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WFH Series

WFH Series
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Aluminum Housed Wirewound Power Ohmite’s new flat core winding technology allows for wirewound heatsinkable resistors affording a very low profile, and superior thermal transfer characteristics when compared to conventional aluminum housed wirewound resistors. Close mounting of heat sensitive components is possible due to only a slight rise of the temperature on the aluminum profile. No heat sink compound is required because of large mounting surface. • Solder lug, wire, and “Fast-on” amp terminations • Multiple resistors in one profile possible • Custom wire lengths available *at 40°C base plate temperature Power Dissipation Power rating 90W-330W Resistance ±5%, ±10% tolerance Dielectric 2500 VAC peak strength Temperature Normal: 50ppm - 150ppm Coefficients Low ohmic values: 400ppm Test voltage 6000 VAC Lead wire (wire terminal version only): XLPE, 600V, 125C, 18 AWG stranded Insulation Silicone Rubber & Mica. The Silicone is UL-recognised (UL 94 HB) to a working temperature of 220°C. Temperatures of up to 300°C can be endured for shorter periods. This may however cause an expansion of the silicone rubber with a possibility of reducing the dielectric strength. Min. Storage & -40˚C Operating Temp. This graph shows the maximum wattage rating for each possible resistor of standard size corresponding to the heat sink temperature. It is assumed that all resistors are equally loaded. 80° 100° 120° Heat Sink Temperature, °C Thermal Resistances Thermal Resistance (°C/W) between different measuring points resistor surface to heat sink aluminum housing to heat sink wire to aluminum housing THIS PRODUCT IS DESIGNED FOR USE WITH PROPER HEATSINKING. Maximum base plate temperature of the resistor must be monitored and kept within specified limits to establish the power rating. Best technique is to attach a thermocouple to the side of the base plate of the resistor. Temperature of plastic housing or heat sink cannot be used to establish rating of the resistor.

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Aluminum Housed Wirewound Power d e s i g n i nfo r m a t i on The following equations are applied in the dimensioning of the resistors at stationary load. If more information is required please consult Ohmite. It is assumed that the air around the resistors is stationary (worst case). See ohmite.com for more examples. 1. WFH is mounted on a heat sink: A. The thermal resistance RTH of the heat sink is known, T = WMAX x (RTH4 + RTH) Check that: TMAX = WMAX x (RTH + RTH3 + RTH1) + TAMB <220°C B. The Temperature of the Heat Sink is known, T = WMAX x RTH4 + TH Check that: TMAX = WMAX x (RTH1 +...

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