Ohmite’s new flat core winding
technology allows for wire-
wound heatsinkable resistors
affording a very low profile,
and superior thermal transfer
characteristics when compared
to conventional aluminum
housed wirewound resistors.
Close mounting of heat sensi-
tive components is possible
due to only a slight rise of the
temperature on the aluminum
profile.
S P E C I F I C A T I O N S
Power rating: 90W-330W
Resistance 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
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.
WFH Series
Aluminum Housed Wirewound Power
12.6
0.8
L
6.3
ø4.5
8
13.6
Amp
terminal
41 51
No heat sink compound is
required because of large
mounting surface.
15
F E A T U R E S
• Solder, wire and “Fast-On”
Termination
• More resistors in one profile pos-
sible
B
A
10
• Custom wire lengths available
P O W E R D I S S I P A T I O N
300
350
4.8
ø2.2
0.6
300
WFH330
250
6
200
WFH230
150
WFH160
100
WFH90
50
Lug
terminal
Wire
terminal
0
40°
60°
80°
100°
120°
140°
Heat Sink Temperature, °C
This graph shows the maximum wattage rating for each possible resistor
of standard size corresponding to the heat sink temperature. It is as-
sumed that all resistors are equally loaded.
Power
Resistance
Range
(Ω)
0.22Ω – 6.8K
0.47Ω-18K
0.82Ω-27K
1Ω - 39K
Rating*
Dimension (mm)
Type
WFH90
WFH160
WFH230
WFH330
(watts)
90
160
230
330
A
B
L
70
140
210
39.7
80
2x 80 193
53
123
T H E R M A L R E S I S T A N C E S
resistor
Thermal Resistance (°C/W) between different
measuring points
surface to
heat sink
aluminum
housing
to air
280 2x 100 263
R
TH
4
WFH90 WFH160 WFH230 WFH330
*at 40°C base plate temperature
R
TH
2
R
R
R
R
2
1
0.75
2.75
0.03
0.1
0.5
2
0.02
0.085
TH
1
TH
2
TH
3
TH
4
D E S I G N I N G
6.8
0.1
0.3
3.9
0.05
0.17
R
TH
1
R
TH
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.
3
wire to
aluminum
housing
aluminum
housing to
heat sink
O R D E R I N G I N F O R M AT I O N
1. WFH is mounted on a heat sink:
RoHS
A. The thermal resistance RTH of the heat sink is known,
Compliant
� ꢀ ꢁ ꢂ ꢃ ꢄ ꢅ ꢆ ꢇ ꢈ ꢉ ꢊ
T = WMAX x (RTH4 + RTH
)
Tolerance
J = 5%
K = 10%
Check that:
Series
Wattage
Terminal Type
Ohms
at 40°C
L = lug terminals R47= 0.47Ω
TMAX = WMAX x (RTH + RTH3 + RTH1) + TAMB <220°C
base plate A = amp terminals
temp.
W= 30cm insulated wire cable 18AWG
B. The Temperature of the Heat Sink is known,
T = WMAX x RTH4 + TH
Check product availability at www.ohmite.com
S TA N D A R D P A R T N U M B E R S F O R W F H S E R I E S
Check that:
TMAX = WMAX x (RTH1 + RTH3) + TH <220°C
2. WFH is mounted without a heat sink:
Check that:
WFH90L4R7KE WFH160LR47KE WFH160L1K0JE WFH230L100JE WFH330L50RJE
WFH90L10RKE WFH160L1R0KE WFH160L5K0JE WFH230L150JE WFH330L75RJE
WFH90L25RJE WFH160L2R0KE WFH160L10KJE WFH230L250JE WFH330L100JE
WFH90L50RJE WFH160L10RKE WFH230L1R0KE WFH230L1K0JE WFH330L150JE
WFH90L100JE WFH160L27RJE WFH230L2R0KE WFH230L1K5JE WFH330L250JE
WFH90L470JE WFH160L50RJE WFH230L5R0KE WFH230L2K5JE WFH330L1K0JE
WFH90L750JE WFH160L75RJE WFH230L10RKE WFH330L1R0KE WFH330L5K0JE
WFH90L1K0JE WFH160L100JE WFH230L27RJE WFH330L2R0KE WFH330L10KJE
WFH90L2K7JE WFH160L150JE WFH230L50RJE WFH330L10RKE
TMAX = WMAX x (RTH1 + RTH2 ) + TAMB <220°C
Where:
WMAX = Maximum reguired load in resistor
TMAX = Maximum hot spot temperature reguested in resistor (TMAX
<220°C) The lower TMAX the higher reliability and lifetime.
TAMB = Ambient temperature
RTH = Thermal resistance. Refer to table Thermal resistances
TH
T
= Heat sink temperature (chassis).
= Temperature on top of the Aluminum profile.
WFH90L5K0JE WFH160L250JE WFH230L75RJE WFH330L27RJE
30
Ohmite Mfg. Co. 1600 Golf Rd., Rolling Meadows, IL 60008 • 1-866-9-OHMITE • Int’l 1-847-258-0300 • Fax 1-847-574-7522 • www.ohmite.com • info@ohmite.com
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