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help reading circuit for welding machine (negative switching supply)

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coppercone2:
I am hoping that someone good at reading circuit pathes can answer a simple question for me

https://forum.millerwelds.com/filedata/fetch?id=615167&d=1622257766

In the middle of this circuit diagram, you can see the output for -15V . The description says its regulated from the 30V

If you just feed 30V into the board, should that -15V rail start up? The -15V is generated by Q21 and inductor L1 I think.

Does it have any other input requirements for starting to generate that rail?


I bought the pomona banana to round pin adapters (great purchase BTW, the kit of them allows you to connect banana cables to adapters that go into pin sockets or over pins, and they have necks that are flexible) so I can plug a supply into the board. When I add 30V, the 15V is made, but the -15V shows +0.5V.

I need to know if I am missing some input that enables the negative voltage rail to start working. I don't think so, but its a big circuit and I thought I might be missing something.  |O

I circled the area in red which I am confused about

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Andy Chee:
Your circled area appears to be an inverting buck-boost regulator, driven by an oscillator made from a schmitt trigger NAND gate.

But I can't work out how the regulation feedback functions, suffice to say, it looks like the designer has had some major difficulty in converting a negative voltage feedback into a positive voltage feedback signal.  The entire regulation circuit is non-intuitive!

coppercone2:
yeah I loathe this blasted board

coppercone2:
Around the area, all the 15, -15 and 28 points are connected to the pins labeled as such.

I ignore the Q11 and R13 area, because that is biased at 15V, so its just a 15V diff between30 and 15, aka its between two low impedance points, so that Q11 can't do anything to the +15 V rail, and it measures at +15V when its on.


Then you have the R64 tap and the R55 path, which go into some annoying bullshit that can mess up the gate some how. I don't know what that area is doing, it looks like its on because the difference is the 3 ohms on the 28v RAIL??? ????

coppercone2:
what is supposed to oscillate this circuit. ???

The transistor driver is biased between +30V and +15V to get +15V full scale. That is correct because I measured it.

The transistor driver is controlled by Q25. All the oscillator signal must go through R55 to power the transistor base.
R55 is connected to the NAND gate.

THen it gets fucking confusing.

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