help reading circuit for welding machine (negative switching supply)

[Andy Chee]

Pin #1 of U#2 is 1.8V, #2 = 15V, #3 = 15V

Ok, with pin 1 @ 1.8V that suggests there's something wrong around Q12.  Don't forget to measure the collector of Q12, which is connected to pin 1, and should read the same because they should be connected together. 

I assume you've been supplying +28V during these tests?  With +28V & +15V connected, the voltage at U2 pin 1 should be 15V, not 1.8V.

With the schematic drawn as is, I'm not exactly sure how Q14 is supposed to turn on (thus turning on Q12 as well).  D14 & D12 look like zener diode symbols, so that should theoretically allow Q14 & Q12 to turn on. 

Do you know what voltage zener diode D14 & D12 are?  The voltage isn't marked on the schematic.  Note that if you inserted conventional diodes in these positions, the circuit will not work.

[coppercone2]

nah I left the diodes alone. I just measured continuity both ways.

OK to be extra  cautious I got my other power supply with is 30V on the mark. The last test actually turned out to be a little low (26.5V)


Now I do see a square wave!

I measured the two transistors on the tester and they look OK. I need to reattch them and check it again.

[Andy Chee]

Before you reinstall anything.  Connect oscilloscope to Q23,Q24 emitter.  There should be a squarewave here as well.

Reinstall Q15.  Leave Q21 out.  Connect oscilloscope to Q23,Q24 emitter again.  The squarewave should still be there.

[coppercone2]

The square wave on the gate of the removed power transistor (23 + 24 emitter) is a square wave from 19 to 30 volts,

so its floating on a DC

when I reinstalled the little transistor it looks the same

[coppercone2]

Ok so it actually works. Maybe the 26.5V test voltage was a little low or maybe there might have been a tiny short circuit somewhere because of conformal coating remains. The inductor sings and makes the -12V rail (I forgot to mention that it actually is/was -12V according to my previous measurements and other forum reports about this card)

There is a good chance its something on the motherboard then. Oh I loathe that Skiip IGBT construction. The 30V rail must be acting up.

At least that narrows it down by a huge amount.

Actually I can test to see what the UVLO is on this rail

It makes the power supply cranky at 27V turning on and off. I damaged the display on my 40V linear supply (thanks micro-gear-pump) (this is done with the 250V supply) and I thought I could get away using the 25V dual supply. So it was just a red herring.


BUT the noise that it makes, at 27V, where the supply is fluctuating/unstable is the noise the welding machine was making alot. I guess it means unstable negative rail, or unstable 30V rail, or low rail voltage, or fluctating rail voltage. It is a annoying CRT "dancing/squirming" noise. That is a good thing to know. I always suspected the machine was supposed to sound "more stable". Sort of like a angry fly trapped some where.



But holy crap that circuit for the negative rail is ludicrous IMO. I can't believe they did not try to integrate that a little more. It seems insane. Did they really need that much crap there? Its like 1/3 of the board.


It's like 40 parts, and it does not even feel like its built good. I can understand if there was protection parts and shit like that but it just seems like a big freaking mess. I mean if they had like filters and clamps and stuff I can see the parts count getting high, but this is just like wut?


Do you see some...  strength of this power supply? I am not buying that they were scared that a negative regulator IC is gonna be a supply chain limitation, they are too common.

I don't even think it could be considered reliable, because they share the NAND gate with other circuits. So they are sharing a NAND gate connected to a big inductor power supply, and basically linking the power supply to a whole bunch of other control circuits. I think that gate in the feed back path should be its own gate, not piggybacked, if you wanted this design to be 'good'. I mean there is a signal path in that chip to the 810VDC rail for christ sakes. Its sharing a NAND gate between a -15V instrumentation supply and the mains connected 810V down regulator. that seems like cascade failure to me. And its freaking controlling 810VDC through a header connector and that is for a iGBT control. I think this thing is built like the death star for self destruct. Why the hell is that regulator not placed locally. to me its bananas. I think thats fitting like a freaking   primary nuclear reactor control system in the mail room of a building

[Andy Chee]

But holy crap that circuit for the negative rail is ludicrous IMO. I can't believe they did not try to integrate that a little more. It seems insane. Did they really need that much crap there? Its like 1/3 of the board.

