Carbon mass resistance
A carbon ground resistor is one type of electrical component resistor .
General
Carbon mass resistors are components from the beginning of the 20th century, some of which were used until the 1970s. They were mainly used in tube radios and tube televisions but also in more professional electronic equipment. They are rarely used in newer technology.
Before the advent of this type of construction, resistors were realized as wire resistors (winding resistors), which firstly were relatively expensive and secondly had their disadvantages, especially in high-frequency applications.
In the English-speaking world, these resistances are called “carbon composition resistors”, “carbon composite” or “carbon comp” for short.
The term “coal mass” is slightly misleading, since this mass is a mixture or composition of several materials.
construction
Resistance material
The resistance-determining material consists of three components:
- Finely ground coal
- Finely ground ceramic (often aluminum oxide, Al 2 O 3 )
- plastic
Depending on the desired resistance value or the maximum planned power loss of the resistance body, this mass is pressed into a cylindrical shape in different mixing ratios. This results in an inhomogeneous internal structure of conductive carbon and non-conductive ceramic grains of different sizes, which touch with different surface sizes. This structure results in the instability of the parameters of the carbon mass resistance (see below).
Housing and contact
In the early 20th century, these cylindrical "resistor blanks" were initially not surrounded by an insulating cover. The contact was made by wires wound around the edges, which lay directly on the ground. The resulting wire eyelets were soldered. Possibly. varnish was applied in different colors to encode the value of the resistance.
In later versions, the connecting wires were pressed into the resistor body or the contacts were made using caps at the ends. The resistors were increasingly encased in ceramic because, on the one hand, the resistance material is hard, porous and therefore brittle and, on the other hand, environmental influences on the resistance body (moisture, dirt) quickly change the electrical values.
Parameters
Resistance value
Carbon mass resistors were (and are partly) available in all common values from the ohm to the megohm range according to the E series E6, E12 and E24.
Tolerances
Carbon mass resistors were offered with tolerances of 20%, 10% and in some places even 5%.
Since the manufacturing process was difficult to control, the manufacturing tolerances were very high. That is why the resistors were simply manufactured and then measured and sorted.
stability
The stability of the resistance value leaves a lot to be desired in all areas of carbon-mass resistors measured by today's standards. In addition to the above delivery tolerance of up to 20%, further tolerances can be added very quickly. Even when not in use, the resistance value can increase by around 5% per year due to aging processes. When used above temperature (max. 150 ° C) even 15% can be achieved here. The temperature coefficient is negative and can be up to −1200 ppm. With a temperature change of + 100K, this means a change in resistance of −12% (for comparison: today's resistances are usually in the range of ± 50 - ± 100ppm).
Noise
Due to the internal structure, carbon-mass resistors tend to be very noisy (current noise, composed of shot or flicker noise). This can be very disturbing in signal paths (e.g. audio or measurement technology)
Linearity
Depending on the voltage across the resistor (approx. 100 V or more), disruptive non-linearities occur, as the current paths within the resistor change due to the different electrical field strengths at the grain boundaries.
Pulse resilience
The coal mass resistance can score here. Due to its construction, the resistor can absorb higher pulse energies than most current resistor types.
Designs
There are currently no known standardized designs. The dimensions were determined by each manufacturer depending on the planned maximum permissible power loss and were therefore at most similar
Areas of application
The above-mentioned pulse load capacity ensures that these resistors are still in use today. They are (and were) used for the following areas of application:
- Current limiting circuits
- (Discharge) protection circuits
- High voltage power supplies
Some of them are also used in defibrillators, where under certain circumstances they have to absorb the entire energy of around 30 joules.
Voices and opinions from practice
Among electronics enthusiasts and hobbyists, carbon mass resistors were and are disparagingly referred to as “coal bunkers”. Probably with reference to the loose pieces of coal lying around in the coal bunkers, which are just as inhomogeneous as the resistance material in the coal mass resistors.
When restoring older audio technology, this old (resistance) technology is sometimes still used. Here there may be “originality” versus “better (signal) quality”.
literature
- John Watkins: Modern Electronic Materials. Butterworths, London 1971, ISBN 0-408-70140-4 , pp. 22-36.
Web links
Individual evidence
- ↑ Carbon Composition Resistors Resistor Guide. Retrieved April 28, 2020.
- ↑ current noise in coal mass resistors www.radiomuseum.org. Retrieved April 28, 2020.
- ↑ Stiny, Leonhard: Passive electronic components: structure, function, properties, dimensioning and application . 3rd edition 2019. Wiesbaden, ISBN 978-3-658-24733-1 .