Bredhurst Receiving and Transmitting Society

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. How great or small the problem is dependent upon the being component designed 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 .

Components with a positive coefficients will increase in value.

The design of the associated circuit will have to take into account these changes.

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 !!).