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0-30V Stabilized Power Supply

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I built it (3A version)

Which version? Which opamps?

I have a few issues:
1) current limiting LED is always on (if I turn current pot to max value it even lights more)

Opamp U3 us used as a comparator with a voltage gain of about 200,000. Its output switches on and off, not as a linerar fader. It turns Q3 off when the output current is less than the setting of the current-setting pot.

I just can adjust minimum voltage to 50mV with voltage trimmer

If there is a load then the voltage setting pot should adjust the voltage to 0.0V. The offset adjust trimpot sets the offset so the output can go to 0.0V when there is a load.
C7 is an electrolytic type that produces "dielectric absorption" which produces an output voltage when there is no load. A film type of capacitor for C7 fixes it.

what can I adjust with other two trimmers?

I told about the offset adjust trimpot. There is a voltage trimpot that sets 30.0V when the voltage pot is at max and there is a current trimpot that sets 3.0A when the current setting pot is max and there is enough load.

However, PSU works fine, the maximum output voltage with no load is 40V.

You should adjust the voltage trimpot so that the max output voltage is 30.0V so that the circuit regulates properly without ripple when it has a load.

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Which version? Which opamps?

The newest version with mc34071 opamps.

Opamp U3 us used as a comparator with a voltage gain of about 200' date='000. Its output switches on and off, not as a linerar fader. It turns Q3 off when the output current is less than the setting of the current-setting pot.[/quote']
I don't understand this part.. ???


With RV2, I can set the maximum output voltage. If current pot is set to maximum, I can regulate it to minimum 30V. But If I turn current pot to minimum, I can regulate it to minimum 40V. Something is wrong here...

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I don't understand this part.. ???

Opamp U3 is a comparator. It compares the voltage at its inputs and its output is either high at about +24V or low at about -0.6V.
When its output is low then it reduces the output voltage through D9 until the current is the same as the setting on the current-setting pot P2. It also turns on Q3 which lights the LED.
When its output is high then it does nothing.

Its (-) input measures the voltage across R7 produced by load current on the circuit. Its (+) input voltage is set by the current-setting pot. When the (-) input voltage exceeds the (+) input voltage then the output of U3 goes low.

With RV2, I can set the maximum output voltage.

But RV2 on my latest schematic adjusts the output offset voltage a little. P1 adjusts the voltage and P2 adjusts the current.
Whose schematic did you use?

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You've disunderstood me. I am also adjusting voltage with P1 and current with P2, but with RV2 I can set the maximum output voltage.

I used schematic that you posted it one page back.
PS.: I still don't know, why LED is always on. What is the reason, what can I do?

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I've noticed something new:
If I have pot for voltage adjustment set to maximum value and I am changing current limit pot value from minimum to maximum, something strange happens. At it's minimum, output voltage is 40.5V. Then I am slowly raising it. The output voltage stays 40.5V for about 1/4 of a circle, then it starts to fall to 30V, and then again raising to 35V, and than finally falls to 24V at maximum value of pot for current adjustment. I don't understand. Is my pot broken?

PS.: And I am using 24V/250VA transformer.

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I've noticed something new:
If I have pot for voltage adjustment set to maximum value and I am changing current limit pot value from minimum to maximum, something strange happens. At it's minimum, output voltage is 40.5V. Then I am slowly raising it. The output voltage stays 40.5V for about 1/4 of a circle, then it starts to fall to 30V, and then again raising to 35V, and than finally falls to 24V at maximum value of pot for current adjustment. I don't understand. Is my pot broken?


So you have used Audio's schematic ?, Did you redraw the schematic and then transfered the design over to the PCB package then once you laid it out you ran the DRC rule checker ?
or Who's PCB layout you used ?,
The only reason I ask is if you have just laid out the PCB yourself just from the schematic then you could have errors in your PCB.
When I did mine I redrew the schematic check it and double check it against the orginal then transfered the design internally to the PCB package, Then Laid out then design then ran the DRC rule checker this tells me that it is 100% routed and error free.  Mine worked first time has it should whenI built mine.

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boy I swear
        This thread is starting to read like a Linux man page with all the misinformation floating  around, can some please post the right schematic and parts list for the 3amp supply.  I think audio's is the right one but with all the European influence its hard telling ( just like Linux ).  Not the 5amp just the 3amp.  **I thought a 5amp thread was started years ago.**

gogo 

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This is exactly the one I built. So, audioguru, can you answer me, what can I do about my issues to solve them. I also noticed, that PSU has no power. If I connect a load on it, the voltage drops unnormaly.

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I think, there is no problem with this schematic and PCB.

I didn't look at your pcb but the schematic is correct.
But zener diode D8 has only 5.6mA in it so i specified a BZX79 one that is tested at 5ma. You used a BZX85 zener diode that is tested at 45mA and it might have poor regulation at only 5.6mA.

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No, that's just on my schematic becouse of Kicad (doesn't have BZX85), but actually, I used BZX85C.
What could be the reason also? Why is voltage changing when I turn current adjustment pot? I double checked PCB and it's completely short connection free. It's soldered good and everything is checked with Kicad, so all connections are fine. There must be something with components, or my transformer is the problem (24V/250VA).

