2A1 Understand component tolerances and the effects they may have in circuit operation.
We have learned in the Intermediate course about tolerance of resistors. Actual values may vary according to the 4th band. This is usually 5% or 10%. Capacitors also have tolerances either stamped on them in actual figures or with a value code. They are usually marked with their voltage ratings. It is important not to exceed these ratings for electrolytic and tantalum capacitors. Inductors are usually printed their value and tolerance.
However all these component values can change with temperature and in some case humidity. Temperature increases can be due to currents through the components causing them to heat up. Heat can change the value of a capacitor and humidity can change values of air-spaced capacitors. Inductors can change due to current heating effects. Transistors, integrated circuits, and other semiconductor devices actual ratings can change due to temperature changes. Some devices have what we call negative temperature coefficients. We will see in a moment how this can happen.
Temperature has an effect on the value of components. Those with negative coefficients will reduce in value as temperature rises whereas those with positive coefficients will increase in value. Understand the effect this will have on tuned circuits and remedial measures.
All components that are passing current have an associated heating problem. The design of the component will have a effect as to how great or small the heating effect affects the component. The component must be chosen to operate at the level of heat being generated by itself or nearby components.
Coefficients of heating
Components with negative coefficients will reduce in value as temperature rises whereas those with positive coefficients will increase in value. The design of the associated circuit will have to take into account these changes and where possible balance out a positive with a negative so that changes are minimal.
If the component is designed to operate at a higher than ambient temperature (ie the normal temperature of its surroundings) then few problems should occur.
The tuned circuit is a critical area where changes in values can give a dramatic effect as they can cause changes in the received or transmitted frequency. The values can and do change as the result of temperature changes.
The remedial methods include :-
Screening with thin metal sheet is effective in keeping heated circuitry away other sections, and it also helps to reduce unwanted radiation from equipment and/or between stages within equipment.
It is often necessary to keep heat and RF away from parts of a circuit or to prevent coupling of one part of the circuit with another. This is often the case when dealing with tuned circuits. The part of the circuit that needs protection is placed in a thin metal enclosure which is connected to the earth of the circuit.
When used with tuned circuits the "screening can", as an enclosure is sometimes called, must be more than 1 1/2 times the diameter of the coil otherwise the Q of the coil will be degraded.
Also a screen may be needed to prevent a strong signal feeding back from one part of the circuit to an earlier stage.
Aluminium, copper, or tin are suitable materials at RF.
The origin of some of the text on this page is from the RSGB with additions by the web master