0-30V Stabilized Power Supply

[red baron]

Hi AudioGuru
Finally the project is near completion - pics attached.  Thank you for the help getting to this stage.
I have two problems:
1) The current limiting circuit does not function correctly and I need some help with trouble shooting.  With P2 turned down to about 1kohm (near minimum) the LED comes on, but turns off one current limit is set higher.  Setting the maximums of 5A and 30V, the LED starts to turn on faintly and then gets brighter, but only at the max.  In between all works well.  Could Q3 have blown?
2) From the pics you will see that I have used Picmaster pcb, but I believe there is one mistake - the polarity of C10 (in your circuit C3) has the +ve on the pcb away from ground and -ve connected to ground.  Surely not correct?
Regards

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[red baron]

More pics

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[audioguru2]

1) The current limiting circuit does not function correctly and I need some help with trouble shooting.  With P2 turned down to about 1kohm (near minimum) the LED comes on, but turns off if current limit is set higher.

Maybe your opamp u3 has a high i9nput offset voltage that needs to be adjusted to near zero.

Setting the maximums of 5A and 30V, the LED starts to turn on faintly and then gets brighter, but only at the max.  In between all works well.  Could Q3 have blown?

Q3 is simply an on-off switch. The opamp that drives it could be defective since without negative feedback its gain for DC is supposed to be nearly 1 million times.

2) From the pics you will see that I have used Picmaster pcb, but I believe there is one mistake - the polarity of C10 (in your circuit C3) has the +ve on the pcb away from ground and -ve connected to ground.  Surely not correct?

You are correct. The positive wire of my C3 should be connected to 0V so that the few associated parts make a negative supply.

 

[redwire]

red baron,  Outstanding build!  Very tidy and professional looking-Congratulations.  How about a photo of the front face and sides?  Where did you get the case from?

 

[red baron]

Thanks Audioguru, I will buy some replacement TLE2141s and see whether this fixes the problem, but I have my doubts, since I have tried moving them around with no change in results.  I have also discovered that the input offset pot RV1 has no impact on any of the readings - does this tell us something?
Redwire, your positive comments are most appreciated.  I have many pics, and will include a few more.  The housing is RS Components item 223-988 and costs $89.00 here in Sydney.

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[audioguru2]

I have also discovered that the input offset pot RV1 has no impact on any of the readings - does this tell us something?

Most opamps have an input offset voltage that causes the output to be a little away from the voltage it is supposed to be. RV1 is adjusted so the output is 0V when the proect is set to 0V.

 

[redwire]

red baron,

I have also discovered that the input offset pot RV1 has no impact on any of the readings - does this tell us something?

Many builders have experienced the same.   This is common when the negative rail is only -1.3V.    The PS I built with the 5.6 v zener instead of the two diodes in series allows the voltage to be set to zero, whereas the PS I built with -1.3 rail will only go down to about 0.3 volts and adjusting  RV1 will not bring it to zero.    I did not put the second one in a case, just connected the parts for a test.

When I turn the current seting to the minimum, the light will come on letting you know your voltage can not be increased  because it is being controlled by the current.

You identifed the output transistors as 2N3055's.  Is that a TO-3 case?  

 

[red baron]

Hi Redwire
My mistake, the power transisters are TIP3055 in TO-218 configuration.
Thank you for the input regarding the input offset question.  For some reason, I have +1.6v wherever others have +1.41V and I wonder whether this could be causing the problem.  I was unable to get R7 at 0.27ohm here, so I have used 0.33ohm which at 5A has a volt drop of 1.65V, but at zero output current, this should not cause the LED to light.
I am also concerned that when I turn the current limit P2 to a higher level, say more than 200mA, then when the voltage is increased to trigger the LED, it does not 'switch' on as it should but increases in brightness from 0 to bright over a voltage increase of about 5V eg. if I set the current limit to 2.5A, with a load of 6ohm, then the LED starts to shine at 14V and is bright at 16V - any ideas?

 

[redwire]

red baron, 

1.  can you do a test with two 0.33 ohms in parallel on R7 to see if the voltage drops.
2.  Do you have the 10V zener in backwards
3.  Q3 could be bad
4  I noticed that when the 10V zener was added, I changed R22 to 1.2K  I can't remember if it was becasue I have 2 leds in parallel (one on the pcb and one for the front panel) or it was becasue of the reduced voltage due to adding the 10V zener.
5.  Do you have 5.6v zener that you could use to test in lieu of D2 and D3

 

[audioguru2]

If the negative supply is higher than 1.6V then the total supply for opamp U2 is higher than its absolute maximum of 44V when there is no load or a low current load.

 

[redwire]

Audioguru,  not sure if I'm understand how the absolute value of U2 is above 44V with a 28V transformer.  U2 is not tied to the negative rail. 

 

[audioguru2]

Audioguru,  not sure if I'm understand how the absolute value of U2 is above 44V with a 28V transformer.  U2 is not tied to the negative rail.

U2 is the only opamp with an input offset voltage adjustment.
You said that the adjustment doesn't work unless U2 has a -5.6V supply. Then the total supply for U2 is too high.

