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CHAPTER 4 - Active components > 4.3 BIPOLAR TRANSISTORS - Pg. 165

4.3 Bipolar transistors 165 equation is a safety factor. Remember to check the resistor's power rating, allowing for pulse derating. The diode should have the same voltage rating as the triac or thyristor, but the half-cycle surge current rating need only be two or three times I L as it only conducts for a short period at each turn-off. A triac is controlling a 1-kW cartridge heater at 240 V. Assume the heater cold resistance is roughly one-tenth its hot resistance, which translates to an R L of 6 U and a peak turn-on current of 56 A. The triac could be a TIC226M, which has a non-inductive dv/dt rating of 500 V/ms and an I TSM of 80 A. This gives a C of ½0:63 Â 340=500 Â 6 ¼ 0:07 mF; so use the next highest value of 0.1 mF. R can be calculated from the first equation to be ½340=ð80 À 56Þ Â 0:5 ¼ 28:3 U Using a 27-U resistor would give a di/dt of 4.7 A/ms. Because R is significantly higher than R L , use of a parallel diode and putting the snubber in a bridge would be advisable. Note that the selection of triac can have a large effect on the required values of the snubber components. For instance, a larger device, though unnecessary from the strict applications point of view, would have a larger I TSM and could therefore get away with a lower R, which in turn might obviate the need for a diode. However, this might increase the stress on the capacitor which would need