These are guidelines only. These are some of the reliability factors the electronic device manufacturer should consider during the design phase and if not considered, could easily lead to a defective device. When evaluating the proper use of resistors in a design, temperature is one of the most important considerations as overheating is the major cause of resistor failure. The effect of too much heat is usually not immediate, but if sustained usually results in a deterioration over a period of time until at some point the resistor fails, usually resulting in an open circuit. If the resistor is a critical component in a device, it may result in catastrophic failure of the device into which it is incorporated.

In addition to contributions from the environment, resistors generate their own internal heat because they present a resistance to current flow. This internal heat is an energy or power loss which the resistor absorbs and dissipates. The energy loss is measured in “watts” and each resistor is rated in watts according to how much power it can safely dissipate. This “power rating” is usually established at an ambient temperature (usually 25 C) and takes into consideration how much the internal temperature of the resistor will rise with rated power applied.

Although most electronic component manufacturers specify their product’s electrical parameters at 25 C very few components actually operate at this low temperatures after incorporation into an operational device. This is particularly true with power circuits, such as used in power supplies. In finished electronic devices, normally electronic circuits are arranged in some sort of small enclosure. The combined heating effects of all the circuit components inside the enclosure soon raises the internal air temperature well above 25 C. Often the resistor is the major contributor of this heat, especially when large power resistors are used when power supplies are a part of the device.

When resistors are required to carry substantial currents they should be positioned with consideration given to the effects their self-generated heat will have on neighboring components. The heat from a hot resistor may cause a neigh-boring borderline component to fail prematurely.

Power resistors, which must dissipate a lot of heat, must be properly “heat sinked” and situated at the edge/outer side of the board, so cooling air circulates freely about the resistors.

Heat-sinks are usually “fluked” or “vaned” metal fixtures on which components are mounted to aid in removal of heat from the device by conduction. Sometimes components are mounted directly onto the device’s metal chassis and the chassis acts as the heat sink. Sometimes a cooling fan is necessary in addition to the heat sinks. Preferably, resistors should be mounted so that dissipated heat can be immediately exhausted, and not blown across other components.

An electronic component that operates in a cool environment will last much longer than a hot component and the device’s reliability will be improved.

When power supplies are incorporated into a device or high voltages are generated, “heat distribution” studies should be made within the device enclosure during the prototype design stage. If hot spots or excessive temperatures are measured, cooling fans, air vents, power supplies, etc., should be located properly to eliminate the adverse conditions.

If the finished electronic device is to be used in an operating room, where explosive gases are used, flammability of resistors may be an important factor to consider. If they get hot enough, some resistors will actually burst into flame. An example is the carbon composition resistors which are used in all electronic devices. If flammability is a factor, the designer should specify a requirement for flammability resistance when ordering components.

All electronic components, including resistors, should be mounted so that they are restrained from movement relative to the selected mounting base. When electronic devices are subject to vibration, and if not mounted securely, components can short circuit against neighboring components or leads and connections may be weakened or broken. If components designed to be mounted horizontal to the mounting surface must be stood on end, the leads should be insulated to prevent short circuits. Components should also be mounted to preclude collection of dirt and moisture between conductors that might result in short circuits.

Electrical parameter changes due to other environmental variations and aging must be taken into account when designing an electronic device. Fluctuations may cause the restricted tolerances of a critical component to drift out of prescribed limits resulting in the device performance varying outside its operating limits.

The resistor is a simple component in that it performs no active function, and it has been historically the most reliable component used in electrical circuitry. But in the past few years, due to the economic situation and increases in cost of materials, a multitude of resistive materials have been introduced for use in resistors, especially thick and thin film. Often the user does not know the identity of the materials used, as some are proprietary. All resistors can not be expected to function reliably unless their reliability has been established through long use in the selected application or through extensive qualification and testing.