Bredhurst Receiving and Transmitting Society


Components

9C1   58  Recall that temperature has an effect on the value of components. Those with a negative coefficient will reduce in value as temperature rises whereas those with positive coefficients will increase in value.

The resistance to current flow of a conductor, changes with the size of the conductor, the resistance of the conductor also changes with change in its operating  temperature. As the temperature rises the conductor will expand, and as the temperature goes down the conductor will reduce in size.

Materials that are classed as CONDUCTORS tend to INCREASE their resistance with an increase in temperature, so temperature up = resistance up and this is the case with the component called a  resistor it resistance rises as it heats up until it is destroyed by the heat. So the rise in temperature is not self limiting due to the rise in resistance !!.
However  INSULATORS (glass, plastic etc)  are likely to DECREASE their resistance with an INCREASE  in temperature but the temperature has to be very high for any marked change in resistance so for most amateur radio us the are likely to remain a good insulator.


Looking at components those with a negative coefficient will reduce in value as temperature rises, and  those with positive coefficients will increase in value, this is an important factor when considering components for a project.

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9C1  58  continued  Understand the effect this will have on tuned circuits and remedial measures.

The remedial measures that can be taken is the insulate the components where changes in temperature causes different values and the insulation then needs to keep the components as a known temperature and this is sometimes achieved by using a local "oven" to control changes.

Please to remember that  a change in the value of a component in the tuned circuit will consequentially change the frequency at which the tuned circuit operates, that is the way one changes the frequency by changing either the coil or capacitor values or both.

So to have these changes happen due to changes in temperature will be detrimental at least,  and at worst could result in out of band transmissions thus invalidating the conditions of your licence..

Some remedial measures that could be taken would be to insulate the components, where changes in temperature could cause different values and to keep the components as a known constant temperature then insulation is needed, sometimes achieved by using a localised  "oven" box.


The difficulty occurs when you might be asked to assess what the change in frequency which would occur if an inductor has a Positive temperature  coefficient.

Firstly you need to understand that the calculation is not straight forward so not easy to take a guess at the right answer.  

On your formulae sheet there is a formula for resonant frequency note that the L ( for inductance is below the 1 and this tells you that an increase in L will yield a decrease in frequency.
So if in the answers to a question give rises as possible answers then you would be able to rule them out straight away.

An example might be that there is a 10ppm/℃ ( 10 parts per million parts per degree Centigrade) with a 10
℃ rise in temperature.
So ignore the square root and thus the for
10  rise a 10ppm/℃ will give a 100 (10 x 10) ppm decrease in frequency.
If we are working on 10MHz ( 1 million Hz) the change would be 10 ( from the 10 MHz )  x 100 ( from the
10 rise a 10ppm/℃ ) so 10 x 100 = 1000 or 1kH decrease frequency change.
BUT we have not taken into account  the Square root and as square root of numbers greater than 1 results in a smaller number the result you would look for in answers would be less than 1kH.


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NOTE: Questions may include simple calculations.




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