LM723 PSU with 0V lowest voltage

[earckens]

Hi spec and Les,

I studied all the ideas of the last few hours and have a few questions, just to confirm before I start building tomorrow (and spec: I promise not to wait until the prototype is ready and finished, I try to start on the Eagle drawing tomorrow):

I refer to the drawings in post 113 and 119:

1. I miss the voltage feedback circuit to -comp:
a. no need to recalculate the voltage divider 18k / 82k?
b. value for the voltage regulator pot: remains at the original 2k2?
c. maximum 46V gets fed back to -comp via the 82k resistor: +comp remains unchanged compared to the original circuit; surely this is not right?

2. 18V zener regulator: currently based on 40mA (both zener bias and total load for 18V).
a. Is this sufficient?
b. Why 18V and not for example 30V? 36V?
c. Why not a series regulator with a transistor? (if it can be solved with a zener, sure why not?)

Thanks a lot!!
Erik

 

[earckens]

Edit 2. I have just has a look at your new circuit. It will subject the collector of the internal pass
transistor to the full 46 volts. Also the 39 volts you are using to power the LM723 is closer to its maximum rating than I would like.
Les.

Full agree, 39V is too close to the LM723 max ratings.
What about using 36V zener?

What is the problem of connecting the collector of the internal pass transistor for the series regulator to the rectifier output?

 

[Les Jones]

Transistors have a maximum collector/emitter voltage rating and if it is exceeded they can fail. I think 36 volts should give a safe enough margin.
spec's design is a much better solution.

Les.

 

[earckens]

Transistors have a maximum collector/emitter voltage rating and if it is exceeded they can fail. I think 36 volts should give a safe enough margin.
spec's design is a much better solution.

Les.

Sure, I think too. But what about the voltage feedback circuit (-comp at pin 4, etc...): is there no need to adapt this (resistor values)? And the voltage pot: can/may 5k be used? Changes needed there?

Thks,
Erik

 

[spec]

Fully agree, 39V is too close to the LM723 max ratings.
What about using 36V zener?

Yes , a 39V +-5% Zener diode, for example, could have a nominal voltage of 41V and the voltage could be even higher when all factors are taken into account.

As a general design guide, only use maximum of 80% of the components voltage rating. So in the case of the LM723, 40V *0.8 = 32V

What is the problem of connecting the collector of the internal pass transistor for the series regulator to the rectifier output?

Not sure what you are saying here John. What is the 'series regulator'?

spec

 

[earckens]

Not sure what you are saying here John. What is the 'series regulator'?

spec

spec, I meant this as a reference to my post where I would have substituted your proposal for a series resistor with 18V zener diode (the 18V regulator of your schematic in post 113), with a series regulator where the transistor collector would be exposed to the full 46Vdc. Les' answer was that it exposes the transistor to the full 46V at the collector-emitter junction.

Spec, is the voltage feedback circuit (-comp on pin 4) not in need of adjustment (your drawing post 113)? Or will the regular 18k/82k voltage divider with the 2k2 voltage pot remain?

And can/may I substitute the 2k2 pot with a 5k pot, and what adjustments would be needed if I did?
(I happen to have a 5k multiturn pot available).

Thks
Erik

 

[spec]

Hi earckens,

I refer to the drawings in post 113 and 119:

1. I miss the voltage feedback circuit to -comp:
a. no need to recalculate the voltage divider 18k / 82k?
b. value for the voltage regulator pot: remains at the original 2k2?
c. maximum 46V gets fed back to -comp via the 82k resistor: +comp remains unchanged compared to the original circuit; surely this is not right?

I have not analyses the voltage feedback. I will need to redraw that area to understand what is going on. You can get into a terrible pickle with the LM723 type voltage feedback if you are not careful.

2. 18V zener regulator: currently based on 40mA (both zener bias and total load for 18V).
a. Is this sufficient?

Yes: the LM723 takes 7mA worst case, and the base of the TIP142C requires 7 mA max, which means that there will be 40mA - (7mA + 7mA) = 26mA minimum flowing through the Zener diode.

b. Why 18V and not for example 30V? 36V?

The LM723, will be happy at 18V.
An 18V supply line will have better stability and less ripple than a 36V supply line.

c. Why not a series regulator with a transistor? (if it can be solved with a zener, sure why not?)

If you mean a standard three terminal regulator, like an LM317, they will not take 56V.
The drop-out voltage of a three terminal regulator ia around 3V so that would further limit the PSU output voltage range.
The circuit would be more complex.

spec

 

[earckens]

Hi spec, is it then not risky to implement your proposal of post 113 not knowing if the voltage feedback circuit is still adequate and would work?

