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0-30V Stabilized Power Supply

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Hello!!
I'm from Austria, so please excuse if I don't speak english well  ;)

One month ago I built the version of the PSU which is on the homepage and it worked very well. But few days ago it broke down and I don't know how it works and so I'm not able to repair it without your help.
The problem is that the voltage doesn't rise higher 2V....  :-\
I hope you can help me to repair it and please speak nice english, else I wouldn't understand it  ;)

Thanks
Clemens

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Hi Clemens,
The original version of this power supply had many parts that were overloaded. So I fixed it.
I would guess that the overloaded Q2 transistor (2N2219) on your project has failed but it also could be the output transistor Q4 or any of the opamps that have a supply voltage too high for them.
Maybe one or more overloaded resistors has failed.

Even the power transformer is overloaded so maybe it has failed.

With the voltage turned up, measure the output voltage of opamps U1 and U2 and U3 then measure at the emitter of Q2 and report here the measurements.

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Hey
Thanks for the reply
Here are the voltages:
Output: U1 = 11,24V  that voltage is constant no matter of the setting of the potentiometers
            U2 ~ 31,5V    that is at highest setting of the voltage potentiometer and if I turn down the voltage below  0,8V (meanwhile 0,8V is the maximum voltage, at first it was 2V and it sinks permanent)
            U3 ~ 32,9V    if I activate the current control it sinks heavily but i think that is normal, right?
Emitter:
            0,32V and it sinks permanent also if I don't do anything

When I built the PSU I measured the supply voltage on the opamps but the voltage wasn't higher then the maximum voltage of them so I thought that the post on the homepage about the supply voltage of the opamps wasn't right.

This is the post i mentioned ".If you connect them as it is shown in this circuit diagram that voltage will be about 45VDC, so they will burn down immediately."

Clemens

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Hi Clemens,
1) The 11.24V output of U1 is perfect.
2) The output of U2 goes to +31.5V which is good. Then the output of the project should go to +30.5V without a load and to maybe +25V with a 3A load.
3) The output of U3 is normal.
4) The emitter voltage of Q2 is very wrong. It is an emitter-follower of the output of U2. Its emitter output should be about 0.5V less than the output of U2 without a load and should be about 3V less than the output of U2 with a 3A load. Q4 is also an emitter-follower. Its emitter output should be about 0.5V less than the emitter of q2 without a load and should be about 1V less with a 3A load.

Maybe Q2 is burned out or the pins of Q2 or Q4 are connected backwards?
If the 2N2219 is burned out then replace it with a BD139 transistor on a medium size heatsink.

With a 24VAC transformer that is fully loaded at 3A AC, its peak voltage is 24 x 1.414= 34V and the rectifier bridge reduces it to 32VDC for the positive unregulated supply. The opamps also have a -5.6V negative supply so they are operating with a total supply of 37.6V which is higher than the maximum of 36V on their datasheets.
When there is no load then the 24V transformer will be about 26VAC then the total supply to the opamps is about 41V which is much too high and will be higher if your mains voltage is high.

If the opamps burn out then replace them with MC34071 or TLE2141 opamps that are rated for a maximum supply of 44V.

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Thanks thanks for all the answers

So you mean I should change the 2n2219 to a bd139 with heatsink?
That's good because I still have a bd139  :D
It's very good that the opamps are not broke down but if they will break down I will change it to the better opamps you mentioned.

I will tell you when I repaired the PSU and it work.

Thanks anotherone  ;D

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I'm going to do this PSU in 3A version with 30V 250VA transformer (with an idea for +/- PSU). So I have some questions before i start constructing it. How big should be the heatsink fot the TIP3055 transistros, and should the BD139's heatsink be separated from TIP;s one? Should the heatsink be separated from transistors' body, and from the Ground.
P.S. These are my home made PCBs :

post-59332-14279144289209_thumb.jpg

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Hi Morpheous,
You should attach your photos here with your reply because ImageShack takes too long to load.

The latest version uses 2N3055 transistors in a TO-3 steel case because their max allowed dissipation is 115W each (with a perfect heatsink). Your little TIP3055 transistors are in a plastic TO-220 case and have a max allowed dissipation of only 90W each. So the heatsink for your little transistors might need a fan or a 3rd transistor.
Calculate the thermal resistance needed then look for that number on heatsink spec's.

Q2 is a BD137 or BD139 which is an NPN type, not a BD138 which is a completely different PNP type. It is on its own medium size heatsink. Calculate the thermal resistance needed for its heatsink.

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Hello audioguru , What is the role of negative voltage?  Why negative voltage to as low as-1.3V, or so could use a 3V  zener diode instead

in the latest version, the output of U3 drives D9 to reduce the output voltage to zero when the output is shorted. D9 has a forward voltage drop of about 0.7V so the output of U3 must be able to go a little below -0.7V. I used two diodes to make the negative supply for U3 at about -1.3V.

