Understanding Carbon Composition Resistors: 101 explanation

Описание: Are carbon composition resistors totally outdated, or are they useful, even today? In this video I provide a depiction of dive into their construction, their properties and see how they are invaluable even in today's high tech electronics environment.

Carbon composition or composite resistors are one of the oldest types of resistor. They've been used for many years - since the early days of radios.

The resistors are available even today as leaded components, and their power ratings available are often between about a quarter of a watt and two or may be five watts. Tolerance figures are often 5, 10 or 20% - back in the heyday of these resistors, 20% components were by far the most common.

Resistance values of a few ohms up to a megohm and more are available.

So what are they like inside? If we break into the resistor, we see that there's a central core and an outer protective coating. The core is made of carbon that's held together with a ceramic binding agent - the composition proportions being one of the major determining factors for the resistance, along with the dimensions.

This cylindrical core has a cap added to each end to which the leads are connected. Then the whole resistor assembly is coated with an insulative coating to act as physical protection.

Despite the protective coating moisture can often enter the resistive core and this can change the resistance and this is why carbon composition resistors are very sensitive to moisture.

Even today there are some advantages to carbon composition resistors.

In view of their construction they have very little inductance - there's no helical cut in a film or there's no wound wire in them.

Another advantage is that they can withstand high transient levels - short periods of high overloads and they perform better in this respect than most other forms of resistor.

However prolonged exposure to heat and excessive power causes changes in the rod that lead to non-reversible resistance changes. Those taken from vintage radios can show really major changes in resistance, and even modern versions are subject to this.

These resistors also produce high levels of noise which is similar in many respects to semiconductor shot noise. They should most certainly not be used in any circuit where noise levels are important.

And of course there is the change due to moisture ingress.

Despite the fact that carbon composition resistors are often written off as old technology, they are still used in some areas.

They can be used in circuits where the resistor may need to withstand high transients. Power supplies and various flyback types of circuit are good examples

They are also used in areas where a resistor with low inductance is needed as in some RF circuits.

They may be also used by some vintage radio enthusiasts to replace old carbon composition resistors. But even many of these people may use more modern metal film varieties because of the long term reliability.

So despite the limitations of carbon composition resistors, they are still used in a number of modern design areas.

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