Electronic Restoration >> Repair Audio Amplifiers

There are two different ways that MnO2 promotes self-healing. First, there is a thermodynamic tendency for oxygen in the dielectric to slowly diffuse into the pure tantalum substrate metal that supports it. This tendency is encouraged, exponentially, Repair Audio Amplifiers by temperature and the presence of electric field in the dielectric.

Of course the presence of electric field is the normal condition for operating capacitors. Oxygen vacancies resulting from this oxygen diffusion degrade the insulating properties of the dielectric Repair Audio Amplifiers and lead to increased leakage current and ultimately to dielectric breakdown. MnO2 is an oxygen rich material that readily supplies oxygen to the dielectric to replace oxygen lost to the substrate via diffusion. 

Thus MnO2 impedes this degradation mechanism, improving reliability.Second, MnO2 can be converted into a less conductive oxide, typically Mn2O3, Repair Audio Amplifiers upon exposure to hightemperatures, generally higher than 450oC. Very high local current densities exist at small flaws in thedielectric (cracks, impurities, etc.) when voltage is applied to the capacitor. 

The result is localized heatingof the dielectric. This heat is conducted to the MnO2 coating, converting it into the less conductive Mn2O3.This process limits the local current flow Repair Audio Amplifiers and "heals" the flaw site.Improved Electrical Performance It was mentioned earlier that a weakness of wet-electrolyte capacitors is poor performance at lowtemperatures as the electrolyte becomes less conductive. 

The capacitor characteristics directly affected bylow electrolyte conductivity are capacitance roll-off with frequency, dissipation factor, and ESR.Because of the porous slug structure of the tantalum capacitor, Repair Audio Amplifiers electrical signals from the external circuitmust be conducted to the inner parts of the capacitor's structure through the electrolyte. 

AC voltage dropsoccur along the way in the electrolyte's resistance and these voltage drops limit the percentage of the signalthat reaches the core of the capacitor. If little of the external circuit's signal reaches the capacitor's core,then capacitance that exists there is largely invisible to the circuit Repair Audio Amplifiers and the effective capacitance is reduced.

At very low frequencies, Repair Audio Amplifiers there is enough time for the capacitance inside the slug to fully charge to theexternally applied voltage and all of the slug's capacitance is "seen" by the external circuit. But at higherfrequencies, the rate of charging inside the slug is limited by RC time constants along the electrolyte's path,and the slug's internal capacitance cannot "keep up" with time-varying changes of circuit voltage. 

A goodelectrical model of this behavior is the RC ladder transmission line. The RC ladder model predicts afrequency beyond which the effective capacitance of the structure begins to predictably decline, aphenomenon known as "capacitance roll-off." A schematic view of the internal structure of a tantalum capacitor element, Repair Audio Amplifiers highlighting the physical originof the RC ladder and the capacitance roll-off effect, appears in Figure 5. 

Computer models of the highfrequency behavior of Repair Audio Amplifiers tantalum capacitors are based on this model with individual values of R and Cchosen to reflect the specific geometry of the capacitor element in question. Such models can predict thefrequency-dependant characteristics of tantalum capacitors with high accuracy.

Capacitance roll-off behavior is easily verified in the laboratory. The effect is demonstrated by observingcapacitance decrease while increasing the resistance of the electrolyte, increasing the measurementfrequency, or both. Alternatively, Repair Audio Amplifiers reducing the resistance of the electrolyte increases the frequency whereroll-off begins. 

Typical wet-electrolyte capacitors tend to experience substantial capacitance roll-off aboveroughly 1 kHz at room temperature and at much lower frequencies at very low temperatures. In contrast, Repair Audio Amplifiers conventional MnO2 tantalum capacitors have substantially stable capacitance until well above 10 kHz because of the increased conductivity of MnO2 versus wet electrolytes.

Another direct consequence of the resistance along the electrolyte's path is elevated energy loss. An idealcapacitor is a perfect energy storage device and will return all of the energy stored in it. Real capacitorsalways have some resistance in addition Repair Audio Amplifiers to their capacitance, and this resistance is the root cause of energyloss. 

Energy loss in capacitors is quantified by the parameters dissipation factor (DF) and ESR.DF is the ratio of energy lost to energy stored in the capacitor over a full cycle of applied alternatingvoltage, Repair Audio Amplifiers while ESR is the equivalent value of resistance that if connected in series with an ideal capacitorwould perfectly simulate the energy loss mechanisms in the capacitor. 

Both of these parameters grownumerically as the conductivity of the electrolyte falls. At low temperatures, wet electrolytes are very poorconductors and wet-electrolyte capacitors demonstrate very high DF and ESR at low temperature.Conventional MnO2–based solid tantalum capacitors have Repair Audio Amplifiers much better (lower) DF and ESR at low temperatures because of the improved conductivity of MnO2 at low temperatures versus common wetcapacitor electrolytes. 

MnO2 still loses conductivity at low temperatures, just not as quickly as wet electrolytes do. At -55oC, DF and ESR of a conventional MnO2 tantalum capacitor are not likely to Repair Audio Amplifiers degrade by more than a factor of two, but many wet-electrolyte capacitors become essentially nonfunctional at that temperature and are not generally intended for use below roughly -40oC. 

In contrast,MnO2 tantalum capacitors retain useful performance as low as -80oThe remaining essential electrical property of conventional tantalum capacitors is DC leakage current, or Repair Audio Amplifiers rather the absence of it. One important characteristic of capacitors is that they should block DC currents.Real capacitors do not perfectly block DC current, but the current that "leaks" through the dielectric isusually very small. 

Typical values of leakage current for MnO2-based tantalum capacitors usually fall well below catalog limits which are commonly set at 0.01CV, where C is the capacitance in µF Repair Audio Amplifiers and V is therated voltage. For example, a 100µF, 10V capacitor would have a room-temperature leakage current limitof 10µA, but would typically have a measured DC leakage at 10V of less than 1 µA. 

Typically measuredleakage currents for MnO2-based tantalum capacitors at rated voltage range from roughly 0.1 nA to about100 µA.Defining Characteristics of Tantalum Capacitors Made with Conducting-Polymer ElectrolyteMuch was said earlier about the benefits of solid electrolytes (no loss of electrolyte over time) and of moreconductive electrolytes (less capacitance roll-off, lower DF, Repair Audio Amplifiers and lower ESR). 

The primary reason for theexistence of tantalum capacitors made with conducting-polymer electrolyte (called "tantalum polymercapacitors") is that conducting polymers are Repair Audio Amplifiers significantly more conductive than MnO2, perhaps up to 100times more conductive. The direct benefits of this increased conductivity are lower ESR and improvedhigh-frequency performance.


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