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1 hour ago
Hi Guys
You cannot build a linear circuit without resistors, and there are some less known parameters that need to be considered when selecting these essential devices. For our purposes building audio circuits, we tend to use axial-lead carbon-film, metal-film, metal-oxide, wire-wound, types in different circuit locations or to achieve different sonic results.
Resistors in small-signal areas will be carbon-film or metal-film and may be 1/8W up to 600mW. Power resistors from 1W to 3W tend to be metal-oxide, with 5W+ being wire-wound. London Power does not use carbon resistors.
Resistance & Tolerance
The resistance value is the most important parameter and you will always see this on schematics. Resistance is in 'ohms', denoted by the Greek letter Omega. There are historic ranges of values that seem odd today, with newer ranges popularised as manufacturing methods improved allowing greater accuracy in achieving the expected value.
The value charts tell you how many values there are per decade. The decade is denoted as 'E' and the number of values plainly as a number. For example, E12 has 12 values per decade - not very useful. E24 has 24 values per decade allowing the huge gaps of E12 to be filled in a bit better. E96 offers the best overlap and complete coverage of common and less-common values. Within each chart, the basic values are given and these can be multiplied by any exponent (hence E) of 10 over a range that covers fractions of an ohm to millions of ohms.
With each of these value ranges, the resistance tolerances improves, becomes tighter and is represented by an ever-smaller percentage.This percentage represents how far off the value can be. For example, a 1k resistor with a 10% tolerance can be as high as 1,100-ohms or as low as 900-ohms. If its tolerance is 1%, then its value can be between 990 and 1,010 ohms.
E12 is 20%
E24 is 10%
E48 is 5%
E96 is 1%
Material
Resistors can be made from various materials that give them unique characteristics.
Cracked-carbon resistors have the worst characteristics, including: poor tolerance, high noise, high distortion, high voltage sensitivity causing distortion with varying voltage.
Carbon composition has finer particles and all of the problems of cracked carbon to a lesser extent.
Carbon-film takes the carbon issues down another level.
Metal-film has low-noise, very good temperature stability and low-distortion for varying voltage.
Metal-foil is slightly better than metal-film all around but with higher price and greater bulk.
Metal-oxide has higher power capability than metal film but is slightly noisier.
Thin-film and thick-film resistors have characteristics between carbon-film and metal-film.
Wire-wound uses a metallic element usually covered in ceramic.
Power Rating
Self-explanatory, the power rating tells how much heat the resistor body can dissipate in free air. Like most devices, most of the heat is dissipated by the component leads. Smaller body sizes will have a higher surface temperature for a given power than will a larger body size.
Some ranges of resistors with power rating of 1W+ may have shock thermal ratings allowing their continuous power rating to be exceeded briefly.
Temperature Stability
Resistors generate a small amount of heat internally, but their actual temperature rise depends also upon ambient temperature and the ability to remove the heat. the stability is listed in ppm (parts per million) per Celsius degree © or Kelvin degree (K), which are equivalent. Power resistors will tend to have poorer temperature stability than low-wattage devices, but there are exceptions at both extremes.
Voltage Rating
Yes, resistors have a voltage rating. Resistors have two leads and are constructed either axially or radially. The space between the leads has a voltage limit based on the exterior coating of the resistor and the shape of the body. Ridges on the body can increase the effective distance (creepage distance) between the leads, providing better voltage withstand.
Package
Through-hole resistors have leads arranged either axially or radially from the resistor body. The leads provide support for the device and the through-hole mounting on a PCB provides large solder connections. power devices should be elevated from the board by 6mm.
Through-hole resistors are the historic format used in hand-wired assemblies, on circuit cards, on turret boards, and on most printed circuit boards even today.
There is a type of power resistor that has its own heat sink with solder-lug leads and attachment holes. The smallest type is rated for 20W provided it is bolted to a proper sized heat sink.
Surface-mounted devices allow smaller overall circuit assemblies and are very difficult to handle manually, and thus are the mainstay of highly-automated assembly, particularly for computers, monitors, cell phones, portable devices, and generally every consumer electronic item considered to be disposable. It is an irony of our times that such sophistication is so devalued.
