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Syllabus Sections:- Temperature Effects 3m.1 1 Recall that 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
a 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
surrounding) then few problems should occur.
Tuned circuits
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 :-
using properly rated components
maintaining the temperature of the tuned
circuit by the use of thermostatically controlled heaters - since a crystal
can be treated as a tuned circuit and also is affected by temperature changes
high stability equipment will commonly employ a "crystal oven" to maintain
the crystal at a constant temperature above ambient.
isolating the tuned circuit from other
components that might cause local heating - for example DO NOT mount the
tuned circuit near the voltage regulator (because the regulator get hot !!).
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