Tantalum Capacitor Sizes|
|Package designation||Size (mm)||EIA designation|
|Size A||3.2 x 1.6 x 1.6||EIA 3216-18|
|Size B||3.5 x 2.8 x 1.9||EIA 3528-21|
|Size C||6.0 x 3.2 x 2.2||EIA 6032-28|
|Size D||7.3 x 4.3 x 2.4||EIA 7343-31|
|Size D||7.3 x 4.3 x 4.1||EIA 7343-43|
SMD tantalum capacitor markings
The markings on SMD tantalum capacitors normally consist of three numbers. The first two form the significant figures, and the third is the multiplier. Values are in picofarads. Therefore the SMD tantalum capacitor shown below has a value of 47 x 105pF, which works out to be 4.7µF.
Sometimes values will be marked more directly as shown in the example below. The value is obvious from the markings.
Tantalum capacitor summary
The table below provides some of the salient features about tantalum capacitors to take into consideration when designing circuits or replacing old components.
|Tantalum Capacitor Summary|
|Typical capacitance ranges||1µF to 100µF|
|Rated voltage availability||From around 1.5V to 20V.|