Capacitors (Misc.)
This text is machine translated.
About Rotary Capacitors
Although nobody is quite aware of it, many people have used a variable capacitor. At the latest if a station has been set for an older radio with scale pointer. We explain was a variable capacitor and what technology is behind it.
-
What was a variable capacitor?
-
Who invented the variable capacitor?
-
How does a variable capacitor work?
-
What was the difference between a trim capacitor and a variable capacitor?
-
Frequently asked questions about variable capacitors
What was a variable capacitor?
A capacitor is a passive component that can store electrical energy statically in the form of a charge.
In the simplest case, a capacitor consists of two metal plates facing each other. The larger the plates and the smaller the distance, the greater the capacity or storage capacity of the capacitor.
A variable capacitor or variable capacitor is a variable capacitor whose capacity value can be set mechanically continuously. The value of the variable capacity or the minimum and maximum capacity are determined by the design.
The change in capacity is achieved by mechanically twisting the moving plates. This increases or decreases the effective surface of the opposite capacitor plates.
Who invented the variable capacitor?
On June 8, 1892, Desider Korda from Paris received a patent for his electric capacitor from the Berlin Imperial Patent Office. With this capacitor, the capacity change was made by the position change of the semicircular capacitor plates. A principle that has not changed for more than 100 years.
Since the capacitor presented at that time had to have a high electrical strength and a high capacity, Desider Korda was still working with a liquid dielectric (insulator between the plates).
Dr. Adolf Koepsel has developed the concept further and successfully used the variable capacitor for frequency tuning in oscillating circuits. For this reason, it is often incorrectly referred to as the inventor of the variable capacitor.
How does a variable capacitor work?
Even though the classic variable capacitor is no longer in use, it can be explained with its help very nicely.
As with an electric motor, a variable capacitor has a fixed part (stator) and a rotating part (rotor) . Both parts are made of fan-shaped plates mounted at an exact distance from each other. The plates of the rotor are mounted in a rotating position on a common shaft. Thus, the rotor plates can dip into the gaps between the stator plates without touching each other . .
The surrounding air serves as a dielectric. For this reason, these variable capacitors are also referred to as air variable capacitors.
The capacitors had a maximum value of only a few hundred picofarad (pF), which was perfectly sufficient in practice.
In addition to the air variable capacitors, there are also the film variable capacitors . With these adjustable capacitors, thin plastic films are used as dielectric between the stator rotor packages.
This increases the dielectric strength and makes it possible to assemble the plates more tightly. In addition, the design of the variable capacitor shrinks.
For frequency adjustment, film capacitors are partly equipped with integrated trim capacitors.
Film rotary capacitors were preferably installed in portable transistor radios and radio recorders. Today, they are still occasionally found in retro radios, which are offered as kits.
When a rotary capacitor was used in a radio, the rotor was mechanically connected to a rope wheel. The scale section , on which the scale pointer was also attached, ran over the rope wheel. When the transmitter adjustment knob was turned, the scale pointer moved and the variable capacitor was adjusted.
Due to the different diameters of the pulleys, the variable capacitor only made a minimal movement, even if a complete revolution was made at the adjustment knob. With high-quality variable speed capacitors, some of the reduction gears were also installed without play (see gear in figure 1). Thus, the radio's reception frequency could be finely adjusted to the respective transmitter frequency.
In today's world, digital technology and capacitance diodes have completely displaced the variable capacitor from our receivers. Regardless of this, however, we still use trimming capacitors, which ultimately work according to the same principle.
What was the difference between a variable capacitor and a trim capacitor?
Rotary capacitors are designed for frequent operation or continuous change of the capacitance value by the device user. As a result, they can be adjusted from the outside using controls, rods or cables.
Trimming capacitors or trimmers are part of the group of variable capacitors, but are not intended for continuous adjustment. Trimmers are installed during device manufacture and precisely adjusted (adjusted) at completion. Normally, it is no longer necessary to adjust a trimmer.
Only during maintenance and service work or when replacing components, it may be necessary to adjust a trim capacitor later on. This should only be done by specialists who know exactly what was to be done.
Adjustment screwdrivers made of plastic are required for the adjustment of trimming capacitors. Especially for capacitors with small capacitance values, setting with a metal screwdriver can lead to incorrect results.
In addition to foil trimming capacitors, ceramic trimmers are now preferably used. Especially when the trimmers are needed as surface-mounted SMD trimmers.
Since the ceramic layer between the stator and rotor can be kept extremely thin, the trimmers achieve quite high capacitance values despite their compact design.
Frequently asked questions about variable capacitors
What was to be taken into account when selecting a variable capacitor?
The most important selection criterion is the capacity specified in Picofarad (pF). One pF corresponds to 1 x 10-12 farad (F). The replacement capacitor must have the same value in pF as the defective original part.
The second important selection criterion is the electrical strength. The maximum permissible voltage specified in the technical data must never be exceeded.
Only when the above criteria have been met can one check whether the replacement variable capacitor also fits mechanically in the circuit.
Can a variable capacitor be tested for measuring purposes?
If the variable capacitor has a short circuit, you can easily determine this with a resistance measurement. Many multimeters also offer the possibility of measuring capacitance. This works without any problems with capacitors with higher capacitance values. However, if the variable capacitor has only a few pF capacitance, the measuring leads can already have a negative effect on the measuring result.
If a variable capacitor does not fit into the circuit ich can I extend the connections?
When extending the rotating co-connections, there is a risk that the connecting wires will have a negative effect on the function of the circuit. That is why this is not recommended. Such "DIY solutions" must definitely not be used for the professional repair of devices. Suitable original parts must be used here.
Can an air variable capacitor be repaired from an old radio?
Basically, yes, if you have the appropriate experience. You have to be very careful with the variable capacitors to avoid bending the plates, was that a change in capacity would result. This can have a negative effect on synchronous operation, especially with multi-circuit receivers. Specialists adjust the synchronous operation by specific deformation of the outer plates of the rotor. For this purpose, the outer plates are divided into segments by slots (see figure 2).