Adjustable power supply questions

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Re: Adjustable etc

janneman said:
Hi Roady,

To understand the outboard pass, assume that initially the pass transitor does not conduct. Now you put on a load, and the 317 starts to supply current to the load to keep the output voltage up. This current flows through the (in)famous R1 (33 Ohms). When the current reaches about 18 mA, the Vbe of the pass transistor gets high enough to cause Ic to flow, starting to add to the load current. If this current is not enough for the set output voltage, the 317 will start to conduct more, which in turn will turn on the pass transistor more etc: the output will be regulated.

BTW, I agree with the analysis that the 317 only suppllies a very small part of the total current. Why it is a K-version, beats me. The small price difference between TO3 and TO220 in those days may indeed have something to do with it.

Jan Didden

Exactly and also the way it is explained in most documents describing the pass transistor. As for the K version I think the reason as I mentioned simply is because it is one of the versions in this datasheet. Nothing to complicated, it just happens to be the K version drawn. Simple enough.

Thanks for filling in Jan.
 
If this is a lab supply, here are some points to consider:

1) by biasing the ADJ pin to -1.25 volts you can get down to "0".

2) these LM317 circuits were OK when folks used analog meters with 1% or 2% acuracy. Now, with a 10 bit ADC you should be able to get close to 0.1%, with 12 bits you will get better than 0.1% etc. If the supply is going to drive some audio circuitry, say a phono-preamp, or to breadboard and test a preamp, you should really do better than 0.1%. Consider using the noise-cleanup circuitry that Wenzel describes on their site:<p>http://www.wenzel.com/documents/finesse.html
 
jackinnj , a properly designed amp (phono or otherwise) should not affected by variations in the supply voltage, of 1% or even more. This is assuming you mean LM317s suffer from 1% DC inaccuracy.
Also, why would you want to drive an amp with a variable supply anyway? (except during experimenting, but then you should have a proper design, supply and all, before starting to test it for noise).
The ADCs used in audio circuits are usually driven by an opamp at the input, which will have a high PSRR. PSRR at the top end of the audio band for most opamps is of the order of 30dB, worst case. This will drive the already low power supply noise to inaudible levels, if not, below the noisefloor.
 
Yes, you were right UrSv

I did not know about the Current Limiting thing.

My logic said me, that if you have 2 TO3 devices
it is bad design to let one of them carry all the burdon
in this case power/heat.
If you have two TO3 parallell you divide the power
so that both devices operate safe, clear within
their limits.

But if Current Limit is only in one of the rails
of course you direct as much of the current
as possible into that one.

That circuit was a sample. But I guess any sensible
designer wouldn't use a TO3 just for controlling purposes.
As TO220 and TO92 has better performances
when it comes to such tasks.
They are mostly both faster and have other prestanda
that TO3 don't have.

So, Excuse me UrSv for not checking up facts better

I am not "a bad loser".

/halojoy
in the north of our beloved country
fredagen den 17:e januari 2003
 

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