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Linear regulator boost with NPN transistor
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sureshot:
Thank you again for the help with this. So figure 7.4 is the way to go, if it's an npn transistor as the final power element ? And the pnp transistor drives it. Are R2 and R3 replaced with the 50 ohm and 75 ohm resistors ? And is the diode on the input for an intended voltage drop, as I can't see it's for protection, being it's forward biased. Thanks again for the help, and circuit schematics.
David Hess:

--- Quote from: sureshot on July 09, 2018, 06:51:26 pm ---Thank you again for the help with this. So figure 7.4 is the way to go, if it's an npn transistor as the final power element ? And the pnp transistor drives it. Are R2 and R3 replaced with the 50 ohm and 75 ohm resistors ? And is the diode on the input for an intended voltage drop, as I can't see it's for protection, being it's forward biased. Thanks again for the help, and circuit schematics.
--- End quote ---

R1 and R2 have nothing to do with the 50 and 75 ohm base-emitter shunt resistors.  The base-emitter shunt resistors are needed to remove charge from the base-emitter junctions and bypass Icbo leakage and R3 corresponds with the 50 ohm resistor shown and fulfills the same purpose in the power PNP version of the circuit.  R1 and R2 control the ratio of current between the output transistor and regulator so that when the regulator goes into current limit, the transistor current is well defined.

The diode roughly compensates for the base-emitter voltage drop of the PNP transistor.  Without it, the current limit will not be as accurate.
sureshot:
I think i get that. Thank you for taking time to explain that, that's where i struggle a bit with the maths. i need to find a decent book for these equations that doesn't go to far down the complex physics route. I will probably give both the first circuit I posted a go, and the national semicoductor schematic you kindly posted. 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.
Thank you to all that helped me out, appreciated.  :)
sureshot:

--- Quote from: T3sl4co1l on July 09, 2018, 10:34:44 am ---Yah, you've hit all the main points.

Dropout voltage (minimum input = output + droppout) is ~0.7V higher.

Regulation is worse by the "softness" of the diode curve.  It's not inaccurate by a diode drop, because of the diode compensating it.  But the diode's current is ~constant (whatever the regulator GND current is), while the transistor's Vbe depends on current draw.  This, plus transistor resistance, gives maybe 100mV of variation.  Worse than a 7812 -- but not bad for most purposes.

No current limit, or rather it's hFE limited, which is a bad idea all the same.  If you need that capability, expect to add more components to implement it.  Note that it's not as simple as pulling down the base with a current sense resistor and transistor, because the 7812's current is what has to be sunk.

Nor can you quite pull it off with a LM317, which has 1.25V output, more than the Vbe drop so you can't quite completely turn it off by pulling ADJ down to GND (with a few transistors wrapping around to do that).

Tim

--- End quote ---
I often go back over posts in case I've missed something, and I think I have, form Tim's post to David's post. The mention of the work the regulator does in the national semicoductor schematic. I see it uses resistors to adjust shared parameters between transistor and regulator. Yet the schematic I posted in my opening at the top, what stops the 7812 from not drawing over 1 Amp of load current. Especially closer to 3 Amps, or is it's internal circuitry of the 7812 that current limits the regulator ? Sorry if it's a lot of questions, that one is not immediately obvious to me. Shouldn't guess really. But I think it's the 7812's circuitry,  please correct me if I'm wrong with that thought.
T3sl4co1l:
Yes, it's only limited by the 7812's internal circuitry.  That current is amplified by the following transistor, so the current limit at the output terminal will be vastly higher.

Tim
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