Low noise DC regulator problem

[mravlca]

Hi!

Today I made low noise regulator from this scheme"LOW NOISE" and now I'm having some problems. The positive rail doesn't respond, regardless on the trimmer value, on the negative rails I have positive voltage but it responds to trimmer value. Any idea where have I gone wrong?

Regards Aleš

[janneman]

Originally Posted by mravlca

Hi!

Today I made low noise regulator from this scheme"LOW NOISE" and now I'm having some problems. The positive rail doesn't respond, regardless on the trimmer value, on the negative rails I have positive voltage but it responds to trimmer value. Any idea where have I gone wrong?

Regards Aleš

The neg reg apparently tracks the pos, so if the pos is wrong, the neg is wrong. Forget the neg for now, it's the pos you want to get going first.
What is the pos output value, and do you have a circuit diagram?

jd

[mravlca]

The positive value is around 16.90 and scheme is on the link I posted. I had checked pcb and the scheme and I couldn't find the error. Is it possible that IRF640 is making trouble?

Regards Aleš

[janneman]

Originally Posted by mravlca

The positive value is around 16.90 and scheme is on the link I posted. I had checked pcb and the scheme and I couldn't find the error. Is it possible that IRF640 is making trouble?

Regards Aleš

What are the voltages on the inputs and the output of the opamp for the pos regulator? That will give us a clue what is wrong.

jd

[mravlca]

On out pin there is 19.5V, on the IN- 8.1V and IN+ 11V, the last one is changing depending on the trimmer positon. I don't know what happened, but now are changing positive and negative rails while I turn the pot, though the negative rail is still above GND not below.

Regards Aleš

[janneman]

Originally Posted by mravlca

On out pin there is 19.5V, on the IN- 8.1V and IN+ 11V, the last one is changing depending on the trimmer positon. I don't know what happened, but now are changing positive and negative rails while I turn the pot, though the negative rail is still above GND not below.

Regards Aleš

OK, think it through. The -IN should be half of Vout, can you see why?
But it isn't. Keep that in mind, but first things first.

There's a solder pad in the schematic, close that so that +IN gets about 6.9V from the ref.

How does it look now (forget about the neg reg for now).

Edit: turn the wiper of thetrim pot to the top of D15.

jd

[JensH]

I think I have found the problem.

Pin 4 on the op-amp is connected to GND in your layout. It should be connected to the negative supply.

It is still strange that the positive regulator does not work though. There should not be +11V on the reference diode. Have you checked that the diode is connected properly? Try to measure the voltages directly in the diode leads.
Also try to measure the voltage across R42 to check the current.

What are the unregulated supply voltages by the way? Since you have measured 19.5V on the output of the op-amp it sounds like you have a supply voltage around 21.5V. If that is the case yopu have to be carefull since the maximum rating of the NE5532 is +/- 22V. You should stay well below the maximum rating, otherwise the op-amp may be destroyed.

[janneman]

Originally Posted by JensH

I think I have found the problem.

Pin 4 on the op-amp is connected to GND in your layout. It should be connected to the negative supply.

It is still strange that the positive regulator does not work though. There should not be +11V on the reference diode. Have you checked that the diode is connected properly? Try to measure the voltages directly in the diode leads.
Also try to measure the voltage across R42 to check the current.

What are the unregulated supply voltages by the way? Since you have measured 19.5V on the output of the op-amp it sounds like you have a supply voltage around 21.5V. If that is the case yopu have to be carefull since the maximum rating of the NE5532 is +/- 22V. You should stay well below the maximum rating, otherwise the op-amp may be destroyed.

Yes, there's more wrong in that schematic. I was trying to get him to get the pos reg work first. That should work with pin 4 at gnd.

Funny that such obvious errors are in a published schematic; probably never build it.

jd

[JensH]

The schematic from lcaudio is actually correct regarding the negative supply for the op-amp, but the layout is not.

[janneman]

Originally Posted by JensH

The schematic from lcaudio is actually correct regarding the negative supply for the op-amp, but the layout is not.

Indeed. But for correct tracking, R37 and R38 should be equal, with R37 connected to the pos output and R38 to the neg output. The way it is now, makes the tracking depend on the pot setting.

jd