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Please help me understand the voltage regulator with external bypass BJT?
Please help me understand the voltage regulator with external bypass BJT?
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Old 15th November 2012, 11:18 AM   #11
east electronics is offline east electronics  Greece
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the particular circuit is mod as presented in the LM 317 application it is one of the most clever circuits and mother of many others in electronics .

beyond being very quite its extremely tolerant to many loads and that will include a lot of RF applications and you will find similar circuits driving many RF CB amplifiers and many medium power 50-300W FM linear amplifiers ..

Additional features is that the Lm 317 will preserve thermal stability if common heatsinked with the output transistor ( S )

And the trick to preserve 1000% regulation is to remove feedback lead of the LM 317 in this case the output of the LM317 to sense voltage as close to the load possible .

suppose you have the schema 1 of the power supply in 13.8 version and a draw of 5 A with 2 transistors ....Suppose also the you used 0.75mm cable to drive the load in a distance of lets say 10m expect to loose almost half of the voltage due to cable loss ...

Simply if the LM 317 senses the voltage on the load will see that voltage is dropped and increase the output until the voltage on the load is exactly as set to 13.8 volts ..

Of course the cable will boil and the intention of this example is understanding how critical feedback might be in circuits like that ...

One may think the combo of the LM317 +2955 as an sziklai amplifier ...there is a lot of juice in this thought and only a few people advanced thoughts like that in the forum ...most of people are focused in the first stages and in EFP outputs ...think people !!!!

Happy regards
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Old 15th November 2012, 06:59 PM   #12
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Originally Posted by jerluwoo View Post
I'm sorry to inform you but that schematic and its intended use are incorrect. The circuit in post 1 is a short circuit protection scheme and should not be used as a "current boosted regulator" , for one it will inject all the noise and ripple from the input straight into the output, making the whole point of using a regulator moot, unless you simply want a voltage and dont care if its clean.
Attached is the correct way. Note c1 and c2 are solid tantalum types and should be included with these types of regulators.
I am sorry, but You are wrong sir.
The cirquit You present here will need to be corrected with an extra diode in the common leg of the regulator IC, as the IC detects the voltage at the output-transistors base. This will give an error to the output voltage that actually varies some with the temperature of the 2N3055 and the current drawn. And the diode in the common leg of the 7812 would only make a minor correction to get the voltage wanted.

The cirquit in the start of this thread will give a much more stable output voltage as the 7812 will sense the voltage directly to the output of the complete regulator.

The ripple-amplification done by the TIP2955 is actually negligeable, and could easily be filtered out.

I agree however of the recommandation of the capasitor 0,33uF on the input of the 7812. I also would remove the 470uF ellyt there.
Sooner or later you end up with TANDBERG
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