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Since this PSU was published by Elektor I assume that some of you might heard or read about this project.

http://www.retro.co.za/zs1ke/projects/PrecisionPowerSupply/PrecisionPSU-Elektor-Dec-1982.pdf

This PSU is capable of delivering from 0-33V at 3A. Its voltage is stabilized, can be regulated and has current limiting control + short-circuit protection.

This is also my final school project. I made two of these PSU's in one housing. First one works perfectly fine. But second one causes a problem on the output that couldn't been solved yet.

Me and my mentor (professor) did all kinds of measuring on the not-working PSU - measuring connections, short circuit possibilities, voltage measurement on specified components and with oscilloscope too.

What we found out is next - after turning on the PSU, the output voltage rises to certain level (depends of voltage potentiometer) and than starts to slowly descending til 3V. Then the voltage changes slowly from 3V-5V. When we measured the Op-amps and LM723, on certain pins there was voltage also slowly changing. And if I connect load on the output the voltage slowly descends till 0V (everything seems like there should be a capacitor making these problems)

I changed Op-amp, LM723 and a few capacitors (every of them but smoothing capacitor - my professor said that it wouldn't be making such problems).

*Capacitors are older ones (but not used yet), all other components are new (smoothing capacitor too - the big one).

I also invested a lot of money and time into it and I don't want to end like this (with only half of it working - I made two in one with separated transformer stages so I can get negative voltage from +33V to -33V).

I hope someone has any ideas or has worked on similar project so I can solve my problem (which stayed unsolved till now).

One of two PSU's

Some badass heat sinks

Circuit schematic

Front panel (switch is missing in the middle)

enter image description here

All of these measurements were measured to the common GND of circuit in that part of circuit (stabilizer stage). This stage has a transformer source of 10V/0/10V (TR1). TR2 is power stage of 26V.

NOT-WORKING PCB

C1= 10,7V (it should be more(Uin times square root of two))

C2= 10,7V (-||-)

C3= fluctuates around 9V

C4(IC1/pin 13)= fluctuates around 10V

C5= fluctuates around 9V

C6(R7)= fluctuates around 5V

C7(IC2/pin 6)= fluctuates around 8V

C8(R14)= fluctuates around 8V

C9= fluctuates around 8V

C10(*to GND of power stage)= 35,5V

C11(R23)= fluctuates around 9V

IC1(stabilizer):

1= 0V

2= 9,6V-9,8V

3= same

4= same

5= same

6= same

7= 8,8V-9V

8= 0V

9= 4,7V-4,8V

10= 9,6V-9,8V

11= 10,7V

12= same

13= 10,6V-10,8V

14= 0V

IC2(op-amp):

1= 10,8V

2= 7,2V-7,5V

3= 5,3V-6V

4= 10,8V

5= same

6= 7V-9V

7= 10,6V-10,8V

8= 0V

IC3(op-amp):

1= 10,8V

2= 8,8V-9V

3= same

4= 10,8V

5= same

6= 10,3V

7= 10,8V

8= 0V

WORKING PCB

C1= 13V

C2= 13V

C3= 7,2V

C4(IC1/pin 13)=7,3V

C5= 7,3V

C6(R7)= 35V

C7(IC2/pin 6)= 12,5V

C8(R14)= 0,15V

C9= 0,15V

C10(*to GND of power stage)= 35,5V

C11(R23)= 0,15V

IC1(stabilizer):

1= 0V

2= 7,3V

3= same

4= same

5= same

6= same

7= 0V

8= 0V

9= 1,2V

10= 7,3V

11= 13V

12= same

13= 8,6V

14= 0V

IC2(op-amp):

1= 13,1V

2= 0,04V

3= 0,43V

4= 13,3V

5= 13,1V

6= 12,5V

7= 13,1V

8= 0V

IC3(op-amp):

1= 0V

2= 0,15V

3= 0,5V

4= 13,1V

5= same

6= 12,4V

7= 13,1V

8= 0V

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  • 3
    \$\begingroup\$ Since you have a working one, did you compare the non-working one to the working one? \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 4 '17 at 18:33
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    \$\begingroup\$ Most likely solder connection errors. Compare all pin voltages at threshold of failing including Vcc,Vee \$\endgroup\$ – Sunnyskyguy EE75 Mar 4 '17 at 18:38
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    \$\begingroup\$ In particular OA's pins 2,3 must latch for output in linear range which lowers output on pin 6 to cut output. If LM723 Reg. output oscillates , then you have problems pulling up output. \$\endgroup\$ – Sunnyskyguy EE75 Mar 4 '17 at 18:48
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    \$\begingroup\$ Never use old capacitors especially electrolytic ones - they don't age that well. Listen to what @IgnacioVazquez-Abrams said too. \$\endgroup\$ – Andy aka Mar 4 '17 at 18:56
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    \$\begingroup\$ " When we measured the Op-amps and LM723, on certain pins there was voltage also slowly changing." Now there's the heart of the problem, and you don't think we need to know exactly what was happening. Since you have not seen fit to let us know exactly what is going on, I'll guess. Start by removing the diode from the current limit op amp. If this fixes it, you know where to look next. Otherwise - did you say you have 2 supplies configured for both + and -? And are you sure the - supply is properly isolated? How about contact of the heat sinks? How, exactly, are the two supplies connected? \$\endgroup\$ – WhatRoughBeast Mar 4 '17 at 20:00
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@Keno - "Yes, heart of the problem is 100% "the changing of voltage" Right. So stop beating around the bush and tell us what the voltages ARE. And if the 723 voltages are changing, then isolate the 723 section by removing the 3 base drive components (R9, D4 and D5), and find out why the 723 is acting up. Come on, this isn't hard. Isolate the problem section, determine the error, and fix it. Stop beating around the bush with generalities.

