FABLE OF A FARAD

 

 

Of all the various electronic components, capacitors seem to come in the greatest variety of sizes and shapes, perhaps because of the shortcomings of each type.Correct capacitor selection, both in original circuit construction and for repair replacements, can be vital for proper device performance.

 

Aside from proper voltage and current ratings, and physical size and shape, capacitors can be distinguished by their dielectric material.Itís the dielectric that largely determines the capacitorís characteristics.

 

Let me show you what I mean:if you need a filter capacitor for a power supply, you might need a unit of a few hundred microfarads, with good current carrying capacity.Most likely, just because of issues of size and weight, youíll end up with an aluminum electrolytic capacitor.Be sure to pay attention to the amount of current cycling in and out of the capacitor, as well as providing for some voltage margin.As far as packaging is concerned, be aware that manufacturers are stressing radial capacitor production, so axial lead units are getting harder to locate, and their prices are running up quickly.

 

Now your electrolytic cap packs a lot of capacity in a small space, but performance is generally optimized for 120 Hz or so, the internal resistance and inductance may be quite high, and the device is generally unipolar.So-called computer-grade electrolytics allow higher circulating ripple currents and offer lower equivalent series resistance (ESR).An electrolytic cap is fine for a DC power supply filter; not so great if RF or transient performance is important.And its value isnít stable, so forget about using it in a tuned circuit of any kind.Dipped tantalum and solid tantalum caps are similar to aluminum electrolytics, but offer some performance improvements in density and ESR.

 

The oil-filled capacitor uses oil-impregnated paper for a dielectric, often in a metal can outfitted with leads.These are used where youíd like to use an electrolytic cap, but canít because the voltage is too high, or you need an AC device (phase delay for AC motor windings, and power factor correction for AC motor loads, to offer two examples).You should assume any of these manufactured before the mid-80ís is impregnated with PCB-bearing oil, and so must be tagged and disposed of properly upon failure to avoid legal and environmental issues.Their modern replacements look similar but use mineral oil or mylar in their dielectrics to protect the environment from dioxins and furans.

 

Ceramic or monolithic caps are small and inexpensive, and good for non-critical bypass and filter applications.Often youíll find them in parallel with electrolytics in power supplies, to smooth out transients that are too quick for treatment by electrolytics.But their stability can be even worse than the electrolytics, so you canít use them in any precision applications.An exception is the so-called plate ceramic cap, which is quite stable, but 99.9% of ceramic caps shouldnít be used when youíre timing or tuning.

 

Polystyrene and polyethylene caps do offer precision and stability, but they can be bulky and are, by their nature, inductive. Good for tuning, but not at RF frequencies.Mylar and polyester caps can be used for moderate precision, and they offer good stability and fairly low inductance.The granddaddy of stability for capacitors is the silvered mica cap, which is great for RF and precision applications, but you may find it quite expensive and hard to obtain.

 

Of course, in broadcasting, we often run into high power RF applications, and so we use a few types of capacitors that arenít seen much elsewhere:there are high power versions of the ceramic and mica caps, and then there are the vacuum and vacuum ceramic types: the only types even more expensive than silvered mica.

 

Often an RF capacitor can be improvised out of available materials for a particular use:the so-called plate blocker in a tube transmitter is often a Teflon or Mylar sheet, used as a bypass capacitor at the plate of a PA tube to shunt parasitics, harmonics and other electronic miscreants and troublemakers to ground right at their source.