 

[red baron]

Great help, guys.  I am trying as fast as I can to check your suggestions.  Since the last thread has raised the topic of maximum supply voltages, can you help on this?
1) the supply transformer has input 220V and 240V, and output 0-24V or 0-30V.  a) When I use 220V (and Australia is 240V off the street) I get 30V output on the 24V tapping (no load).  b) When I use 240V input, then I get 34V on the 30V tapping.  Which combination is the best for the regulator circuit?  Using combination a) so far I can only get 29.7Vdc through the power supply at 5A, but I can live with that - 30V would be nice.
2) I checked on the effect of reducing R7 from 0.33ohms to about 0.25ohm.  No noticeable change, and LED behaviour the same.
3) 10V zener is correct. I get 39.3V on the bridge rectifier +ve, and 29.3V at the emitter of Q3.
4) I am busy replacing Q3
5) I will reduce R22 from 3.9k to say 2.8k and see if this changes the LED behaviour at all.
6) The voltage drop across the 2 x 1N4148 is 1.52V (D2 and D3 are not labelled)
7) Some other observations:  U3 pin 4 = -1.52V, U3 pin 3 varies 0 to 1.5V depending on P2 setting, U1 pin 6 is correct at 11.2V

What other readings can I check?

 

[mendimano1]

Hi red baron, congratulations for a good work, im posting some pictures of power supply 5 ampers version  that i have build it one year ago, it is working perfectly and acuratly thanx to audioguru and redwires tips schematics and parts list, i have used 3 power tranzistors MJ11016 and 10 amp 30 volt transformer, i have tested with 30 volts 8 ampers load it worked without any problems, some tests and measurments you will have to do it in your own becouse it is imposible to see from here what is wrong with your power supply,
but anyway im encouraging you to finish your power supply .

very best regards

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[red baron]

Hi Mendimano
It is amazing how closely our projects look in the end.  Thanks for the photos.
I have invested too much time (and money) to give up now, so hopefully with the help of others it will get finished and work according to expectations.
Regards

 

[audioguru2]

Great help, guys.  I am trying as fast as I can to check your suggestions.  Since the last thread has raised the topic of maximum supply voltages, can you help on this?
1) the supply transformer has input 220V and 240V, and output 0-24V or 0-30V.  a) When I use 220V (and Australia is 240V off the street) I get 30V output on the 24V tapping (no load).

Then your transformer is very cheap with low quality.
I have a Chinese 9VDC wall-wart that measures 18.5VDC without a load. At its rated current its output is 9VDC but with a lot of Ac ripple and boy-oh-boy it gets HOT!

A Hammond power transformer has good spec's. Rated for 30VAC its output with no load is 31.0VAC that produces 42.4VDC. Their 28V transformer is even better.

b) When I use 240V input, then I get 34V on the 30V tapping.  Which combination is the best for the regulator circuit?  Using combination a) so far I can only get 29.7Vdc through the power supply at 5A, but I can live with that - 30V would be nice.

Your elecricity voltage varies too much and your transformer has terrible regulation.

2) I checked on the effect of reducing R7 from 0.33ohms to about 0.25ohm.  No noticeable change, and LED behaviour the same.

I don't know which opamps you are using. The typical DC voltage gain of a TLE2141 is 220 thousand and the transistor it drives multiplies the gain by at least 100. So the LED should be either ON or OFF, not dimmed. Your circuit must be oscillating.

3) 10V zener is correct. I get 39.3V on the bridge rectifier +ve, and 29.3V at the emitter of Q3.

Perfect. Some people use a power zener that does not drop 10V at the low current used here.

4) I am busy replacing Q3

Make sure you have not swapped its emitter and collector pins.

5) I will reduce R22 from 3.9k to say 2.8k and see if this changes the LED behaviour at all.

It will change the brightness a little.

6) The voltage drop across the 2 x 1N4148 is 1.52V (D2 and D3 are not labelled)

Thjat is fine.

7) Some other observations:  U3 pin 4 = -1.52V, U3 pin 3 varies 0 to 1.5V depending on P2 setting, U1 pin 6 is correct at 11.2V

Perfect.

 

[redwire]

You said that the adjustment doesn't work unless U2 has a -5.6V supply. Then the total supply for U2 is too high.

U2 is not tied to the negative rail in my PS, what I meant to say is that the PS output will go to 0V (with no load )on the one that has the 5.6 zener.    That one is driven by a rewound MOT that puts out about 44V resulting in about 50V output. It is certainly outside the TLE's range but has been working fine for the last several years.

    The other PS that is not in a case, is based on the latest design and uses the 2 diodes, and will go down to about  0.4V output.  It is the one shown in the picture on the 1st page of this thread.  If you look closely at the picture, lower left hand corner of the PCB you will notice I had the boards built to use the zener or two diodes (D2 and D3).        Now it is possible that the display and internal fan on the cased unit puts enough load to bring it to 0V whereas this one not in a case will not go below 0.4V regardless of any adjustment on RV1.  This issue has been noted by several builders with several suggestion such as going to a different cap or using a resistor.    Just wondering if U3 is over riding something and allowing a small voltage to show.

 

[red baron]

Audioguru/Redwire - thank you both for all the input and help.  While trying to make further voltage measurements on the pcb I managed to blow up the regulator circuit, and have now had to replace the 10V zener, Q3, and LED.
The PS now operates as before, without the 'dimming' of the LED at high currents, but still with the LED illuminated when the current limit (P2) is turned down to nearly off.
Regrettably I cannot afford more time to fix this 'bug' and since the PS works fine, I will live with it as it is.
Once again, thank you for your interest.

 

[mendimano1]

Hi red baron , try to replace the R17 from 33 Ohm 1/4 watt rezistor to 47 Ohm 1/4 watt rezistor.

 

[red baron]

Hi mate
Thanks for the idea, but already changed to 56ohm with no effect.  I suspect that there is something wrong somewhere but cannot find it.