A 3V zener diode or even a 5.6V zener diode can be used instead of the two diodes.

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in the latest version, the output of U3 drives D9 to reduce the output voltage to zero when the output is shorted. D9 has a forward voltage drop of about 0.7V so the output of U3 must be able to go a little below -0.7V. I used two diodes to make the negative supply for U3 at about -1.3V.

A 3V zener diode or even a 5.6V zener diode can be used instead of the two diodes.

Thanks~
I did a  circuit, but found ineffective regulator, for example, in the 3A maximum current range, adjust the voltage of 20V, but the addition immediately after the load into 16V. What is the reason?

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I did a  circuit, but found ineffective regulator, for example, in the 3A maximum current range, adjust the voltage of 20V, but the addition immediately after the load into 16V. What is the reason?

For a DC output of 3A, the transformer must supply 4.2A AC. C1 must have enough capacity to filter properly.

Did the warning LED for the current regulator turn on? Current regulation is done by reducing the output voltage. Maybe the current setting was too low for 3A.

U2 is the voltage regulator. It drives emitter-followers Q2, Q4 and Q5 to supply enough output current.
U2 has a typical open-loop voltage gain of about 200,000 so if the output voltage tries to drop 4V then U2 amplifies the error which produces a drop of only about 0.001V to 0.01V.

Maybe you connected the emitter and collector pins of Q2 backwards or maybe Q2 is shorted.
Maybe Q4 or Q5 has its pins connected backards or is shorted. 

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I want to ask, can I meke the circuit and use a transformer (for example 22V) just to try if everything is OK with the board. And after that to order the transformer which will be used. Is that possible, and if is, what to expect from the circuit to be sure all is OK.

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The circuit will work with a 22VAC transformer but its max output will be about +28V with no load and about +23V with a 3A load. When you turn the voltage setting pot higher then the output will be full of ripple when there is a load.

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The circuit will work with a 22VAC transformer but its max output will be about +28V with no load and about +23V with a 3A load. When you turn the voltage setting pot higher then the output will be full of ripple when there is a load.

My aim is only to check the boards, if they work I'll take the nessessary transformer.

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The latest version uses 2N3055 transistors in a TO-3 steel case because their max allowed dissipation is 115W each (with a perfect heatsink). Your little TIP3055 transistors are in a plastic TO-220 case and have a max allowed dissipation of only 90W each. So the heatsink for your little transistors might need a fan or a 3rd transistor.
Calculate the thermal resistance needed then look for that number on heatsink spec's.

Q2 is a BD137 or BD139 which is an NPN type, not a BD138 which is a completely different PNP type. It is on its own medium size heatsink. Calculate the thermal resistance needed for its heatsink.

I didn't want to use 2N3055 because of the TO-3 package. I'm not familiar with it, and I don't know how to connect it. As I saw the collector is the steel case. So, should the steel case be separated from the heatsink or not, and how to connect it(especially collector)? The BD139's heatsink should be separated from the others one, but should it be separated brom BD's case?

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I didn't want to use 2N3055 because of the TO-3 package. I'm not familiar with it, and I don't know how to connect it. As I saw the collector is the steel case. So, should the steel case be separated from the heatsink or not, and how to connect it(especially collector)? The BD139's heatsink should be separated from the others one, but should it be separated brom BD's case?

Mounting kits are sold for TO-3 or for TO-220. Mica insulators are also sold for both.

Depending how you will mount the heatsinks, the transistors can be bolted directly to the heatsink then the entire heatsink can be insulated from touching ground, or the transistors can be insulated from the heatsink. Use thermal compound for both ways.

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This is PicMaster's PCB. I have a question about the PDF. The white print on page 3 and the PCB connection on page 6 aren't same. The parts names and their places are different. I think the white print on page 3 is the correct one, but am I right?

I hate the confusion caused when somebody takes an existing circuit then changes all the parts designation numbers.
I don't know which one is correct and don't have time to sort it out. Ask Picmaster.

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This is PicMaster's PCB. I have a question about the PDF. The white print on page 3 and the PCB connection on page 6 aren't same. The parts names and their places are different. I think the white print on page 3 is the correct one, but am I right?


I will look into it and let you know, Your the first one to spot it.

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For what purpuse are the 3 trimpots? For what ajustments? And is there a way for fine ajustment of the Voltage and Current.

The trimpot beween pin 1 and pin 5 of U2 nulls its input offset voltage. With the voltage set to 0V then the trimpot is adjusted for ezactly 0.0V.

With the output voltage set to maximum then the voltage calibration trimpot is set to 30.0V.
With a 3A load and the current regulation set to maximum the current regulation calibration trimpot is set to 3.0A.
I didn't compare PicMaster's schematic to mine because it might have the trimpots with different values and in different locations.

I think using coarse and fine adjustment pots will be a nuisance. one pot will always be adjusted wrong.
Simply use a big knob on the pot that is about 5cm in diameter.

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