capacitors

Above: old (top) and new style tubular capacitors

Capacitors (old name: condensers) are made in many different sizes and shapes, from a variety of materials: but they all have certain things in common. 

There is a metallic conducting material used for the plates and there is a non-conducting material which is used to keep the two plates electrically separate. This insulator is known as the dielectric. For the greater part of the valve era, this was made from paper, mica, rigid plastic or air. 

Capacitors store electricity. They can be charged up and can hold the charge for short though useful periods.  They are used in a number of ways.

In radio power supplies, they can be used to store power and give it back as needed, thus acting as a reservoir, usually in conjunction with a second 'smoothing' capacitor and a choke or resistor, the standard system used to filter out out the slight AC ripples left after rectification in all mains sets. Electrolytic capacitors are used in these circuits and whenever large values of capacitance are required. These use ‘damp' paste electrolytes and are usually polarised. They must be connected correctly or they will 'leak' - pass DC. This could lead to rapid, even catastrophic breakdown, at the high voltages used in radio power supply circuits. The use of an electrolyte saves physical size. Without this, large values would be physically very large.

At audio frequencies (LF), capacitors can effectively pass alternating signals such as analogues of speech or music, at the same time blocking any direct current that the alternating signal may be carried upon.

Left: the old term 'condenser' is commonly found in valve circuitry

At radio frequencies (HF), variable types are used in tuning and to bypass unwanted RF signals after detection.  In tuned circuits they work together with an inductance to select wanted signals and reject unwanted ones. This is the way the tuning works - large adjustable air-spaced ganged capacitors wired in parallel across the inductance of a tuning coil allow the frequency selected to be varied across a given band.  Pre-set capacitors allow  ‘trimming up’ of tuned circuits for maximum response. In conjunction with resistors, capacitors form so-called 'time-constant' circuits, allowing timing functions, integration and differentiation of waveforms. This type of circuitry can be found in television equipment.

Above: Mu is the symbol for 'micro'. It is shown as an image because of the limitations of some web software in representing such font symbols. 

Capacitors are measured for their amount of capacity by a unit called the Farad, named after Michael Faraday. In electronics, the Farad is a very large unit and we mostly work in MicroFarads, NanoFarads and PicoFarads of which the largest measurement is Microfarad. Most electrolytic capacitors are in microfarad values. The symbol ‘Mu', shown symbolically above and pronounced ‘mew’is used to represent micro. The letter N (nF) represents nanofarad and the letter P (pF) represents picofarad. 

Typical values for electrolytic capacitors used in transistor circuitry might be 1 microFarad to 1000 microFarad. Paper and plain (i.e. non-electrolytic) capacitor values range from 1 microFarad to 0.0001 microFarad. Tiny values of capacitors used in RF stages of radio and TV receiving circuits might be as low as a few picoFarad. Capacitors add in value when in parallel, unlike resistors. Example: two 100 microFarad capacitors in parallel = 200 microFarad. 

Note that in the valve era, the nanoFarad was not in common use.

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