                                                    THERMAL PARAMETERS

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.

 Symbols ∆T = Temperature Rise (°C) Wa = Watts / Unit Exposed Surface Area (W.cm -2) v = Volume / Disc (cm 3) cm = Specific Heat Capacity of Active Material = 2J. cm -3. °C -1 Do = Disc Outside Diameter (cm) t = Resistor Thermal Time Constant (s)

Wa = 0.00026 (∆T) 1.4

(∆T = 50 °C to 175 °C, Do = 10 mm to 151 mm, Ambient 25 °C)

Thermal Conductivity

0.04 W / cm2. °C / cm

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)

Recommended Operating Temperatures

200 °C (Infrequent Operation)

150 °C (Continuous Operation)

Temperature Rise from Energy Injection

∆T (°C) = Joules (per Resistor) / (v x cm)  (Free Air)

Thermal Time Constant

t (s) = Max Joules @ 25 °C / Max Watts @ 25 °C

Full Cooling

≥ 4 t

De-rating for other ambient Temperatures (Ta °C)

Multiply Max Joules @ 25 °C & Max Watts @ 25 °C by the ratio (150 - Ta) / 125

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:

 If ∆Tp (°C) = ∆T x ( 1 - (e - ( t / t ) )n ) / ( 1 - e - ( t / t ) ) ............... 1 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 If the number of impulses (n) → ∞ (i.e. continuous duty), then equation 1 can be simplified thus: ∆Tp (°C) = ∆T / ( 1 - e - ( t / t ) ) ............... 2

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