audioguru Posted February 15, 2010 Report Share Posted February 15, 2010 why do I need 3 Transistors for a 5 amp supply?I checked the datasheet for the MJ11015, and it states it can handle 200W.From my calculations, I get : 30V * 5 Amps = 150 W...I should say one transistor is enough...It can dissipate 200W if its case is cooled to 25 degrees C somehow (liquid nitrogen spraying on a huge heatsink?).Your math is wrong. The load dissipates 150W not the output transistor.The 30VAC transformer has a peak voltage of 42.4V. The rectifier bridge drops 2V and the main filter capacitor might have ripple as low as 1V. The current-sense resistor has 1.4V. Then the output transistor has a collector voltage of 38V and dissipates 38V x 5A= 190W.The heat from the transistor will heat the heatsink so the case of the transistor will be hotter than 25 degrees C so you must reduce the power to keep the transistor from over-heating.With a huge heatsink, one 200W transistor will be at its max temperature dissipating 120W. With a huge heatsink and a noisy fan the transistor might survive power of 150W.But do you want to risk operating one transistor at its absolute max rating when adding a second or 3rd transistor is best? Quote Link to comment Share on other sites More sharing options...
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