RESISTORS – ELECTRONICS
Saturday, July 17, 2010
What is resistance?
Resistance is one of three basic quantities in electrical or electronic circuits:
Current is the flow of electrons through a circuit. It is the principle quantity because it does work, and accomplishes the desired results. We measure current in Amperes.
Voltage is the force that causes current to flow in a circuit. In fact, we sometimes call voltage "electromotive force" or "EMF." We measure it in Volts.
Resistance controls the flow of current. We measure it in Ohms.
These three quantities are so basic to electrical and electronic circuits, a simple equation called Ohms Law relates them.
What are resistors?
Resistors are electronic devices that resist the flow of electrical current. A resistor obeys Ohm's Law, which states that the voltage (or potential) across a resistor is proportional to the current flow through the resistor. In formula form, V=IR, where V is the voltage across the resistor, R is the value of the resistor in ohms, and I is the current flow through the resistor in amperes (amps).
What is Ohm's Law?
Ohm's Law states that the current flowing in a circuit is proportional to the voltage applied to the circuit, and inversely proportional to the resistance of the circuit. In other words, for a given voltage, the current in the circuit will decrease as the resistance increases. Mathematically, Ohm's Law is: I = V/R, or current equals voltage divided by resistance. This simple equation can be transformed to find voltage or resistance, given the other two quantities. (For example, if you know the voltage and current in a circuit, you can calculate the circuit resistance by dividing the voltage by the current.)
What are the major design considerations for using resistors?
The designer / salesman must take into account the following parameters;
1. Circuit minimum and maximum operational characteristics i.e. Power, voltage, current, other component values, duration of pulses (if any) and frequency.
2. Environment into which the parts are to be fitted. Temperature, humidity, mounting type and relation to other heat sensitive/generating parts.
3. Short term / long term requirement in terms of electrical performance. Stability / life etc.
4. Design criteria, safety specifications and the level to which the part should be approval tested.
5. Cost band of resistor versus other alternatives if there are any.
6. Connection and terminal configuration.
A decision made against each of the above questions will reduce the available product list until the most suitable is left. Unfortunately, a recommendation often has to be made balancing a number of undesirable parameters. This is where good product knowledge will enable the best practical decision for the Customer. To specification and budget.
Resistance Tolerance
Resistor tolerance is the deviation from the nominal value in any production run. It is expressed as a ±%, measured at 25°C with no load applied. Some resistor designs have extremely tight tolerances. For example, precision wirewound resistors are made with tolerances as close as ±0.05%. Film resistors typically have tolerances of ±1% to ±5%, however, very close tolerance High Voltage resistors are available for special applications. In applications like precision voltage dividers and networks, the designer should consider resistor sets matched for resistance or ratio tolerances. Often, these matched sets save cost over buying individual resistors with very tight resistance tolerances.
Temperature Coefficient of Resistance (TCR)
Temperature Coefficient of Resistance (TCR) specifies the maximum change in resistance in response to change in temperature, it is expressed as “parts per million per degree Centigrade” (ppm/°C). A wide range of TCRs are available to the designer (typically from ±5 ppm/°C to ±6700 ppm/°C in very low values) for specific applications.
Specifying TCR is important in applications where the change in resistance with temperature changes must be small. Equally important may be applications where a specific TCR is required (temperature compensation circuits for example). Typically, there are two contributors to temperature-related resistance changes; the resistor’s temperature increases as it dissipates power and also, the resistor’s temperature is affected by the ambient temperature.
Often matching TCRs for pairs or sets of resistors is more important than the actual TCR itself. In these cases, matched sets are available which assure that resistance values of the set track in the same magnitude and direction as operating temperature changes.
Power Rating
Power ratings are normally specified the maximum dissipation rated for a device under reference conditions, normally +25°C. this will be derated as ambient temperature increases, a graph or curve is often used to represent this relationship. Since these parameters are application dependent, power derating curves should be considered general rather than absolute. Power ratings are based on many factors. The safest designs use the largest physical size operating at conservative temperatures and power ratings.
Temperature Rating
Temperature rating is usually the maximum internal operating temperature of the resistor. An operating temperature range is often specified: for example, -55°C to +200°C. Internal resistance figure are available for calculation. However, it is recommended that reasonable safety margins are applied to maximise reliability and stability in application.
