Reprinted with permission from the November 2009 issue of ...
High Voltage Carbon Film Resistors
By Darryl K. Mayo, VP Sales & Marketing, Electro Technik Industries, Largo, FL
I n today’s business climate, many manufacturers arelooking for cost-effective resistors that have high reli-ability. And with the virtual extinction of the carboncomposition resistor, many of these OEMs are turning tocarbon film.Carbon film resistors are designed for high tempera-ture and high voltage applications. Theyare actually made in a vacuum by break-ing down hydrocarbon gases at high tem-peratures. This in turn forms a carbondeposit on the substrate. The process isaccomplished by heating methane orpropane gas in a process called high tem-perature pyrolysis. When the heat isapplied, molecular condensation resultsand hydrogen is released. The end resultis carbon. This is a simplified explana-tion, but the following is the chemicalequation for the process.
CH 4C + 2H2
The main advantage and reason forusing the carbon film resistor is thatthese components are designed to with-stand high voltages as well as high tem-peratures. The highest voltage thedevices can operate up to is 15kV with a nominal temper-ature of 350°C. They are available with tolerances of 2 per-cent, 5 percent, 10 percent and 20 percent. Cutting a helical grove in the carbon film createsthe resistive tolerances. The desired resistance value isachieved by regulating the pitch of the helix. The thinnerthe carbon layer, the finer the pitch and the higher theresistance value. After the helixing or spiraling is com-pleted special alloy contact caps and tinned electrolyticcopper connecting wires are pressed onto the ends of theresistor body. The final step involves coating several lay-ers of tan lacquer or using a glass film as a shield. Thisis done primarily for electrical protection, but also forshielding from the climate. to temperature.Carbon film resistors are a good choice because theyhave a small size for such high ratings and they have awide resistance range as well. The resistance can be any-where from 500 Ohms to 100M Ohms. Another plus isthat they have no outgassing. This means they will not
deposit any vapor into the environment during opera-
tion. Outgassing is a condition that causes high materi-al vapor pressure in the device to exmit contaminationdeposits into the air. It corresponds directly to the tem-perature. These deposits can cause problems with thesurrounding equipment.advantages, however, seem to outweigh most of the disad-vantages, especially in specific applications.Many ApplicationsCarbon film resistors are used in many products.Some uses are more optimal than others are, since theywithstand higher temperatures and higher voltages.They are ideal for use in mercury vapor lamps, metalhalide lamps, high pressure sodium lamps, radar, x-rays,lasers, test equipment, electrical appliances, and highvoltage power supplies.Tepro, a subsidiary of Electro Technik Industriessince 1982, was incorporated in 1958 to manufacturewirewound resistors. The company made only wire-wound resistors for 35 years; in 1993, Tepro acquiredVamistor Corp. and with it the Tenn-R product linewhich makes carbon film resistors. Now, Tepro manufac-tures, not only its traditional conformal coated and chas-sis mount wirewound resistors, but metal film resistors,carbon film, thin and thick film networks, and currentshunt resistors.Of all the different products Tepro offers, one of thefastest growing is the carbon film resistor. Our Tenn-R,carbon film resistor does not use a tan lacquer, as men-tioned previously; the Tenn-R uses a glass film to shieldthe resistor rather than a lacquer. This helps to eliminatethe outgassing problem. In addition, the Tenn-R resistorsare available with a high temperature silicone coating,where outgassing is not a concern.The carbon film resistor will continue to be theworkhorse of the resistor world. With so many benefitsto offer and so few drawbacks, it is a good choice in mostof today’s high-voltage and/or high-temperature applica-tions. It is a resistor that has a long life, and with goodreason. It operates at high voltages and high tempera-tures, but at a low cost.Contact:Tepro of Florida, Inc., P.O. Box 1260,Clearwater, FL 33757 727-796-1044 fax: 727-791-7425, Web: www.tepro-vamistor.com
Cutting a helical grove in the carbon film creates the resistive tolerances. Thedesired resistance value is achieved byregulating the pitch of the helix.Some of the other advantages are that they haveless stray capacitance and inductance, so they are betterat high frequencies. Also, they have a high stability ofperformance when compared to carbon compositionresistors and, of course, they are flame proof. Electrical NoiseElectrical noise is another factor when choosing aresistor. There are two main types of electrical noise,thermal and shot. Thermal noise is the product of theBrownian motion of ionized molecules. This noise cannotbe eliminated because it is fundamental to resistance.Carbon film resistors have less of this thermal noisethan the carbon composition resistor. The lower valuestend to be noise free while increasing with the highervalues. Shot noise usually results from the flow of elec-trons through a highly charged field. It is more prevalentin solid state devices. It is not significant in carbon filmresistors and until recently was not even known to exist,with respect to resistors.There are also some disadvantages when using car-bon film resistors. In the first place, they are limited toabout 1 percent accuracy. Secondly, they exhibit drift withtemperature and vibration. The resistors can have a TCR(temperature coefficient of resistance) range of -250 to -1000 ppm/°C, depending on the resistance value. The
Reprinted with permission from the November 2009 issue of ...
