AC transformer welder open circuit voltage step-up

[eneuro]

Hello,
I've in my hands this quite bylky trafo  230VAC -> 24VAC and it is used in spot welder project, but its oryginal 24VA secondary is not removed, so I thought I could try upgrade spot welder software on its primary to be able support AC welding.


Maybe I will add synchronous rectification to get DC output as an option.
For the moment spot welder primary is driven by those SCRs in anti-parallel configuration


It has reinforced 5mm AC mains PCB tracks with 3mm2 copper wires + solder and SCRa are rated at 25A each, but in this anti-parallel configuration each of them conducts only within 10ms period at 50Hz mains (50%).


Looking for a way to step-up welder transformer output open circuit voltage to lets say 60V-70V from its ~35Vmax, I guess.

I've found quite interesting description of solusions to (maybe) similar problem there:
Patent US 6156999 A: Method and device for welding arc ignition for arc welding apparatus

and there are some concepts, but unsure if AC welding voltages are supported 

Anyway, I want try first this concept taken from this patent analysis:


I've got some idea to use classic 12VAC<-230VAC transformer where while reversed and powered its secondary with lets say ~5VAC at given frequency (maybe higher than 50Hz or synchronized with AC mains) output up to 100V on its primary, but low current since I will provide low ~5VAC current , so it will go down even more on HV side since estimated tradfo ratio is 230VAC->12VAC: ~19:1, so 20ma 5VAC tranformed to 100VAC but 1mA only, which how do you think, could be safe input to 1:1 transformer with a few turns of wire to connect  it in series with welder transformer secondary and provide HV pulses to help ignite arc?

This is similar concept to this shown on image taken from this patent description linked above, but first trying to figureout how to step-up this voltage it in a safe way, while of course we have electrodes and metal base which can be touched by humans, so close to 100V open circuit have to be at very low output current, so trying to figureout for example how big this output capacitor might be ?
Anyway I have AC, not DC output for the moment, so any ideas welcome how to step-up this opencircuit voltage and what could be safe current levels lets say in the case of 50Hz synchronized with AC mains or something more tricky like providing 1kHz or more of low current close to 100VAC via 1:1 trafo, but we do not want of course to many turns there, while welder electrodes need to be thick link copper wires 

I like this concept with additional trafo (I have spare bulky MOT E core to make some experiemnts), but as always question is howto do it in a safe way, so my idea is start with low voltag elow current input in DC-DC converter (transformer), so I hope I will be able provide even less current but a few times higher HV independent of welder transformer 35Vmax open circuit ? 

[eneuro]

I've found quite interesting description of solusions to (maybe) similar problem there:
Patent US 6156999 A: Method and device for welding arc ignition for arc welding apparatus

and there are some concepts, but unsure if AC welding voltages are supported 

Nope,  after reading summary of this invention we can find there among other something like this:

According to further features in preferred embodiments of the invention described below, the high voltage short pulse is transmitted from the first high voltage power source within 1 to 10 microseconds, at a voltage of 0.5 to 15 kV and at a current of 1 to 10 A.

According to still further features in the described preferred embodiments the long high voltage pulse is transmitted from the second high voltage power source within 0.1 to 100 microseconds, at a voltage of up to 1 kV.

According to still further features in the described preferred embodiments, the open circuit voltage of the welding power source is preferably 60 to 100 V.

Yes, they says their method is capable to ignite arc in AC and DC welders, but... they already have decent high voltage open circuit 60V to 100V 

Maybe, this trafo will be useless alone as welder, but maybe plasma speaker could help ignite arc? 


However, they use tricky ZVS flyback drivers... but a few centimeters discharges looks quite cool and... this plasma music... awersome 

There plasma music better than... from my old Creative speakers 


Imagine, welder plays you plasma music  in the background of welding sparks 

[eneuro]

Nope, I've put into circuit simulator concept of two transformetrs and it might work for 20kHz DC-DC switching frequency, but unfortunatelly we have very low inductance in MOT in practice, so it shorts DC-DC output and only works ahen MOT coils inductance is 1H which is probably no chance to get with this MOT and a few turns of wire:


However, when we limit 230VAC mains  lets say to 1mA, than rectify and put this as input to step down HF push-pull (two IRF840) DC-DC 25:5, so we could expect ~46VAC on 5 turns MOT secondary @ current <5mA, so this is what we put into sim right now and we'll see what happends.

Anyway, while we have +/-32Vmax on wedler transformer, than by adding high frequency low current +/-46VAC, we should get +/-78Voc (max), which looks, good but still I'm not sure if such low HV current will help ignite arc, but lets draw custom PCB with push-pull and check it out if we get >70Voc between electrode and material base and what short current from MOT will be when we add 300 Ohm load (with turned off welder trafo only MOT) simulating bad case scenario from human body model (immersed in water or broken skin, etc) according to this Conduction of Electrical Current to and Through the Human Body: A Review   

Safety first 

[oldway]

You seem to have reinvented the wheel!... 

The high frequency high voltage arc starter device has been known for decades and is used on TIG welders for starting arc without contact between tungsten electrode and the metal of the workpiece.
Early systems used a resonant circuit and a spark gap to generate a damped wave train.

It is also used to maintain the arc when welding aluminum with TIG AC welding machine .
Note that in AC, the aluminum welding current is asymmetric and can cause saturation of the transformer.
It is therefore recommended to use a capacitor in series with the transformer secondary.

Most TIG welding Inverters also have this feature, but the high frequency is generated by discharging a capacitor through a thyristor or triac in an oscillating circuit.

[eneuro]

The high frequency high voltage arc starter device has been known for decades and is used on TIG welders for starting arc without contact between tungsten electrode and the metal of the workpiece.

