Solid-State Switching Devices for Resistive Loads Solid-State Relays 3RF21 solid-state relays, single-phase, 22.5 mm $ % & ! $ ' & $ ( ( ) ) Type current 70 A (3RF21 70, 3RF20 70) ! " $ % & !
$ ' & $ ( ( * ) ) * ! " Type current 90 A (3RF21 90, 3RF20 90) Arrangement example Given conditions: I e = 20 A and T a = 40 C. The task is to find the thermal resistance R thha and the heat sink overtemperature dT ha . From the diagram on the left → P M = 28 W, from the diagram on the right → R thha = 2.0 K/W. This results in: At dT ha = 56 K the heat sink must therefore have an R thha = 2.0 K/W.
dT ha = R thha Ч PM = 2.0 K/W Ч 28 W = 56 K. 13 Siemens · 2009