The maximum power rating of TO-style power film resistor is specified with the case temperature (TC) at 25°C. This is the same method proven and established by the power semiconductor industry. Case temperature is the temperature measured at the center of the resistor mounting surface which is in contact with the heat sink, while the resistor is under electrical load. Case temperature is different from the molded body temperature, the tab temperature, the lead temperature, or the ambient temperature (as shown in Figure 1).
Using case temperature we can determine the temperaure of the resistor film (TJ). This is crucial factor since early device failures are usually traced to excessive temperatures of the resistor film. Excessive film temperatures will cause a drift of the resistance value or reduced component life. Proper thermal design followed by temperature measurements to verify the design, and consistent mounting procedures will avoid these problems.
Due to variations in the mating surfaces between the resistor package and the heat sink air voids are created. These voids will substantially degrade the performance of TO-Style device. Thus, it is important to use a thermal interface material to fill these air voids. Several materials are available to reduce thermal resistance between the resistor and heat sink surface.
Proper hardware is an extremely important consideration in a good thermal design. The hardware must maintain firm even pressure on the device, through thermal cycling, without deforming the heat sink or the device.
Direct Electronics TO-Style power film heat sink mountable resistors, TO-220 and TO-247 Style Packages, are designed for intermediate power applications and combines performance with an economical price.
TO-220 Power Resistors, TO-247 Power Resistors RMG series are ultra precision and high power resistors encapsulated in the TO-220, TO-247 style power package. Power resistors are manufactured in 20W, 30W, 35W, 50W and 100W. Resistance element is electrically insulated from metal heat sink mounting tab. When properly mounted Direct's RMG** TO220/TO247 packaged power resistors provide up to 50/100 watts of steady state power. These very low inductance resistors are ideal for many industrial applications: power supplies, power controls and inrush/bleeder resistors.
Download Entire Heat Sink Mountable TO-220, TO-247 Power Resistors Catalogue in PDF file (628KB).
Direct's TO-Style Series satisfy demanding applications for accurate and stable power resistors housed in the convenient TO-Style case.
The resistance element is isolated from the mounting tab by an alumina ceramic layer, providing very low thermal resistance and ensuring high insulation resistance between terminals and tab.
These isolated resistor element are constructed and packaged in a high temperature plastic case with a single screw metal tab for easy mounting to the heat sink. The non-inductive design makes these products especially useful in high frequency and high speed pulse applications.
Direct's TO-Style resistors are designed for use in pulse-loading applications, as bleeder or snubber resistors in switching power supplies, industrial power drives, medical, test equipment, high power equipment such as uninterruptible power supplies (UPS), and other power distribution and power conversion applications.
The Power Film Resistors use an optimized process of Direct's thick film technology on an alumina substrate to achieve tolerances as low as ± 0.5 %, and up to ± 10 %. The Non-Inductive design and resistance values as low as 0.05 ohms are also ideal for current sensing applications.