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When you turn the current adjust pot P2, measure the output voltage of opamp U3. It should stay high at about +22V so it cannot cause D9 to reduce the voltage at the input of opamp U2.
Maybe D9 is leaking and possibly caused by light shining on it.

Look at the datasheet for a BZX85C5V6RL, it is tested at 45mA and its max dynamic resistance at 1mA is pretty high so it regulates poorly. Why don't you use a BZX79C5V6RL that is tested at 5mA?

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When you turn the current adjust pot P2, measure the output voltage of opamp U3. It should stay high at about +22V so it cannot cause D9 to reduce the voltage at the input of opamp U2.
Maybe D9 is leaking and possibly caused by light shining on it.

Look at the datasheet for a BZX85C5V6RL, it is tested at 45mA and its max dynamic resistance at 1mA is pretty high so it regulates poorly. Why don't you use a BZX79C5V6RL that is tested at 5mA?


The voltage on U2 output raises and falls like output voltage if I turn current adjustment pot. The lowest voltage on outpu of U2 is 17V, the highest is 35V (I was meassuring pins 6 and 4??)..

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The voltage on U2 output raises and falls like output voltage if I turn current adjustment pot. The lowest voltage on outpu of U2 is 17V, the highest is 35V (I was meassuring pins 6 and 4??)..

That is normal when there is a load on the output of the power supply.
When you turn down the current-setting pot then the current regulating circuit reduces the output voltage to keep the output current at the setting of the current adjustment pot. The LED lights to warn you that the output voltage is not regulated anymore because the current-regulating circuit is reducing the output voltage.

Your circuit has a serious problem if the output voltage drops when you turn the current adjustment pot and there is no load current.

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You are trying to make the original circuit that does not work properly and has many overloaded parts.
We fixed it and changed it and the new schematic and new parts list have been posted here many times.

Your 25VAC transformer produces a positive unregulated voltage of only 34VDC which is too low. Your supply will produce a maximum regulated output of pnly 26VDC at 3A, not 30VDC.
The new parts list has a 28VAC or 30VAC transformer and new 44V opamps. The new opamps work with no negative supply and a low voltage negative supply.

Your idea has a positive 34VDC supply and a negative 5.6V supply which is a total of 39.6VDC when there is a full 3A load and a total of maybe 43VDC when there is no load.
But the original TL081 opamps and the ones you are talking about have an absolute maximum allowed total supply of only 36VDC.

The new circuit has a positive supply that is +37.6V to +42.4V. It has a negative -1.3V supply for one opamp. Then its maximum total is less than the 44V max of the new TLE2141 or MC34071 opamps.
The old opamp and the opamps you are talking about do not work without a negative supply or a negative supply as low as -1.3V.





Thank you for your reply~
Regards ~
bg8dmy

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That is normal when there is a load on the output of the power supply.
When you turn down the current-setting pot then the current regulating circuit reduces the output voltage to keep the output current at the setting of the current adjustment pot. The LED lights to warn you that the output voltage is not regulated anymore because the current-regulating circuit is reducing the output voltage.

Your circuit has a serious problem if the output voltage drops when you turn the current adjustment pot and there is no load current.


Yes, it has and if I connect a load on it the voltage drops unnormally and there is no power. What could it be?

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Your circuit has no output power maybe because you connected the collector and emitter of an output transistor or the driver transistors backwards.

U2, Q2 and the output transistors are an amplifier that can have a load of 3A and the output voltage will drop only 5mV (0.005VDC) which is caused by the voltage drop of the wires.
If the emitter-collector pins of a transistor are backwards then the output voltage will drop when there is a load.

Double check the value of R7. It should be 0.47 ohm and some people say R47.

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I made a mistake when I was drawing schematic. I didn't connect - of C1 to gnd. I corrected PCB now, and PSU works when I connect a load. But only in some cases, when I turn pot for current regulation, the voltage drops. LED is still on the whole time. Now the voltage on output of U3 is 24V but not all the time. In some cases it's 17V when I turn current adjustment pot.

sch.pdf

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I made a mistake when I was drawing schematic. I didn't connect - of C1 to gnd. I corrected PCB now, and PSU works when I connect a load.

Of course it didn't work without C1.

when I turn pot for current regulation, the voltage drops.

That is normal. it regulates the output current by reducing the output voltage which reduces the output current according to Ohm's Law.

LED is still on the whole time.

The LED should turn on only when it is reducing the output voltage.

Now the voltage on output of U3 is 24V but not all the time. In some cases it's 17V when I turn current adjustment pot.

That is normal. The output voltage of U3 drops when it regulates the output current and it turns on the LED.

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Of course it didn't work without C1.


That is normal. it regulates the output current by reducing the output voltage which reduces the output current according to Ohm's Law.


The LED should turn on only when it is reducing the output voltage.


That is normal. The output voltage of U3 drops when it regulates the output current and it turns on the LED.


All this I wrote applies to the case, when I have no load on output...

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Then it is easy to fix.
Without a load, there is no current in R7 then it has no voltage drop so the (-) input of opamp U3 is at 0.0V. But The current-setting pot P2 has a small positive voltage from R18 and the current trimpot so the (+) input of U3 is at a positive voltage which makes its output go high which stops current regulation from reducing the voltage at D9 and stops transistor Q3 from lighting the LED.

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