What would you propose: the theoretical approach: analyse the feedback circuit to calculate correct component values, or me building the circuit and start with fault analysis and try to resolve from there?

 

[spec]

Spec, is the voltage feedback circuit (-comp on pin 4) not in need of adjustment (your drawing post 113)? Or will the regular 18k/82k voltage divider with the 2k2 voltage pot remain?

And can/may I substitute the 2k2 pot with a 5k pot, and what adjustments would be needed if I did?
(I happen to have a 5k multiturn pot available).

I will have a look at the voltage control, but it will need a bit of work to untangle what is going on.

Almost certainly the circuit can be configured for your 5K multiturn potentiometer.

spec

 

[earckens]

I will have a look at the voltage control, but it will need a bit of work to untangle what is going on.


spec

Tell me how I can return the favour!

 

[spec]

Hi spec, is it then not risky to implement your proposal of post 113 not knowing if the voltage feedback circuit is still adequate and would work?

The suggested modification does not affect the voltage feedback.

What would you propose: the theoretical approach: analyse the feedback circuit to calculate correct component values, or me building the circuit and start with fault analysis and try to resolve from there?

I will have a look at the voltage feedback and let you know, but do not worry, it is no big deal.

spec

 

[spec]

Hi Les,

I think having to modify the drawing using Photoshop a reasonable excuse. I avoid using it unless there is something I can't do with PaintShop Pro. And now using Paintshop Pro now means going back to an XP system as the Corel version that runs on windows 7 is not as good.

I started a new thread about graphics editors: https://www.electro-tech-online.com/threads/graphics-editors-which-program.149652/

I did wonder about connecting the base to Vz but the non liniarity as the Zener just starts to conduct may cause instability. In the original version you kept the Zener biased into conduction with a resistor to the positive rail. This would not now be possible using the internal Zener.

You could still use the same approach- just put a resistor between the base and emitter of the new transistor so that the voltage drop across the resistor is not more than about 350mV, so that the additional transistor is not turned on.

The only problem I can envisage is that a grounded emitter transistor will introduce more current and voltage gain, which could make loop frequency stability more difficult, especially with the PNP driver transistor.

I did not look at the modification to look for mistakes but just to see how you had solved the problem.

It would be much appreciated if you did look for mistakes- my stuff needs to be checked.

spec

 

[Les Jones]

Hi spec,
The resistor between base and emitter should work. (Why didn't I think of that.) I think Eric may as well stick with your design to save doing another schematic. I like the way you explained to him about the way to makethe schematic easier to understand. I was mentaly turning the PNP transistor upside down so the emitter was at the top.

Les.

 

[earckens]

The Eagle schematic is almost ready, will be posted the next few hours

 

[earckens]

Schematic attached; upload of .sch not allowed, so I zipped it.

Edit: zip edited to include exit capacitors

Edit 2: I do not understand the drawing in post 132? Where is the output voltage driver?

Edit 3: added the schematic in .png format

 

[spec]

I do not understand the drawing in post 132? Where is the output voltage driver?

The output transistors are the two NPN transistors, as before.

The output NPN transitor driver is the TIP142C, also as before

spec

 

[spec]

POST Issue 03 of 2016_12_09

Hy earckens,

Below a complete schematic for the LM723 power supply, with the following features:
(1) Common base transistor to limit the voltage seen by the LM723 (and not increase loop gain).
(2) Output voltage adjustment from 0V to 36V (but note my previous comment about getting down to zero Volts and leakage currents).
(3) Only a simple constant current limit of 1.5 A, which is the maximum current that two 2N3055 transistors will safely provide, as previously stated.

spec

 

[Les Jones]

Hi spec,
Sorry to do this again but I think the base and emitter of the current limit transistor need swapping over.

Les.

 

[spec]

Hi spec,
Sorry to do this again but I think the base and emitter of the current limit transistor need swapping over.

Les.

Don't apologize Les; your keen eye is much appreciated.

I will update the schematic of post #137

spec

 

[spec]

Hi earckens,

Schematic attached; upload of .sch not allowed, so I zipped it.

Edit: zip edited to include exit capacitors

Edit 2: I do not understand the drawing in post 132? Where is the output voltage driver?

Edit 3: added the schematic in .png format

I just had a look at the .png image of your schematic of the LM723 power supply- nice work. that is much easier to follow.

spec