0-30 V PSU negative supply not working

[st2]

Hi folks. I built this PSU in its original version, with 081 opamps and 5.1V zeners. I understand the circuit is rather old and has been improved over time, but I already had the PCB and the whole thing is getting me to learn something, so I'm trying a troubleshoot before tossing it into the waste.

I have 23.4 VAC on input and 30.4 VDC on C1. Unfortunately the negative supply around D7 isn't working. Checking on multimeter and scope (old CRT with no measure function) I have:

on D6 anode, a pseudo sinewave with -1.8/-8.7 V p/p

on D7 anode a jagged, flatted top wave with 0.7/-1.5 V p/p

So no zener regulation is happening. I tried shorting out R2 in order to have more voltage swing, but this brought D7 anode only to 0.8/-2.8 V p/p.

I don't understand why my voltage swing on D7 is so low, starting from 0-30 V p/p on R2. Before starting desoldering things like mad I'd like to understand.

Thanks for helping me if you can. Cheers,

Luca

 

 

[HarryA]

Putting just the front  end into the simulator. The labels in large print map back to the schematic. Using a 4.7 volt zener simulator doesn't have a 5.1 zener.

 

In these traces the green is the voltage on the negative rail and the yellow is between D6 and R3.

 

In this one the green trace is between D5 and D6 while the yellow trace is the current through R2.

And this one is with C3 disconnected. voltage  at the negative rail.

[st2]

Thanks for your kind reply!

I tried simulating that too (Simetrix), sorry for bad image without captions, top is D5 anode voltage, then D6 anode, then D7 anode, bottom: current in mA in negative rail. No zener regulation here too...

[st2]

I had some time to analyse the simulation, it appears there's a problem of current, the negative rail needs 40 mA but the charge pump can provide no more than 9-10 mA before collapsing (with my 23.4 VAC to start from).

On my board I discovered that U3 is too hot to touch, perhaps it's shorted and overloading the rail (I had no more sockets so I soldered it unsocketed... poor stupid, hard work now🤐. Anyway, -5.1 to +30.4 supply seems a hard beating for a TL081, isnt'it?)

Changing the value of R13/R14, could the circuit work with a lower negative rail, say 2V7 or 3V3?

Cheers,

[HarryA]

On 2/19/2021 at 8:23 AM, st2 said:

Anyway, -5.1 to +30.4 supply seems a hard beating for a TL081, isnt'it?)

I must admit the more I look at the circuit the more confused I get. Why the negative rail connects to the TL081 is beyond me. Why it connects to the 30 v rail when it is only rated 18v max is a mystery to me also.

Also there are many different up grades to the circuit I gather; for example I have this one I got from somewhere. Note the differences in connections to negative rail and a voltage regulator not  the zener diode.

Also if you search for "0-30 V PSU not workingy"  on the internet you will find this PSU has been beaten to death numerous times on various forums including this one.

 

[st2]

8 hours ago, HarryA said:

Also if you search for "0-30 V PSU not workingy"  on the internet you will find this PSU has been beaten to death numerous times on various forums including this one.

 

Yes I realized that 😀

Well, I've got a populated board and I'd like to try getting it to work to avoid contributing to electronic waste 😁. The board is small and limits modding and upgrade opportunities. I'll try rigging up 4 AA batteries for -6V negative supply and see what happens. I tried simulating a 7905 but, for this option too, voltage swing is missing for that current. But building a dedicated aux power supply for 40 mA -5V would go beyond any common sense...