h. Plastic capacitors
i. Polyester or PET capacitor:
Polyester or PET capacitors are plastic capacitors available as leaded packages that replace the paper capacitors. These capacitors are made of polyester films which small in size and available at low cost. These have Operating voltages up to 60,000 V DC, operating temperatures up to 125 °C and low moisture absorption. These are mostly used as low frequency signal capacitors and integrators. They are preferred where cost plays an important role because they have high tolerances of 5 - 10 %.
ii. Polystyrene capacitors:
These are large size capacitors available in tubular shape leaded packages. They have high stability, negative temperature coefficient (NTC), high accuracy and low moisture absorption. The operating temperature is limited to +85 C. These are mostly preferred for low frequency applications as the tubular structure induces inductances which degraded the performance at high frequencies.
iii. Kapton polyimide capacitor:
These capacitors are similar to polyester or PET capacitors that are made of Kapton polyimide film. They are expensive but offer high operating temperatures up to 250 C. These capacitors are not suitable for RF applications.
iv. Polycarbonate capacitors:
These are high performance capacitors which are least affected as it ages. These are characterized by good insulation resistance and dissipation factor. The operating temperature ranges from -55 to +125 C. The dielectric constant is 3.2 %, and dielectric strength is 38 KV/mm. The dissipative factor is 0.0007 at 50Hz and 0.001 at 1MHz. The water absorption is 0.16%. These are mostly used for filters, coupling and timing applications. These can be directly replaced by Polyethylene napthalate (PEN), Polyphenylene sulphide (PPS), Polyimide (PI) and Polytetrafluoroethylene (PTFE).
v. Polypropylene capacitors:
These are used where higher tolerances than PET film capacitors are demanded. These are available in leaded packages and are used for low frequency operation. They have high operating voltages and are resistant to the breakdown. However, they get damaged by transient over-voltages or voltage reversals.
vi. Polysulfone capacitor:
These capacitors are similar to polycarbonate capacitors but can withstand full voltage at comparatively higher temperatures. These capacitors are very expensive and are not readily available. The stability is limited as the moisture absorption is typically 0.2%
vii. TEFLON or PTFE fluorocarbon capacitor:
These plastic capacitors are large in size and expensive. Due to low losses and higher stability these are used for some critical applications. The operating temperature ranges up to 250 C. The dielectric used is Polytetrafluoroethylene.
viii. Polyamide capacitor:
These plastic film capacitors are large in size and expensive. The operating temperature ranges up to 200 C.
ix. Metalized polyester or Metalized plastic capacitor:
These capacitors have metalized plastic films which provide self heating advantage and also reduce the size of the capacitor over conventional plastic or polyester capacitor. However, they are limited by maximum current capacity. They are available in leaded package.