Any idea when this board was designed?  If this welder was from the 70s or 80s, integrated chips for dedicated SMPS may have been relatively expensive and hard to source back then.

Another explanation is that the power supply was first initially designed by an undergraduate student doing work experience for the company, and the design was just copied from generation to generation without any thought of revision.  R&D costs money.

Other than that, I agree, what a mess of a circuit design!

[coppercone2]

the miller dynasty is the 2000's flagship state of the art unit from miller at a price tag of 4k+ USD

[coppercone2]

See I thought maybe it had something to do with CMRR or immunity but

I don't see any components I associate with 'protected'

and I see a giant 'circuit'

I think that means its worse. 

Its like really freaking bootleg for a switcher. The output ripple looks crappy too. You would think it would have like filters

This reminds me of the circuit you learn how a DC DC converter works on a bread board in electronics 102 class with some extras, and given the gigantic size, it might have similar performance lol

and it sounds annoying too

[Andy Chee]

I don't see any components I associate with 'protected'

I mentioned several posts ago that R64, Q18 & Q15 form a MOSFET current limiting circuit.  It's pretty rudimentary, but functional.  As current increases through R64, the voltage increases across R64.  The increased voltage turns on Q18, which turns on Q15, which shorts out C18 and causes the oscillator to shutdown, which turns off the MOSFET.

I asked before what was on the -15V receptacle RC1?  Maybe this rudimentary protection is sufficient for whatever is connected to RC1?

[coppercone2]

I still have to investigate that. I think it provides power to the other PCB that is very complicated with lots of CD gates and op amps etc.

The card its connected to is called the inverter control card. this card is called the boost control card

By protection I mean ESD, RFI, awesome parts etc. Current limiting is a basic function of most controllers. Its not 'guilding the lilly"

I just mean you don't get any 'extra' feature from the way they built it, related to the parts count. You would think there should be a good reason behind that circuit being the way it is! I don't think it offers better performance or durability compared to a controller based one.


You know I mean like in the sense of its not  "they had to use this topology because they used extra high reliability hermetic 3904's in a proven mil-spec design with whatever whizz bang cool tech they have in nasa that helps justify the price and complexity over using a unreliable switching integrated circuit with generally superior performance"

Because there are sometimes discrete designs that have a justification. I don't think this is one of them lol


I can't talk too much because I don't work with designing switching power supplies and I am not too interested in them

[Andy Chee]

I just mean you don't get any 'extra' feature from the way they built it, related to the parts count. You would think there should be a good reason behind that circuit being the way it is! I don't think it offers better performance or durability compared to a controller based one.

As I suggested earlier, it's likely their R&D is being performed by an unpaid engineering student intern.

As far as cutting edge technology goes, welding machine control boards are a far cry from designing automotive engine management computers, for example.

[coppercone2]

the price says otherwise 

I am just being careful not to accidentally trash some nice 'analog' or 'discrete' design because it looks 'different'


I think they really could have made it a little better for the market segment they thought it should be in. it seems insane they handed the design of this machine control electronics to a intern.... its a real dilbert's boss type decision.

[Andy Chee]

the price says otherwise 

One shouldn't be surprised just how high product mark ups can be, particularly if there's name recognition behind the brand!

[coppercone2]

the price says otherwise 

One shouldn't be surprised just how high product mark ups can be, particularly if there's name recognition behind the brand!

its supposed to high light american industrial manufacturing might 

B2 bomber. GPS satellite. Space Shuttle. Miller dynasty.

Marketing put it up like a space shuttle . I guess its more like a "ford"