Yep, they are mentioned in previous art in the patent description I've linked above, but anyone knows any specs of its output current levels and frequency as well as voltages used in those HV HF starters? 
Thanks for schematics, but unfortunatelly, no mater how hard I tried type my keyboard I couldn't find those China characters to google ICs used there 

Maybe, this looks like reinventing a wheel, but I haven't got such arc starter and I have no idea how safe are those comercial available, so we made such simulation with rectified 230VAC and push-pul is feeded with limited current to <1.5mA at ~300VDC on rectified caps (10mA limit in 100nF capacitor PSU before diodes) and it looks like we should get HV spikes with short circuit current at expected 6mA range or so, when open circuit Voc close to 100V from addtional DC-DC converter is loaded with 300 Ohm resistor simulating bad case sceanario from human body model mentioned above, which looks good, but is it capable to help ignite arc in this 33Vmax only oryginal welder transformer? 



I've all components needed to build such circuit (IRF840 n-channel mosfets 500VDS), so probably after good looking simulation time to try this in real circuit.

BTW: What about using one of these ZVS flyback drivers?
I mean, it could be nice do it without any MPUs and additional ICs like 555 timers etc 

It looks similar to push-pull, but didn't tried it yet-inductor heater is still on my todo list

[eneuro]

Meantime, designing also custom welder electrode holder, while I might be interested in automatic welding not by holding it in hand by let simple welder robot which will keep given distance from workpiece, so I wonder if this my ~40mm2 of copper pipe and electrode holder pressed by spring and screw in this not finished yet prototype:



maybe has higher contact area with electrode than those not the cheapiest one available on market?
 


It looks like, those electrode holders have suprisingly small contact area with electrode, so when I will add another 1mm copper layer at the front of my holder, than electrode wire will be surrounded from 3 sides of 4 possible with copper at distance ~20mm, so when at each side lets say 0.5mm of copper will touch electrode, we have contact area between electrode wire and welder copper holder estimated 20mm x 1.5mm: 30mm2, even now it is about 20mm2, so it looks like my cheapy DIY electrode holder has much better electic contact than those commercially available? 

What do you think, when we add thermal insulation and replce screw with something easier to change electrode, isn't this custom design easier to hold in hand and more comfortable than those comercial with this bloody spring to replace electrode?

My plan is also add Hall effect current sensor around this copper pipe and... small LCD display to show welding current and maybe allow also change welder parameters using small RF home automation module I'm working on too just by performing required action on this electrode holder without need to go to wedler.
I think, that warning, that we are close to maximum allowed duty cycle limit (per 10 min periods) might be usefull information, but now realized that, maybe it will better display this inside welding helmet, while there will be difficult to see anything on electrode holder while welding, so probably I will leave there a few controll buttons only 

[johansen]

I've thought about adding an amp display to the inside of my welding hood but i don't think i could care.
Its not like i can adjust the current while i'm welding anyway.

I could build a large boost converter and run my stick welding from a 40vac open circuit transformer and rectify it and then boost that with a 4 phase boost converter to get realistic welding voltages.. it would certainly be more efficient but its a lot of work for not a lot of gain.

Your electrode holder is more than adaquate but most of the current is conducted into the electrode holder by the steel welding rod. its not really that much heat either.
you'll also want a 3 cm or larger diameter handle to hold the stinger.

[eneuro]

I've thought about adding an amp display to the inside of my welding hood but i don't think i could care.

I was thinking of testing something like weld current profiles, where current is adjusted in time based on given welding task, so at least I might want have a button to start/stop or choose given welding profile without need to go to welder, just by pressing a few buttons in my custom electode holder or adjust potentiometer to choose welding program I want.

Anyway, I've managed to work in simulator with one of those ZVS drivers used by folks in their flyback plasma generators and in simulation, when powered from 12V battery it doesn't seem to draw too much current, however I'll have to put it into spice since this simple tools doesn't deal well with higher frequencies, so I was unable to check if really this thing switches in zero Vds 



It draws in the range of hundred of mA current from battery only and of course without inductor on battery input there are heavy current rushes in both directions.
I wonder, if it works with my MOT, I guess iron core, but there is also option to make custom toroid core.

From safety point of view even when we get high output voltages (low current) and add to oryginal welder transformer output, still this secondary output is floating, so we get something like isolated mains, which means, unless we touch both output wires and create current flow througth body, there is no path to ground, so It is no so bad like trying to touch wall socket mains wire, where earth wire creates path to ground.

Additionally I think i will add button in my cuctom electrode holder to enable this HV arc welding helper, but only when someone holds it in certain position, so it will break welding process when something strange happends and will not create HV pulses without operator doing this intentionally 

[eneuro]

12V battery ZVS driver version ready on 2inch2 (5cm x 2.5cm) double layer PCB:



Bottom layer SMD stuff and transformer primary three taps connectors:



Power mosfets pads optimized for SMD version, but classic TO220 can be easy installed while, holes for heatsinks screws ready.

This is still prototype PCB-but breadboards sucks  and no plans to use them, so spend some time to make something ready to put into any design as ZVS driver module 

Update: Now I see, that on bottom side I've forgot add a few vias under mosfets drain big pads, so we'll fix this, while it vill allow spot weld heatsinks on the bottom and it will help disipate heat for low power versions 
Update2: Yep, close to final-I need remove mask from a few  viases -we'll see how it will perform.

BTW: Mosfets are in AC mosfets switch configuration, so when we cut this PCB in the middle (1inch2) vertically we have usable AC mosfets switch and we need only connect external gate driver, so yep I've spend some time since I've a few things at once optimized for my projects  requirements

It will be fun play with those mysterious ZVS drivers...  lets demistify them