Engineering Principles V11

Clive W. Humphris


A resistor has just one parameter to be tested, its value. Other measurements such as power rating, type etc are manufacturers and circuit design considerations. The easiest way of checking the component value has to be direct measurement using a calibrated 'Ohmmeter'. Measuring resistors 'in-circuit' should be avoided unless they are of a very low value, as there will most likely be a number of other resistive paths connected in parallel, introducing measurement errors.

Applying Ohm's Law will determine the resistance between two circuit points in circumstances where both the voltage and current can both be found. A third method is calculating the unknown value against a standard component. This method is particularly suited when it's necessary to find matched pairs of resistors. Equal value resistors will develop an equal voltages.

Providing a component type and size is chosen with adequate safety margins for working voltage and current then reliable operation can be assumed, they rarely fail by themselves. Resistors breakdown because they are stressed, causing them to overheat thereby lowering their value causing more current to pass which in turn contributes to their destruction. Resistors never become short circuit, they reduce in value and eventually burn-out.

Typical causes of overheating might be semiconductor short circuit. The resistor then acts like a fuse becoming open circuit, thereby removing the voltage supply.

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