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  • \$\begingroup\$ You can check the edit I made (comparing the two PCBs) \$\endgroup\$ – Keno Mar 5 '17 at 18:42
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    \$\begingroup\$ I am surprised your Prof has not helped you isolate this trivial issue. \$\endgroup\$ – Sunnyskyguy EE75 Mar 5 '17 at 18:45
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    \$\begingroup\$ I am also surprised you accepted this an answer. Any challenge with failures is a good experience to learn. \$\endgroup\$ – Sunnyskyguy EE75 Mar 5 '17 at 19:04
  • \$\begingroup\$ @TonyStewart.EEsince'75 - For what it's worth, I'm surprised as well. \$\endgroup\$ – WhatRoughBeast Mar 6 '17 at 5:27
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I am from Brazil, I am also mounting this source (3 outputs: 2 x 0 to 36Vac >> 47Vcc @ 4A, 1x 8vca >> 9vcc @ 3A) I am also facing problems, but in the part of IC2 (voltage adjustment) already I did a lot of testing and switching components that are new, I believe the LM741 I bought is not true, but I have not yet found another reliable supplier.

My LM723 regulator works well with stable 7.1V as described in the Elektor magazine article, and by the way its not working properly.

I saw through your measurements that some voltages are way out of the way ... I would tell you Check some things:

  • Possible batch of fake LM723?
  • High ESR of electrolytic capacitors (did you check this?).
  • Possible connection (US + and US- inverted sensor cables)?
  • Instability due to the use of lower voltage supply in the 10.7V regulator circuit?

If you can put more pictures, it may be easier for someone to see something wrong and find the defect. Be sure to comment on finding the defect. Forgive me if I wrote some nonsense (I do not speak English and I use google translator). Good luck!

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  • \$\begingroup\$ Probably. Since one PCB worked perfectly and the other one didn't, I made another one with new board and all elements new. Now it works perfectly. I don't really bother what went wrong there. \$\endgroup\$ – Keno Jun 25 '17 at 7:42
  • \$\begingroup\$ Glad your circuit works. Mine still has trouble adjusting the voltage. When I raise the voltage setting the voltage goes up to 32Vdc when it should go up to 47Vdc, when I reduce P1 the voltage goes up and the potentiometer works upside down (min: 47Vdc, Max 32Vdc). Therefore, if I analyze an IC2 parameter (741) is exceeded, it is called "input voltage", which is the voltage between the input and GND of the auxiliary source (not the differential voltage between the inputs), this voltage must not be Higher than the supply voltage of the ampop which \$\endgroup\$ – Fábio Jun 25 '17 at 12:36
  • \$\begingroup\$ ... is around 15.5V, such as 23.5Vdc> 15.5Vdc the behavior of the IC2 is unstable .... you could measure the voltage between pin 3 of the IC2 and GND of the auxiliary source With the maximum and minimum voltage setting so I can make a comparison? I am grateful if you can help (as I used google to translate this message before). \$\endgroup\$ – Fábio Jun 25 '17 at 12:36
  • \$\begingroup\$ I managed to solve it, the defect was because I replaced the original transistors (T2 ... T5) with higher gain ones and this caused instability to the circuit. \$\endgroup\$ – Fábio Jul 6 '17 at 19:17
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Recall yesterday I commented " If LM723 Reg. output oscillates , then you have problems pulling up output. "

Clearly your unregulated supply is overloaded on both sides dropping V from +/- 13 (ok) to +/-10.7 (fail). Thus something is loading it.

This old design uses external compensation for 2nd order effects which in closed loop Op Amps causes loss of phase margin and can cause oscillation is no margin.

A common issue is the thru hole lead inductance of cap and ground connections can cause phase shift and if not compensated properly will oscillate.

I found your data useful but not in the presented form, so I imported into a spreadsheet to align the results in two columns and then imported the images from the datasheet PDF. You can show this on your thesis to demonstrate real world issues. All Power Supplies must be tested for gain-phase margin until proven stable. Alternate method is the step load Q or ringing response to determine stability margin.

Recommendations.

  • Increase the value of C4 until stable up to 10x.
  • Warning this will affect loop bandwidth and step load response time.

enter image description here

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  • \$\begingroup\$ Hmm... I don't quite get the point your compensation thing. If the first PCB works without this compensation then why would the not-working one need it. \$\endgroup\$ – Keno Mar 5 '17 at 19:33
  • \$\begingroup\$ Did you measure phase margin from step load? Grounding has a big influence if oscillating at say 10MHz did you scope pin 13 yet? \$\endgroup\$ – Sunnyskyguy EE75 Mar 5 '17 at 19:37
  • \$\begingroup\$ When we measured with professor, one pin (not sure if 13) was making almost square impulses. I myself don't have the scope..yet. \$\endgroup\$ – Keno Mar 5 '17 at 19:50
  • \$\begingroup\$ That's oscillation. If you have another cap to add, try that and measure DC, AC with DMM \$\endgroup\$ – Sunnyskyguy EE75 Mar 5 '17 at 19:53
  • \$\begingroup\$ Which cap? The smoothing one? \$\endgroup\$ – Keno Mar 5 '17 at 19:54

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