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Polypropylene Film and Electrolytic Capacitors

Polypropylene Film Capacitors for Loudspeaker Crossovers

A squashed polypropylene film capacitor for loudspeaker crossovers

       So what is a Polypropylene Capacitor, also known as a PP Capacitor, and how is it made?
A large number of different materials can be used in manufacturing capacitors, but capacitor types fall into two main-stream families, either Film Capacitors or Electrolytics. There are other much smaller groups but......

     Polypropylene capacitors belong to the "Film" family. Polypropylene sheet, being one of the Polyolefin group of thermoplastics, is precision stretched, with specialised equipment. This stretching forms two matched thicknesses and lengths of long, very thin sheet. NOTE:- Capacitance varies inversely with film thickness, so thinner sheets provide higher capacitances for the same overall component volume. Of course, whilst thinner films mean smaller component sizes, thin films are mechanically less robust and electrically break down more easily. This limits the working voltage of the capacitor and the thickness of stretched film that can be used. The more uniform the film thickness then the tighter the capacitor tolerance will be, a good marketing point.

       At this stage the sheet is known as a dielectric. The dielectric is sprayed, either single-sided or double-sided, depending upon the capacitor design with an extremely thin metallic coating, usually aluminium. This forms the capacitor's electrodes. At this stage the sheet is cut, into small areas and vertically stacked. The metallised stacked layers are alternately offset to provide very short foil layers to which electrical connections are attached at the foil ends. This type of layout is known as a box capacitor.

       Alternatively, two coated sheets are rolled, just like a double layer Swiss roll, with the two layers slightly offset and electrical connections are attached at each end. As absorption of moisture by the dielectric spoils the capacitance value the ends are usually hermetically sealed. However, Polypropylene film has far lower water absorption than a polyester film type cap of similar design and is hence considerably more stable.
As polypropylene film capacitors are also larger than other designs of similar capacitance the rolled format is often squashed. By this technique and correctly orienting the component on a PCB, the footprint is somewhat reduced.

        In summary, the internal design of Polypropylene capacitors provides for low values of ESR and ESL and they have good stability, low tolerances, work well at high frequencies and are well suited for loudspeaker crossovers. These types of designs be it box or cylinder are electrically similar whichever orientation they are wired and are known as Bi-polar capacitors.

       So here is a range of 'good value' Metallised Polypropylene Film Capacitors for DIY Crossovers and rated @ 250Vdc with a tolerance of +/- 5%. Just the thing to get that special speaker project off to a flying start. Made in the UK for QTA Systems. Details:- Wire lead-outs:- (1.0 Diam by 40)mm copper. PVC sheathed. Tolerance :-  +/- 5% Dissipation Factor:- < 0.001 @ 1Khz & 20  +/- 3C
Insulation Resistance:- > 10000Mohms * uF @ 250Vdc & 20  +/- 3C
Rated Voltage:- 250Vdc
Dielectric absorption:- < 0.1% @ 20  +/- 3C
Temperature Range:- -55 to 100C
Over-voltage test:- 375V for 30 seconds, not to be repeated.

Polypropylene Film Capacitors, Specifications. Price Each.
Order Code Spec Dims (L * W * H)mm 1+ 6+ 11+
PPCAP1U0G 1u0 +/- 5% 250 Vdc 28 * 15 * 10 1.40 1.33 1.26
PPCAP1U2G 1u2 +/- 5% 250 Vdc 28 * 16 * 12 1.48 1.41 1.34
PPCAP1U5G 1u5 +/- 5% 250 Vdc 28 * 18 * 14 1.69 1.61 1.52
PPCAP1U8G 1u8 +/- 5% 250 Vdc 28 * 18 * 14 1.91 1.82 1.72
PPCAP2U1G 2u1 +/- 5% 250 Vdc 28 * 20 * 15 2.05 1.95 1.85
PPCAP2U5G 2u5 +/- 5% 250 Vdc 28 * 22 * 17 2.26 2.15 2.03
PPCAP2U7G 2u7 +/- 5% 250 Vdc 28 * 23 * 18 2.37 2.26 2.13

Polypropylene Film Capacitors, Specifications. Price Each.
Order Code Spec Dims (L * W * H)mm 1+ 6+ 11+
PPCAP3U0G 3u0 +/- 5% 250 Vdc 28 * 24 * 19 2.44 2.33 2.20
PPCAP3U3G 3u3 +/- 5% 250 Vdc 28 * 24 * 20 2.58 2.46 2.32
PPCAP3U9G 3u9 +/- 5% 250 Vdc 34 * 23 * 18 2.71 2.58 2.44
PPCAP4U2G 4u2 +/- 5% 250 Vdc 34 * 23 * 19 2.98 2.84 2.68
PPCAP4U7G 4u7 +/- 5% 250 Vdc 34 * 25 * 20 3.11 2.96 2.78

Electrolytic Capacitors

Power supply, electrolytic capacitors

       Electrolytic capacitors fall into the second family of Capacitors and are further split into Aluminium or Tantalum based components. Previously, in film capacitors the dielectric was a stretched film. In Electrolytic capacitors this is replaced with an electrolyte, a thick chemical solution or paste which, based upon its composition, can carry electrical current when correctly polarised. This paste forms one of the capacitor electrodes. The second electrode is chemically grown which can be processed / grown to produce extremely thin metallic films. This feature can produce large capacitances for a given volume. Electrolytic capacitors are usually polarised devices and have the connection polarity, (the negative terminal), marked upon their case, most important this point. Unpolarised electrolytics are marked NP.
     As with most thin film devices the electrolytic film integrity can be destroyed by incorrect use. Namely, over-voltage, electrolytic capacitors have a low voltage threshold before destruction, reverse polarity connection, they don't like that and are of no use in "AC power supplies" where the polarity cyclically reverses and are easily ruined by over temperature.
       Having said all that, they are great for "DC power supplies" and have many uses within the Audio Field as smoothing capacitors in audio amplifiers. Below is a small size / can height Electrolytic for DIY Projects.

New Old Stock:-
Electrolytic capacitors by BHC Aerovox, 10000 uF 50V DC, (57V Surge), Low Impedance.
Case suitable for power supply and audio projects. size (Diam * Height):- 35 * 40mm Nominal impedance (ESR) mOhms @ 20Deg C and 100Hz:- 73mOhms Nominal impedance mOhms @ 20Deg C and 10Khz :- 59mOhms Ripple current amps @ 85Deg C 100Hz:- 4.23A Life Expectancy 15000 Hrs @ 85Deg C.

Electrolytic Reservoir Capacitors. Price Each.
Order Code Spec 1+ 6+ 11+
SP01 Electrolytic Reservoir Capacitors 10000u 50V 3.94 /ea 3.74 /ea 3.55 /ea

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