Frequency Response and Rise Time
Frequency response relates to the change in impedance with frequency, caused by reactive components from the resistors inductance and capacitance. Rise time is an associated parameter, relating the resistors response to a step or pulse input. Wirewound designs use special winding techniques to minimize reactive components. Typical reactive values for these special designs are less than 1µh for a 500 ohm resistor, and less than 0.8 pf capacitance for a 1 megohm resistor. A typical fast rise time resistor has a rise time of 20 nsec or less.
Aryton-Perry Windings
In Aryton-Perry windings, a layer of wire turns is first wound in one direction. After a layer of insulation is applied the next winding is wound in the opposite direction with the turns crossing every 180 degrees over the turns in the lower layer. The net effect of this is that the magnetic forces that are responsible for creating ‘reactance’ are equal and opposite – thereby cancelling each other.
Stability
Stability is defined as the repeatability of resistance of a resistor when measured at a referenced temperature and subjected to a variety of operating and environmental conditions over time. Stability is difficult to specify and measure since it is application dependent. Experience with practical circuits has given us some guidelines; Wirewound and bulk metal designs are best, while designs using composition materials are less stable. For highest resistance stability, it is best to derate critical resistors to generate limited temperature rise.
Voltage Coefficient
Voltage coefficient is the change in resistance with applied voltage. It is normally associated with carbon composition and carbon film resistors, and is a function of the resistor’s value and its composition. In terms of the our High Voltage thick film range of devices, whilst not of the above technologies, the voltage range is so great (up to 150kV), this should be considered as a design issue.
Noise
Noise does not effect the resistor's value, but can generate circuit errors in high gain and sensitive circuits. Wirewound and metal film resistors are best: carbon composition and film have high noise potential. Consideration should be given in applications where there is an EMC (Electro-magnetic Compatibility) consideration.
Thermocouple Effect
The thermocouple effect generates a thermal emf at the junction of two dissimilar metals. In resistors, it is caused by the materials used in leads and the resistive element. It is normally insignificant, but may be important in high gain or critically balanced circuits and low value resistors. Thermal EMF is minimised by keeping the resistor leads and body at the same temperature.
Reliability
Reliability is the statistical probability that a resistor will perform its function. Normally, it is specified as Failure Rate per 1,000 Hours of Operation. Various statistical studies are used at arriving at these failure rates by testing large samples. Reliability is seldom defined for commercial products, but is a common requirement for critical designs such as in aerospace applications.
SOURCE:
http://www.articlealley.com/article_624684_10.html
Resistance is one of three basic quantities in electrical or electronic circuits:
Current is the flow of electrons through a circuit. It is the principle quantity because it does work, and accomplishes the desired results. We measure current in Amperes.
Voltage is the force that causes current to flow in a circuit. In fact, we sometimes call voltage "electromotive force" or "EMF." We measure it in Volts.
Resistance controls the flow of current. We measure it in Ohms.
These three quantities are so basic to electrical and electronic circuits, a simple equation called Ohms Law relates them.
What are resistors?
Resistors are electronic devices that resist the flow of electrical current. A resistor obeys Ohm's Law, which states that the voltage (or potential) across a resistor is proportional to the current flow through the resistor. In formula form, V=IR, where V is the voltage across the resistor, R is the value of the resistor in ohms, and I is the current flow through the resistor in amperes (amps).
What is Ohm's Law?
Ohm's Law states that the current flowing in a circuit is proportional to the voltage applied to the circuit, and inversely proportional to the resistance of the circuit. In other words, for a given voltage, the current in the circuit will decrease as the resistance increases. Mathematically, Ohm's Law is: I = V/R, or current equals voltage divided by resistance. This simple equation can be transformed to find voltage or resistance, given the other two quantities. (For example, if you know the voltage and current in a circuit, you can calculate the circuit resistance by dividing the voltage by the current.)
What are the major design considerations for using resistors?
The designer / salesman must take into account the following parameters;
1. Circuit minimum and maximum operational characteristics i.e. Power, voltage, current, other component values, duration of pulses (if any) and frequency.
2. Environment into which the parts are to be fitted. Temperature, humidity, mounting type and relation to other heat sensitive/generating parts.
3. Short term / long term requirement in terms of electrical performance. Stability / life etc.
4. Design criteria, safety specifications and the level to which the part should be approval tested.
5. Cost band of resistor versus other alternatives if there are any.
6. Connection and terminal configuration.
A decision made against each of the above questions will reduce the available product list until the most suitable is left. Unfortunately, a recommendation often has to be made balancing a number of undesirable parameters. This is where good product knowledge will enable the best practical decision for the Customer. To specification and budget.
