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WFH Series

Type	Power Rating* (watts)	Resistance Range ( ohms)	Dim. A (mm)	Dim. B (mm)	Dim. L (mm)
WFH90	90	0.22 ohms - 6.8K	70	39.7	53
WFH160	160	0.47 ohms-18K	140	80	123
WFH230	230	0.82 ohms-27K	210	2x 80	193
WFH330	330	1 ohms - 39K	280	2x 100	263

*at 40°C base plate temperature

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.

FEATURES Solder, Wire and "Fast-On" Termination More resistors in one profile possible Custom wire lengths available SPECIFICATIONS Material 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. Lead wire: (wire terminal version only) XLPE, 600V, 125C, 18 AWG stranded Electrical Power Rating: 90-330W Tolerance: ±5%/ ±10% Temperature Coefficients: Normal: 50ppm - 150ppm; Low ohmic values: 400ppm Dielectric strength: 2500 VAC peak Working voltage: 1200 VAC Test voltage: 6000 VAC

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). Refer to examples in Application Notes, below.

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 + RTH3) + TH <220°C
2. WFH is mounted without a heat sink:
	Check that: TMAX = WMAX x (RTH1 + RTH2) + TAMB <220°C

Where:
W_MAX = Maximum required load in resistor
T_MAX = Maximum hot spot temperature reguested in resistor (T_MAX <220°C) The lower T_MAX the higher reliability and lifetime.
T_AMB = Ambient temperature
R_TH = Thermal resistance. Refer to table Thermal resistances
T_H = Heat sink temperature (chassis).
T = Temperature on top of the Aluminum profile.

Thermal Resistance (°C/W) between different measuring points WFH90 WFH160 WFH230 WFH330 RTH1 2 1 0.75 0.5 RTH2 6.8 3.9 2.75 2 RTH3 0.1 0.05 0.03 0.02 RTH4 0.3 0.17 0.1 0.085

WFH90L4R7KE
WFH90L10RKE
WFH90L25RJE
WFH90L50RJE
WFH90L100JE
WFH90L470JE
WFH90L750JE
WFH90L1K0JE
WFH90L2K7JE
WFH90L5K0JE
WFH160LR47KE
WFH160L1R0KE
WFH160L2R0KE
WFH160L10RKE
WFH160L27RJE
WFH160L50RJE
WFH160L75RJE
WFH160L100JE
WFH160L150JE
WFH160L250JE
WFH160L1K0JE
WFH160L5K0JE
WFH160L10KJE
WFH230L1R0KE
WFH230L2R0KE
WFH230L5R0KE
WFH230L10RKE
WFH230L27RJE
WFH230L50RJE
WFH230L75RJE
WFH230L100JE
WFH230L150JE
WFH230L250JE
WFH230L1K0JE
WFH230L1K5JE
WFH230L2K5JE
WFH330L1R0KE
WFH330L2R0KE
WFH330L10RKE
WFH330L27RJE
WFH330L50RJE
WFH330L75RJE
WFH330L100JE
WFH330L150JE
WFH330L250JE
WFH330L1K0JE
WFH330L5K0JE
WFH330L10KJE

1. WFH is mounted on a heatsink:
	A. WFH330L10RKE resistor will be used to dissipate 300W. Wakefield Engineering extruded heatsink 392-300AB will be used (no air cooling). Published RTH of this heatsink is 0.33 °C/W. Ambient temperate is 25°C.
		Temperature T is calculated as follows:   T = 300 x (0.085 + 0.33) = 125°C Temperature T seems to be reasonable, however TMAX must be also calculated to make sure it stays below 220°C:   TMAX = 300 x (0.33 + 0.02 + 0.5) +25 = 280°C WHF330L10RKE cannot dissipate 300W because inside temperature would exceed allowed 220°C
	B. WFH330L10RKE is mounted on a metal panel of unknown thermal resistance. In this 150W are applied to resistor and heatsink temperature is measured to be 120°C.
		T= 150 x 0.085 + 120 = 133°C TMAX has to be calculated to assure reliability:   TMAX = 150 x (0.5 + 0.02) + 120 = 198°C WFH330L10RKE can safely dissipate 150W because inside temperature is less than 220°C
2. WFH330L10RKE will be used without heatsink to dissipate 70W; Ambient temperature is 30°C:
		TMAX = 70 x (0.5 + 2) + 30 = 205°C WFH330L10RKE can dissipate 70W without heatsink because inside temperature would not exceed 220°C

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