LM317-based regulated PSU: how does this thing work?

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I drew this schematic by putting together two schematics from Randy Slone's Audiophile Projects Sourcebook, the chapter on power supplies. He says that the transistors are used to enhance the regulation of the LM317/337. I am not familiar with how this works; don't remember seeing anything transistor-enhanced of this sort in any application notes. Can you give me any pointers or explanations?

The exact transistor device choice is mine; Randy says any high-gain low-current signal transistors will work. He generally does not use BC devices, simply because he says the 2N series is more easily available in the US where he picks up his supplies from.
 

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Richard C said:
Are you sure the schematic is correct? The transistors appear to provide 'soft start' rather than enchanced regulation.
Exactly! I've seen this sort of a schematic in the LM317 datasheet, without any explanations; that diagram mentioned "soft start."

I've double checked my schematic against his book; I'm quite certain my reproduction is correct. I'll tell you what Randy writes in his book:
Figure 7.14 illustrates a superior method of achieving high-performance operation from the common LM317/337 regulator ICs. The incorporation of transistors Q1 and Q2 improves the quality of the reference terminals of the IC regulators, by providing beta-enhanced voltage stabilisation. The end result is improved adjustment precision and long-term stability...
The rest of the para discusses the need for the protection diodes.
 
perhaps the transistors are being used as zeners?
Is this normal? If this was the intention from the start, why not use zeners? They have one less terminal to solder. :)

Guys, thanks, but am totally lost. Bottom-line: do you think this circuit will work? Is it worth it trying to build it? I'm not comfortable just blindly building something without any semblance of understanding how it works. At the same time, it's possible I don't understand enough about transistor behaviour to understand how this circuit works. :)
 
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SY said:
From the standpoint of the adjust terminal, the transistors look like emitter followers, with an ac ground reference.


SY,

But they are cut off. They don't do anything during operation. I agree it's soft start. Initially, when the cap at the base is charging, the charge current through the E-B resistor opens up the transistor and it clamps the ref terminal to the cap voltage gnd. So, as the cap charges the charge current gradually decreases, the transistor starts to cut off and the ref terminal rises to its design voltage. Soft start!

Jan Didden
 
I think Randy is wrong by stating that "The incorporation of transistors Q1 and Q2 improves the quality of the reference terminals of the IC regulators, by providing beta-enhanced voltage stabilisation. The end result is improved adjustment precision and long-term stability..."

These transistors with added 47k resistors (R5,6) and 4u7 capacitors (C11,12) are just there for soft starting and will do nothing for improved stability.
Consider the positive half: C11 is discharged at power on. Q1 is shunting the resistive divider R3 and (R1+R7), and the ouput voltage will be Vref (IC2) + Vbe (Q1), so about 2V. Then C11 will be charged via R5 and will slowly rise in voltage. The emitter follower Q1 is less shunting the voltage divider and the output of the regulator goes up. At a certain moment Q1 is not shunting anymore because its base voltage becomes higher than the voltage on the adjustment pin of IC2-0.7V. Then the regulator is on its final output voltage. But C11 will be charged even a little more to make Vbe of Q1 just 0V (neglect leakage). So at that moment Q1 is effectively not in the circuit even more and will not improve the quality of the regulator!

Steven
 
Thanks, guys

Jan, Steven, SY, all of you....

I think I understand the soft-start explanation. My friend Angshu too told me the same thing, off-list. So, you're essentially saying that Randy's explanation may be off the mark, but as a circuit, it'll work.

Fine, then. Is there any better general-purpose adjustable, regulated symmetrical supply circuit I should opt for instead? I need current of 500mA or less per rail, and voltages to be variable from, say, +/- 10-25V. I don't need a super-optimised ultra-ultra-low-noise thing if it increases part count or cost... I'll be driving simple opamp-based circuits or even discrete circuits, with PSRR of 40dB or better (e.g. the discrete buffer I'd discussed here).

Or should I just use this circuit?
 
Ripple suppression?

I would guess that the transistor together with C11 and R5 provides a low pass to the selected output voltage. Hence it should increase ripple suppression - the higher the frequency on the output, the lower it is adjusted. You might be able to achieve this without transitor, choosing a rather large C for a corner frequency in the low hertz range.
Try to simulate power ripple effects!
 
Jan and Steven's objections look quite sound to me- thanks, guys.