You cannot build a linear circuit without resistors, and there are some less known parameters that need to be considered when selecting these essential devices. For our purposes building audio circuits, we tend to use axial-lead carbon-film, metal-film, metal-oxide, wire-wound, types in different circuit locations or to achieve different sonic results.
Resistors in small-signal areas will be carbon-film or metal-film and may be 1/8W up to 600mW. Power resistors from 1W to 3W tend to be metal-oxide, with 5W+ being wire-wound. London Power does not use carbon resistors.
Resistance & Tolerance
The resistance value is the most important parameter and you will always see this on schematics. Resistance is in 'ohms', denoted by the Greek letter Omega. There are historic ranges of values that seem odd today, with newer ranges popularised as manufacturing methods improved allowing greater accuracy in achieving the expected value.
The value charts tell you how many values there are per decade. The decade is denoted as 'E' and the number of values plainly as a number. For example, E12 has 12 values per decade - not very useful. E24 has 24 values per decade allowing the huge gaps of E12 to be filled in a bit better. E96 offers the best overlap and complete coverage of common and less-common values. Within each chart, the basic values are given and these can be multiplied by any exponent (hence E) of 10 over a range that covers fractions of an ohm to millions of ohms.
With each of these value ranges, the resistance tolerances improves, becomes tighter and is represented by an ever-smaller percentage.This percentage represents how far off the value can be. For example, a 1k resistor with a 10% tolerance can be as high as 1,100-ohms or as low as 900-ohms. If its tolerance is 1%, then its value can be between 990 and 1,010 ohms.
E12 is 20%
E24 is 10%
E48 is 5%
E96 is 1%
Material
Resistors can be made from various materials that give them unique characteristics.
Cracked-carbon resistors have the worst characteristics, including: poor tolerance, high noise, high distortion, high voltage sensitivity causing distortion with varying voltage.
Carbon composition has finer particles and all of the problems of cracked carbon to a lesser extent.
Carbon-film takes the carbon issues down another level.
Metal-film has low-noise, very good temperature stability and low-distortion for varying voltage.
Metal-foil is slightly better than metal-film all around but with higher price and greater bulk.
Metal-oxide has higher power capability than metal film but is slightly noisier.
Thin-film and thick-film resistors have characteristics between carbon-film and metal-film.
Wire-wound uses a metallic element usually covered in ceramic.
Power Rating
Self-explanatory, the power rating tells how much heat the resistor body can dissipate in free air. Like most devices, most of the heat is dissipated by the component leads. Smaller body sizes will have a higher surface temperature for a given power than will a larger body size.
Some ranges of resistors with power rating of 1W+ may have shock thermal ratings allowing their continuous power rating to be exceeded briefly.
Temperature Stability
Resistors generate a small amount of heat internally, but their actual temperature rise depends also upon ambient temperature and the ability to remove the heat. the stability is listed in ppm (parts per million) per Celsius degree © or Kelvin degree (K), which are equivalent. Power resistors will tend to have poorer temperature stability than low-wattage devices, but there are exceptions at both extremes.
Voltage Rating
Yes, resistors have a voltage rating. Resistors have two leads and are constructed either axially or radially. The space between the leads has a voltage limit based on the exterior coating of the resistor and the shape of the body. Ridges on the body can increase the effective distance (creepage distance) between the leads, providing better voltage withstand.
Package
Through-hole resistors have leads arranged either axially or radially from the resistor body. The leads provide support for the device and the through-hole mounting on a PCB provides large solder connections. power devices should be elevated from the board by 6mm.
Through-hole resistors are the historic format used in hand-wired assemblies, on circuit cards, on turret boards, and on most printed circuit boards even today.
There is a type of power resistor that has its own heat sink with solder-lug leads and attachment holes. The smallest type is rated for 20W provided it is bolted to a proper sized heat sink.
Surface-mounted devices allow smaller overall circuit assemblies and are very difficult to handle manually, and thus are the mainstay of highly-automated assembly, particularly for computers, monitors, cell phones, portable devices, and generally every consumer electronic item considered to be disposable. It is an irony of our times that such sophistication is so devalued.
