Wirewound resistors may experience open windings due to overheating or stress, or short circuited windings due to accumulation of dirt, dust, breakdown of the insulation coating or high humidity. Film resistors fail for the same reasons as wirewound and composition, but have also failed due to changes in resistive material characteristics resulting in reduction and increase in resistance value. The following information is provided to assist the Investigator in evaluating resistor failures and the proper use and incorporation of resistors into a medical device.
These are guidelines only, as there are no official standards or regulations covering these areas. These are some of the factors the 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, 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 temperatures this low after incorporation into an operational device.
This is particularly true with power circuits, such as used in power supplies. Normally medical device electronic circuits are contained in some sort of 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 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. If the medical 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. Most medical devices are subject to vibration and shock 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. Potentiometers and trimerizers are three-terminal electromechanical parts that contain a resistance path with an adjustable wiper contact. Together with the normal resistance failure modes, mechanical wiper and resistive layer wear, corrosion, surface contamination and mechanical deformations can lead to intermittent wiper resistance changes, which is a problem with the audio amplifiers.
Many types are not perfectly sealed, with contaminants and moisture coming into that part; a particularly common contaminant is the soldering flux. Mechanical deformations such as damaged wiper contact may occur by overturning the casing during welding or mechanical stress during assembly. Excessive stress on the conductors may cause substrate cracking and open failure when the crack penetrates the resistive path.
Google serves cookies to analyze traffic to this site and for serving personalized ads. Improve Junction Reliability Some Users Seek to improve junction reliability in order to prevent solder cracks without compromising specifications such as rated power, or would like to increase rated power by increasing chip size without reducing junction reliability.
In contrast, wide-terminal types reduce the distance between electrodes while maintaining size. Solder cracks did not occur during actual temperature cycling tests. Resistor Sulfuration Sulfur components exist in a variety of forms in the atmosphere, such as in vehicle exhaust gases and gases emitted from hot springs. Previous Next. Electronics Basics What is a Transistor?
What is a Diode? What are SiC Power Devices? What are SiC Semiconductors? What is IGBT? What are LEDs? What is a Photointerrupter? What is a laser diode? What is a Resistor? What is Tantalum Capacitor? Current is the flow of electrons. When electrons meet resistance, as they do in a semiconductive material, they produce heat. Resistors are designed to dissipate the heat so the semiconductive material is not damaged.
When a resistor is placed under a voltage that approaches the upper limits of its power rating, the resistor generates more heat than normal. This is due to the voltage attempting to force more current electrons through the resistor than it is designed to pass.
The resistor will be hot to touch and a faint whiff of burning may be detectable. The burning odor is the breaking down of the components of the resistor: the carbon, the clay binding agent and the color code pigment painted on the resistor. When a resistor has been overloaded with voltage exceeding its power rating, the resistor will become very hot to touch, darken considerably and possibly even melt or catch on fire.
Although a resistor may appear damaged at this point, it can still be functioning.
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