How this linear PSU circuit works, and why it doesn't

[xavier60]

The first one has a 10-turn pot for voltage and a standard pot for current. This time I'll use standard pots with fine/coarse adjustment. The 10 turn pots are chinese "bourns" and not once I had them fail.
I'm not happy about the panel meter, these are the only digital ones that I could find and that display both voltage and current, but don't like them. I'm thinking of building one myself with an MCU.

The few 4 digit meters I have bought lately are much better, the current metering is auto-zeroing.
I make some comment here, https://www.eevblog.com/forum/projects/volts-amps-digital-meters/msg3685030/#msg3685030.
An MCU could be used for other tricks also. Mine toggles the PSU On/Off with the press of a momentary button and controls the fan.
When the 10 turn pots started failing, I added protection that watches for sudden jumps in wiper voltage.
Since switching to Bochen wxd3-13-2w 10k pots, I have had no more open wire type failures. They can go scratchy. Some PAO oil fixes that.
The wxd3-12-2w series pots are 5 turn and shorter.
My MCU also does the transformer tap changing.

[bogdan2014]

I ordered some Bochen pots and tried them. When the voltage pot reaches one end (the 0V one), the BD140 transistor goes shortcircuit between E-C. No idea why, it didn't happen with the normal pots. I'm also investigating using digital pots, and I can also use an LCD with the MCU to measure the voltage and current.

[xavier60]

I ordered some Bochen pots and tried them. When the voltage pot reaches one end (the 0V one), the BD140 transistor goes shortcircuit between E-C. No idea why, it didn't happen with the normal pots. I'm also investigating using digital pots, and I can also use an LCD with the MCU to measure the voltage and current.

That's very strange. Was the PSU loaded? What is the unregulated voltage into the regulator?

[xavier60]

Is the BD140 on a heat sink? How many times has it failed in the same way?

[bogdan2014]

It failed each time I tested it with the new pot (twice until now). It was on a heatsink but it never got hot, and it failed very quickly, I just heard a small pop and that was it, no smoke no heat. No other components were damaged. I teste both with the PSU loaded and unloaded, same thing. I'll investigate further today.

[bogdan2014]

I have two identical transformers (30VAC and about 3-5A) and I'm thinking of making a dual PSU out of them. I will be building two of these boards and run them in series, and I'll have dual pots to adjust voltage and current at the same time. However, I can't fit two voltmeters and two ammeters so I'm thinking of only using one, but measuring the current at the center tap. Is that possible?
The panel meter is one of those chinese digital ones with seven segment displays, with both volts and amps reading in the same unit. It measures current at the return terminal.

[xavier60]

Just curious about how the earlier PSU project is working out?

[bogdan2014]

Just curious about how the earlier PSU project is working out?

It's actually that one that I'm trying to modify into a dual tracking PSU. I gave up on multiturn pots and just used plain old single turn ones and it works as it should.
I even played around with digital pots but that's another story.