Capacitors comprise the largest variety of electronics components. There are many type of capacitors,great variations in their performance, many methods of packaging and making, dozens of major manufacturers, not to mention new types constantly being introduced with specific application and performance.
As a result, capacitors often cause lots of problems for home brewers. Hopefully this blog will take some of the mystery out of the myriad of capacitors available.
Principal capacitor type
- Disk Ceramic Capacitors
These caps are low cost and suitable for many application. Their main disadvantage is high capacitance changes with temperature,except for the "NP0" varieties that are temperature stable.
These caps are the most commonly used for general purpose circuit but the non-NP0 types should be avoided in frequency determining circuit.
- Monolithic Ceramic Capacitors
Alternating layers of electrodes and ceramic dielectric allow higher capacitance in physically smaller packages. Their characteristic are very similar to disk ceramic. They are encapsulated in epoxy to withstand insertion,soldering and solvent cleaning by automatic PCB assembly machines. Introduced for mass production, they are inexpensive and available from surplus dealers.
- Polyester Film Capacitors
Polyester Film use layers of metal and polyester dielectric to make a wide range of capacitance in relatively small packages at low voltages. These have become the standers caps for DC applications. The "rolled" film layers cause high dissipation and capacitance vs. temperature problems, and should be used carefully in high frequency or high current application.
- Polyethylene Film Capacitors
Polypropylene films use layers of metal and polypropylene dielectric films virtually identical to Polyester Film Caps. The polypropylene, however, is a dielectric offering a higher breakdown voltage than polyester, and thus more suitable for high voltage applications,such as switching power supplies. They also have low loss factors and good capacitance stability making them a good choice for high frequency applications, including oscillators and other frequency sensitive circuit. The main disadvantages are a slightly higher cost, and larger physical sizes over other film dielectric capacitors.
- Silver Mica Capacitors
This is a type of capacitor known as metalized film capacitor, in that the electrodes are a metal deposited by a spluttering process onto the dielectric film. Silver Mica's use a mica film dielectric with thin layer of deposited silver forming the electrodes. These are very stable capacitors for high frequency circuits and preferred choice for VFO and oscillator circuits. The main disadvantage are their higher cost, low operating voltages, and sometimes hard to find from hobby vendors.
- Polycarbonate Film Capacitors
These capacitors have become the standers for high stability MIL-SPEC film dielectrics. Their very low dissipation and extreme temperature stability make them almost the ideal capacitor --at a price! They are very expensive capacitors and not available from hobby vendors, but listed here in the event you have the opportunity to appropriate some!!!.
- Electrolytic Capacitors
Aluminum Electrolytic are the most common, inexpensive electrolytic available from hobby vendors. They are made similar to the polyester film, using aluminum foil electrodes and a dielectric material rolled into layers to increase the effective plate area to from high capacitance in small packages. The aluminum foil is "wetted" with a chemical agent to assist in conduction and increase the dielectric property when a DC voltage is applied.
This wetting agent can dry out after long periods of no use, or exceeding the rated voltage, causing a breakdown of the dielectric and component failure(usually a short circuit between the terminals). This is why electrolytic are often found shorted in older equipment that has not been powered for years. This is seldom a problem with equipment that is periodically powered up. These inexpensive aluminum electrolytic caps are suitable in all QRP application.
- Tantalum Capacitors
Tantalum's are a most unusual process that yields a high reliable electrolytic with a long life. Tantalum pent-oxide powder is mixed with manganese dioxide electrolyte and formed into "pellet" forming both the dielectric and the positive electrode plate. Graphite or silver plating forms the negative plate. The "pellet" form a very large effective plate area, and thus very high capacitance in very small packages. Both wet and dry electrodes are used, and called wet or dry tantalum. There are few QRP applications where tantalum's would be must, but if you have them--use them! The chief disadvantages are higher cost due to complicated manufacturing process, and ensuring you never reverse the polarity. A small positive voltage on negative terminal can fuse the "pellet".
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