0-30V Stabilized Power Supply

[audioguru2]

Here is my schematic and parts list for the 3A project again:

View attachment 41306

View attachment 41307

 

[fikic]

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.

 

[audioguru2]

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.

 

[fikic]

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).

 

[audioguru2]

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?

 

[gogo2520]

hello audioguru
    Thank you for the right information.
    gogo

 

[fikic]

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??)..

 

[audioguru2]

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.

 

[bg8dmy]

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

 

[fikic]

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?

 

[audioguru2]

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.

 

[fikic]

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

 

[audioguru2]

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.

 

[petrescv]

Hi,
I'm 16 and I'm new at this. I have to build for my personal use exactly what you did. Can you show me if you have the latest pcb that works?
Thanks in advance! and sorry for my English.

 

[fikic]

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...

 

[audioguru2]

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.

 

[fikic]

I don't understand you. You are saying, like my circuit was perfect. But I am telling you, that LED is always on, without a load...

 

[audioguru2]

I don't understand you. You are saying, like my circuit was perfect. But I am telling you, that LED is always on, without a load...

I know that your circuit works wrong. I explained how it is supposed to work so you can fix it.
1) U3 works wrong so double-check that its inputs are not connected backwards or shorted together.
2) Make sure that the output pin 6 of U3 is not shorted to ground or to -1.3V. What is the voltage of pin 6?
3) Make sure that pin 7 of U3 is about +24VDC.
4) Try replacing U3.

 

[fikic]

I know that your circuit works wrong. I explained how it is supposed to work so you can fix it.
1) U3 works wrong so double-check that its inputs are not connected backwards or shorted together.
2) Make sure that the output pin 6 of U3 is not shorted to ground or to -1.3V. What is the voltage of pin 6?
3) Make sure that pin 7 of U3 is about +24VDC.
4) Try replacing U3.

Thank you audioguru, I am closer now. I figured out, that I turned Q3 backwards. But still, it doesn't work perfect. At current adjustment pot turned to lowest value, LED is not on. It turns on somewhere on the end of turn of current adjustment pot.
Voltage of pin 6 is 28V, when LED is not on. When it turns on, it falls to 13-10V.
Pin 7 is always 24V or higher. All pins of U3 are ok.
The LED is turning on with unknown reason. The voltages on U3 changes like I said with changing value of current adjustment pot. What do you say? Could be Q3 fault?

PS.: all measurements applies to NO LOAD..

 

[audioguru2]

Hi Fikic,
With no load on the project, the output pin 6 of U3 should always be high at about +24V to +28V. So it does not reduce the output voltage through D9 and it does not turn on Q3 to light the LED.

The slider of the current adjusting pot P2 applies a small positive voltage to the (+) input pin 3 of U3. The voltage should be +0.005V when P2 is turned to minimum and should be +1.414V when P2 is turned to maximum. Without a load, there is no voltage across R7 so the (-) input pin 2 is 0V. Then the output pin 6 of U3 should be always be high.
What range of voltage at pin 3 of U3 do you measure when P2 is turned from minimum to maximum?

Maybe the terminals of P2 are connected wrong.
Maybe U3 is defective. Replace it.