CAPACITORS 



Electrolytic capacitors 



Electrolytic capacitors are also characterized by large capacitance in a 

 small volume. Though they are made in working voltages ranging from 6 to 

 600, in practice they fall into two important groups : 



(1) Low voltage types, of working voltage less than 50, for cathode bias 

 resistor bypass, and mains-derived d.c. heater supply smoothing. Capacitance 

 values 20-2,000 ^F. 



(2) High voltage types for HT supply smoothing and decoupling, rated 

 between 300 and 600 V, with capacitance values of 4, 8, 16, 32 or 50 fiY. 



In electrolytic capacitors the positive electrode is aluminium foil and the 

 negative electrode is a paste of glycol and ammonium tetraborate to which 

 connection is made by a second aluminium foil. The dielectric is a very thin 

 layer of aluminium oxide covering the positive foil. This layer is formed by 

 electro-deposition before the capacitor is assembled, and the component must 

 always be wired up the correct way round so that the oxide film is not 

 removed again, that is, the electrolytic capacitor is restricted to applications 

 where the applied voltage is direct, or where, if there is a superposed alternating 

 component, the net voltage never reverses. 



The oxide film is extremely thin, of the order of 0-1 fx, and about 1/100 

 of the thickness of the paper used in impregnated paper capacitors. The 

 capacitance is therefore very high for the size of the component, but so also 

 is the leakage. The capacitance may be further increased by etching with 

 acid the surface of the positive aluminium foil, thus increasing the surface 

 area. This process does not give a commensurate increase in leakage. The 

 leakage resistance of plain foil electrolytic capacitors is about 6 MO-^F, and 

 of the etched foil type, 20 MQ-//F. 



Electrolytic capacitors are somewhat imprecise components ; manufacturing 

 tolerances are commonly —20, +50 per cent, and the temperature coefficient 

 is large and positive, about 0-2 per cent per degree C. They are also extremely 

 liable to loss — tan 6 = 0-15 — and this sets a limit to the magnitude of the 

 alternating component of voltage which may be allowed across them. If this 

 is excessive, the heat generated within the component will volatilize the 

 electrolyte paste and generate a pressure which may cause the component 

 to explode. When this happens the noise is extremely startling and the 

 remainder of the interior of the apparatus becomes plastered with fragments 

 of electrolytic capacitor. 



In most electrolytic capacitor applications the a.c. component is not serious, 

 but there is one important exception; this is in capacitor-input power- 

 rectifier circuits. With some capacitors the maximum permissible a.c. is 

 marked, e.g. '130 mA. a.c. Max', but where this is not done the manufacturers 

 should be consulted. In the highest class of design a paper and foil capacitor 

 is used in this position. Electrolytic capacitors are frequently assembled two 

 or three to a single container; when this is done there is often a printed 

 notice to say that one or other section is intended for high-a.c.-ripple-duty. 



In addition to the maximum direct working voltage rating, electrolytic 

 capacitors are also often marked with a rather larger 'surge' rating. This is a 

 voltage which may be applied to the component for a short time without 

 damaging it. The point is important because in many pieces of electronic 



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