High Voltage Carbon Film Resistors
By Darryl K. Mayo, VP Sales & Marketing, Electro Technik Industries, Largo, FL
I n today’s business climate, many manufacturers arelooking for cost-effective resistors that have high reli-ability. And with the virtual extinction of the carboncomposition resistor, many of these OEMs are turning tocarbon film.Carbon film resistors are designed for high tempera-ture and high voltage applications. Theyare actually made in a vacuum by break-ing down hydrocarbon gases at high tem-peratures. This in turn forms a carbondeposit on the substrate. The process isaccomplished by heating methane orpropane gas in a process called high tem-perature pyrolysis. When the heat isapplied, molecular condensation resultsand hydrogen is released. The end resultis carbon. This is a simplified explana-tion, but the following is the chemicalequation for the process.
CH 4C + 2H2
The main advantage and reason forusing the carbon film resistor is thatthese components are designed to with-stand high voltages as well as high tem-peratures. The highest voltage thedevices can operate up to is 15kV with a nominal temper-ature of 350°C. They are available with tolerances of 2 per-cent, 5 percent, 10 percent and 20 percent. Cutting a helical grove in the carbon film createsthe resistive tolerances. The desired resistance value isachieved by regulating the pitch of the helix. The thinnerthe carbon layer, the finer the pitch and the higher theresistance value. After the helixing or spiraling is com-pleted special alloy contact caps and tinned electrolyticcopper connecting wires are pressed onto the ends of theresistor body. The final step involves coating several lay-ers of tan lacquer or using a glass film as a shield. Thisis done primarily for electrical protection, but also forshielding from the climate. to temperature.Carbon film resistors are a good choice because theyhave a small size for such high ratings and they have awide resistance range as well. The resistance can be any-where from 500 Ohms to 100M Ohms. Another plus isthat they have no outgassing. This means they will not
deposit any vapor into the environment during opera-
tion. Outgassing is a condition that causes high materi-al vapor pressure in the device to exmit contaminationdeposits into the air. It corresponds directly to the tem-perature. These deposits can cause problems with thesurrounding equipment.advantages, however, seem to outweigh most of the disad-vantages, especially in specific applications.Many ApplicationsCarbon film resistors are used in many products.Some uses are more optimal than others are, since theywithstand higher temperatures and higher voltages.They are ideal for use in mercury vapor lamps, metalhalide lamps, high pressure sodium lamps, radar, x-rays,lasers, test equipment, electrical appliances, and highvoltage power supplies.Tepro, a subsidiary of Electro Technik Industriessince 1982, was incorporated in 1958 to manufacturewirewound resistors. The company made only wire-wound resistors for 35 years; in 1993, Tepro acquiredVamistor Corp. and with it the Tenn-R product linewhich makes carbon film resistors. Now, Tepro manufac-tures, not only its traditional conformal coated and chas-sis mount wirewound resistors, but metal film resistors,carbon film, thin and thick film networks, and currentshunt resistors.Of all the different products Tepro offers, one of thefastest growing is the carbon film resistor. Our Tenn-R,carbon film resistor does not use a tan lacquer, as men-tioned previously; the Tenn-R uses a glass film to shieldthe resistor rather than a lacquer. This helps to eliminatethe outgassing problem. In addition, the Tenn-R resistorsare available with a high temperature silicone coating,where outgassing is not a concern.The carbon film resistor will continue to be theworkhorse of the resistor world. With so many benefitsto offer and so few drawbacks, it is a good choice in mostof today’s high-voltage and/or high-temperature applica-tions. It is a resistor that has a long life, and with goodreason. It operates at high voltages and high tempera-tures, but at a low cost.Contact:Tepro of Florida, Inc., P.O. Box 1260,Clearwater, FL 33757 727-796-1044 fax: 727-791-7425, Web: www.tepro-vamistor.com
Cutting a helical grove in the carbon film creates the resistive tolerances. Thedesired resistance value is achieved byregulating the pitch of the helix.Some of the other advantages are that they haveless stray capacitance and inductance, so they are betterat high frequencies. Also, they have a high stability ofperformance when compared to carbon compositionresistors and, of course, they are flame proof. Electrical NoiseElectrical noise is another factor when choosing aresistor. There are two main types of electrical noise,thermal and shot. Thermal noise is the product of theBrownian motion of ionized molecules. This noise cannotbe eliminated because it is fundamental to resistance.Carbon film resistors have less of this thermal noisethan the carbon composition resistor. The lower valuestend to be noise free while increasing with the highervalues. Shot noise usually results from the flow of elec-trons through a highly charged field. It is more prevalentin solid state devices. It is not significant in carbon filmresistors and until recently was not even known to exist,with respect to resistors.There are also some disadvantages when using car-bon film resistors. In the first place, they are limited toabout 1 percent accuracy. Secondly, they exhibit drift withtemperature and vibration. The resistors can have a TCR(temperature coefficient of resistance) range of -250 to -1000 ppm/°C, depending on the resistance value. The