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THERMAL PARAMETERS |
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Heat generated by SR Series Resistors is
dissipated mainly by radiation and convection from the exposed surface
areas. Within restricted domains, mathematical models may be employed to
permit heat transfer estimations. |
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Symbols |
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∆T |
= Temperature
Rise (°C) |
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Wa |
= Watts / Unit Exposed Surface Area (W.cm
-2) |
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v |
= Volume / Disc
(cm
3) |
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cm |
= Specific Heat Capacity of Active Material = 2J. cm
-3. °C
-1 |
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Do |
= Disc Outside
Diameter (cm) |
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t |
= Resistor Thermal Time Constant (s) |
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Radiation
and Convection
Wa
= 0.00026 (∆T) 1.4
(∆T
= 50 °C to 175 °C, Do = 10 mm to 151
mm, Ambient 25 °C) |
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Thermal
Conductivity
0.04 W / cm2. °C / cm |
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Maximum
Insertion Energy Ratings
For a Resistor
initially at 25 °C : 350 Joules / cm3
(Infrequently)
For a Resistor
initially at 25 °C : 250 Joules / cm3
(Continuously) |
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Recommended
Operating Temperatures
200
°C (Infrequent Operation)
150 °C (Continuous Operation) |
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Temperature
Rise from Energy Injection
∆T (°C) = Joules (per Resistor) /
(v x cm)
(Free Air) |
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Thermal Time
Constant
t
(s) = Max Joules @ 25
°C / Max Watts @ 25
°C |
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Full Cooling
≥ 4
t |
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De-rating for other ambient
Temperatures (Ta
°C)
Multiply
Max Joules @ 25 °C & Max Watts @ 25
°C by the ratio (150 - Ta) / 125 |
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Repetitive
Thermal Impulsing
Assuming that
the Heat Transfer Coefficient α (W / cm 2 .
°C / cm) is constant over the operating temperature range, then the Peak
temperature Rise (∆Tp) associated with repetitive impulsing can be
estimated by way of reference to a classical geometric progression: |
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If |
∆Tp (°C) = ∆T x (
1 - (e - (
t
/
t
)
)n
) /
( 1 - e - ( t /
t
)
)
............... 1 |
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Where |
∆T is the Temperature Rise associated with each
electrical impulse (°C)
t
is the Resistor
Thermal Time Constant (s)
t is the
Repetition Rate (s)
n is the number
of impulses |
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If the number of impulses (n)
→
∞ (i.e. continuous duty), then equation 1
can be simplified thus: |
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∆Tp (°C) = ∆T / (
1 - e
- (
t
/
t
)
)
............... 2 |
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