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Power Transistor in Power Supply

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cc1960:
Hi

Can somebody explain to me how the TIP2955/MJE3055 is helping to regulate this powersupply?

http://www.eleccircuit.com/5v-3a-regulate-power-supply-by-tip2955/
or
http://www.reuk.co.uk/High-Current-Voltage-Regulation.htm


I think I understand every other aspect of the circuits

How can it be that the output from TIP2955/MJE3055 is regulated?


Christian

ejeffrey:
The regulator draws a current through R1 that sets the transistor Vbe, which in turn sets the collector current that is transmitted to the load.  If the output voltage is too low, the 78XX draws more current, Vbe increases, and more current is supplied to the load.  If the output voltage is too high, the transistor is turned down.  Thus, the LM78XX controls the voltage, but only up to about (0.6 V / R1) goes through the regulator, the rest goes through the external pass transistor.

cc1960:
Thanks for the explanation.

That makes sense.

Simon:
in the first link the comment about just adding more transistors in parallel for more current is not quite correct. each additional transistor will need a resistor to make the transistors current share. BJT's unlike mosfet won't current share, as one gets a bit hotter it will let more current through and the effect will snow ball until magic smoke comes to the scene.

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