Given that you have pretty good PSR in your circuit, you could do very well with this reg chip, but I'd strongly consider a good sized adjust bypass cap, a good sized ouput cap (big old electrolytic), appropriate protection diodes (discharge path for the caps at power off), and preloading the reg so that it always supplies 100 ma minimum- these chips get quieter and tighter the more current they are called on to supply.
 
I used this slow start circuit for the LM338 regulators for some time and I think I can say that there was certain difference in sound with and without it. The difference was not huge and if I remember correctly it sounded a bit quieter with this circuit applied. The quieter sound often has something with stability, but again it ain’t necessarily so. Anyway, I did not investigate that further. Did simulated though and if memory serves me well, I have not seen any difference neither in PSR nor in any other parameter that I have been observing.

Pedja
 
diyAudio Retiree
Joined 2002
........is paved with good intentions

I am having so much fun reading this that I hate to interupt.:D

First I stongly urge (demand actual) that every body read the data sheet for the LM317! The adustment teminal acts like a current source.The regulator establishes a 1.2 volts voltage between the regulator output and the adjustment terminal, the value of the current being 1.2 V divided by R3 . This current multipled by the resistance to gound added to the 1.2 volts gives the output voltage of the regulator. AC voltage variations in the 1.2 volt reference cause an AC voltage variation across the impedance to ground. The regulator is following the voltage at the adjustment terminal added to 1.2 volts. The value of this AC voltage being the impedance to ground divided the resistance across the current source times the voltage varation between the output and adjustment terminal. That is; the noise at the regulator out put is the noise across the reference voltage times the ratio impedance to ground and the resistance between the adjustment termninal and Vou. For the lowest noise we want this ratio to be as small as possible. It is usually done by putting a capacitor across the resistor from the adjustment terminal to groung to lower the AC impedance while keeping the DC voltage a function of the resistor values.

After lunch....... the rational for the Sloan circuit and it's short comings.

I won't be late for lunch Jim if you are reading this. i am never late for Chinese food!

Everyone read the data sheet in the mean time.
 
diyAudio Retiree
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well lunch and dinner actually

The basic intent was to provide low impedance to ground by having the adjustment terminal looking into the low impedance of an emitter follower. Since there is no base to ground DC path the cap charges up to the point where the base emitter junction does not bias and the emitter follower is doing nothing. Looks like someone made a boo boo. Not very confidence inspiring for one who is supposed to be an amplifier designer......... This circuit could be "fixed" with a few resistor changes (adding one from base to ground for one thing) but DC stability with temperature would be pretty poor. About a quarter of a century ago our esteemed fellow forumer Mr Jung did a supply for a modification of the Dynaco PAT 5 with David White that used a zener to ground from the adjustment terminal to provide a low dynamic impedance to ground for the adjustment terminal. The regulators were tiered positive regulators to get both supply polarities. A capacitor of a few hundred microfarads was also across the zener for noise reduction.

A couple of LM 329s in series with 470uf or more in parallel with each one might be interesting.......... The resistor across the adjustment terminal could be about 1K for more than enough bias current for the L329s.
 
Hi Fred,

I hoped you enjoyed your lunch/dinner.

When you started your exposé in two parts and encouraged everybody to read the datasheets again, I was very curious what kind of hidden shrewdness would be present in the circuit and what new insight you were going to offer. As far as I understood, Slone just copied it from the datasheet, appearantly without further thinking.

After reading the second part, it seems that Jan and I were just right in our posts and nothing more has been added. Actually I'm a little disappointed, but maybe I was just expecting too much.

Steven
 
Shall I proceed to PCB this circuit, then?

I did a PCB layout. Does it look okay? It appears that you're all in agreement that the circuit does not have fundamental flaws, it just doesn't work the way the book explained. On the whole, soft start is a good thing anyway, so no harm done anywhere, I guess.

Will you give me feedback on the PCB layout? Single-sided, with just one jumper (shown in red). Not exactly as compact as some of you guys can make it, I guess, but shows nice symmetry. :)

Should I provide for some load resistors for a steady background load of, say, 100mA on each rail? 100mA x 20V will be 2W. So I guess a pair of 1W carbon resistors connecting each rail to ground should do it.
 

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Re: Shall I proceed to PCB this circuit, then?

tcpip said:
Will you give me feedback on the PCB layout?

Should I provide for some load resistors for a steady background load of, say, 100mA on each rail?

Hi,

C11 and 12 have bigger dimensions (can be electrolytic).
Steady current is too high (why wasting ca 6Watts)?
What about heatsinks on regulators?

Regards
 
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