Resistance Tolerance
Resistor tolerance is the deviation from the nominal value in any production run. It is expressed as a ±%, measured at 25°C with no load applied. Some resistor designs have extremely tight tolerances. For example, precision wirewound resistors are made with tolerances as close as ±0.05%. Film resistors typically have tolerances of ±1% to ±5%, however, very close tolerance High Voltage resistors are available for special applications. In applications like precision voltage dividers and networks, the designer should consider resistor sets matched for resistance or ratio tolerances. Often, these matched sets save cost over buying individual resistors with very tight resistance tolerances.
Temperature Coefficient of Resistance (TCR)
Temperature Coefficient of Resistance (TCR) specifies the maximum change in resistance in response to change in temperature, it is expressed as “parts per million per degree Centigrade” (ppm/°C). A wide range of TCRs are available to the designer (typically from ±5 ppm/°C to ±6700 ppm/°C in very low values) for specific applications.
Specifying TCR is important in applications where the change in resistance with temperature changes must be small. Equally important may be applications where a specific TCR is required (temperature compensation circuits for example). Typically, there are two contributors to temperature-related resistance changes; the resistor’s temperature increases as it dissipates power and also, the resistor’s temperature is affected by the ambient temperature.
Often matching TCRs for pairs or sets of resistors is more important than the actual TCR itself. In these cases, matched sets are available which assure that resistance values of the set track in the same magnitude and direction as operating temperature changes.
Power Rating
Power ratings are normally specified the maximum dissipation rated for a device under reference conditions, normally +25°C. this will be derated as ambient temperature increases, a graph or curve is often used to represent this relationship. Since these parameters are application dependent, power derating curves should be considered general rather than absolute. Power ratings are based on many factors. The safest designs use the largest physical size operating at conservative temperatures and power ratings.
Temperature Rating
Temperature rating is usually the maximum internal operating temperature of the resistor. An operating temperature range is often specified: for example, -55°C to +200°C. Internal resistance figure are available for calculation. However, it is recommended that reasonable safety margins are applied to maximise reliability and stability in application.
Frequency Response and Rise Time
Frequency response relates to the change in impedance with frequency, caused by reactive components from the resistors inductance and capacitance. Rise time is an associated parameter, relating the resistors response to a step or pulse input. Wirewound designs use special winding techniques to minimize reactive components. Typical reactive values for these special designs are less than 1µh for a 500 ohm resistor, and less than 0.8 pf capacitance for a 1 megohm resistor. A typical fast rise time resistor has a rise time of 20 nsec or less.
Aryton-Perry Windings
In Aryton-Perry windings, a layer of wire turns is first wound in one direction. After a layer of insulation is applied the next winding is wound in the opposite direction with the turns crossing every 180 degrees over the turns in the lower layer. The net effect of this is that the magnetic forces that are responsible for creating ‘reactance’ are equal and opposite – thereby cancelling each other.
Stability
Stability is defined as the repeatability of resistance of a resistor when measured at a referenced temperature and subjected to a variety of operating and environmental conditions over time. Stability is difficult to specify and measure since it is application dependent. Experience with practical circuits has given us some guidelines; Wirewound and bulk metal designs are best, while designs using composition materials are less stable. For highest resistance stability, it is best to derate critical resistors to generate limited temperature rise.
Voltage Coefficient
Voltage coefficient is the change in resistance with applied voltage. It is normally associated with carbon composition and carbon film resistors, and is a function of the resistor’s value and its composition. In terms of the our High Voltage thick film range of devices, whilst not of the above technologies, the voltage range is so great (up to 150kV), this should be considered as a design issue.
Noise
Noise does not effect the resistor's value, but can generate circuit errors in high gain and sensitive circuits. Wirewound and metal film resistors are best: carbon composition and film have high noise potential. Consideration should be given in applications where there is an EMC (Electro-magnetic Compatibility) consideration.
Thermocouple Effect
The thermocouple effect generates a thermal emf at the junction of two dissimilar metals. In resistors, it is caused by the materials used in leads and the resistive element. It is normally insignificant, but may be important in high gain or critically balanced circuits and low value resistors. Thermal EMF is minimised by keeping the resistor leads and body at the same temperature.
Reliability
Reliability is the statistical probability that a resistor will perform its function. Normally, it is specified as Failure Rate per 1,000 Hours of Operation. Various statistical studies are used at arriving at these failure rates by testing large samples. Reliability is seldom defined for commercial products, but is a common requirement for critical designs such as in aerospace applications.
SOURCE:
http://www.articlealley.com/article_624684_10.html