Electronics > Beginners

Linear regulator boost with NPN transistor

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sureshot:
I think the npn collector should be the regulated input potential, not sure how there's a third terminal. I didn't notice that until you mentioned it. My full rectifier voltage multiplier mock up had the two terminals from the capacitors + - one from each capacitors.

Zero999:

--- Quote from: not1xor1 on July 10, 2018, 03:09:58 pm ---
--- Quote from: Hero999 on July 09, 2018, 10:43:42 am ---Yes, the base emitter voltage increases the drop-out voltage and worsens the regulation.

No base resistor is required, because the base current is self-limiting. When the transistor turns on, the voltage at the emitter rises, reducing the potential difference between the base-emitter junction, therefore the base current, until an equilibrium is reached. This is an example of negative feedback. Look up emitter follower.

An NPN transistor can be  used, with a PNP transistor, as per the schematic I posted in the other thread.
https://www.eevblog.com/forum/beginners/voltage-multiplier-120443/msg1648847/#msg1648847

--- End quote ---

Hi Hero999
I do not understand how that circuit is supposed to work.
If the purpose is to get a low dropout (through an higher voltage to the NPN base) then the input Voltage labels have been exchanged.

The PNP + the IC should be supplied by the voltage doubler solving the problem of the higher dropout, while the NPN collector (or that of an array of power NPN transistors + ballast resistors) should be connected to the rectified voltage. Then the minimum dropout would be the saturation voltage of the NPN transistor(s + ballast resistors drop).

--- End quote ---
You're right. I got them the wrong way round. Here's the correct schematic.



--- Quote from: sureshot on July 10, 2018, 06:18:08 pm ---I think the npn collector should be the regulated input potential, not sure how there's a third terminal. I didn't notice that until you mentioned it. My full rectifier voltage multiplier mock up had the two terminals from the capacitors + - one from each capacitors.

--- End quote ---
The VDoubler terminal is for the voltage doubler, which can be used to reduce the drop-out voltage to just the tranistor's saturation voltage. If you don't have a voltage doubler, just connect it to the VRectifier and it will work, but with a higher drop-out voltage of about 3V, at full load.

sureshot:
Yes ok I get it, thanks for putting me straight on that, I might, only might, lol have been doing some head scratching somewhere down the the line.

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