Electronics > Beginners

Linear regulator boost with NPN transistor

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David Hess:

--- Quote from: sureshot on July 10, 2018, 12:32:09 am ---And possibly try these with an LM317 just to see if and adjustment for voltage drop under load is needed. But I would prefer to stick with a 7812 for this first npn transistor current boost circuit.
--- End quote ---

The diode to ground in the output transistor version is an easy way to compensate for the base-emitter voltage drop but as T3sl4co1l pointed out, the regulation is "softer" but this is not normally an issue with a 12 volt output.  The 317 can be used the same way by placing the diode at the bottom of the feedback divider which is essentially included inside the 7812.  A resistor can be placed in series with the collector of the NPN transistor on the output to limit the current somewhat.

sureshot:
Thanks for your replys, i had a feeling it was the regulator self current limiting itself. Just because there are no resistors. It probably isn't the best of linear voltage regulator circuits, but I'm curious to see how well it does. The diode to lift the output voltage in the ground path i first discovered in a cb power supply some years ago. I had to do a bit of searching to find why it was in this retail power supply. Thanks you again to everyone for all the help with my questions.  :)

David Hess:
The regulator current limits just fine but the external transistor's current gain is high and by itself poorly controlled.  Adding a resistor in series with the collector allows the collector voltage to drop at high currents limiting current; this is not ideal but it is better than nothing.  The National circuit instead controls the current from the emitter side which works much better.

sureshot:
Yes i can see what you mean, up to 3 Amps current is it necessary to have a resistor in series with the collector ? I'm guessing if it is, something like a 10 ohm 10 watt ceramic resistor. Kind of the same value resistor if it was a pnp emitter series resistor. Not 100% sure that's the right value and power rating for a collector series resistor.

not1xor1:

--- 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

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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).

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