Sunkko 709A Spot Welder Teardown

[pelud]

Hi all,

I have had a bit of an evilbay gadget addiction lately, and I couldn't resist getting one of the Chinese spot welders to play around with.  Other than attaching tabs to some alkaline batteries for some old VTVM's that I'm fixing up, I don't really have a primary use of it. I mainly wanted to see how it worked.

I got the one that has the external leads as I hoped that I could use it to weld random wires together, such as component leads. However, it appears that at least one of the contacts to be welded needs to be relatively high resistance or the wire simply explodes.  I haven't experimented too much with the power settings yet, so perhaps I just need to reduce the power settings when weld copper to copper.

If anybody has advice on using it, it would be much appreciated.

Without furthur adeu, here are the teardown images:














Have fun,
John

[eas]

Thanks, I'm interested in updates as you learn more.

I have a similar interest in being able to spot weld batteries and the like. I was thinking I might try building my own, but I already have too many projects. I thought one of those Chinese welders on ebay might be a good option. Probably enough to get me started, and then tweak and improve when/if I ran into issues.

[pelud]

Here are some more pictures showing the electrodes, in case anybody is interested. 


I've also attached some screen caps from the scope without anything else connected.  The first one is with it set in "2 Pulse" mode (no buttons pressed) with the power set to max, the second is in "4 Pulse" with it again in max, and the last one is with all the buttons pressed (16 Pulse?) and the power set to somewhere in the lower portion.  The unloaded voltage appears to be about 12 to 13V Peak-to-Peak, or approximately 4.5V RMS.  I would expect it to drop significantly when under load.


I've noticed that the firing seems quite inconstant and sometimes very sporadic when in 2 Pulse mode.  The welding seems the best in 4P or 6P with the relatively low power.  There seems to be a lot of trial-and-error to find the best settings.  If the power is too low it doesn't make a good weld, and if too high, the welding "nugget" gets to big and weakens the tab strip.


Does anybody know a good way to measure the current coming out of this thing on on the scope?


John

[SeanB]

Easiest way to measure the current is to pop a low loss ferrite toroid over the one lead, and wind 10 turns of thin wire on it as well. Terminate with a 51R resistor and connect to the scope with some thin coax cable with the scope set to 50R termination and single shot it. Easier than in the old days of setting up with a polaroid camera on the front of the scope, and going through a pack of film doing the single shots, or turning up the brightness and relying on the persistence of the phosphor as a rough setup.

[artag]

It looks as though the secondary winding is connected directly to the probes. Is that correct ?

So any pulse control is being done on the mains side ?

[pelud]

Yes, the secondary is directly connected to the probes, and the primary appears to be controlled by a BCR30AM-12L TRIAC.


Quote from: artag on Today at 11:06:41 AM

It looks as though the secondary winding is connected directly to the probes. Is that correct ?

So any pulse control is being done on the mains side ?

[dutchy]

my apologies for digging this topic up but as my problem is with this machine i think it's best asked here.

my problem lies in the fact that when this machine powers up it blows my circuit breaker instantly. i have already replaced the breaker (16A) with a C curve breaker in the hopes it would give it a tad more time but to no avail.
just to be clear: if i bypass the breaker and run on the 25A main house fuse it works perfectly but we can all agree it is certaintly not anything i want to keep doing.

my idea was that the pulse load of it turning on the first time is so high i could add a couple of microwave caps on the back of the device in order to catch the main surge that blows the breaker.

would adding a few caps actually work of do you (much smarter) guys have a better idea?

[thefamilyman]

You should be using a D curve MCB for an inductive load.

[Jane]

@thefamilyman:
What do you mean by

using a D curve MCB for an inductive load.

[Richard Head]

A D curve breaker is not just for an inductive load. It's for loads that have a high inrush (starting) current such as motors etc.

[dutchy]

I have read on endless sphere that people also have issues even with D MCB's.
 I can always repace the new C MCB with a D but id rather try and "fix" the inrush problem then just slapping on a industrial MCB. It would also fix the obvious issue when i need to use it somewere else.

[poorchava]

Interesting, TBH I always thought that gear like that would just have some super-low-rds mosfets or an IGBT brick or an SCR in series with the secondary...

[dutchy]

Interesting, TBH I always thought that gear like that would just have some super-low-rds mosfets or an IGBT brick or an SCR in series with the secondary...

those are way too expensive for those chinese builders.

did you see the build quality of this thing? imagine how it would look like with a "2 dolla" SCR.


i havd some more thinking about this and obviously my first idea was to put a MOV in there but then i thought of how the power load looks like from a MOV point of view. how long would a average MOV last? 20 seconds? 1 minute?

i need to gracefully handle the inrush and then it needs to handle the same pulses but without triggering. would using a MOV and a triac or relay combined with a delay circuit to bypass the MOV after a few seconds be any good? any ideas on how to implement it properly?

ps: i just noticed i am missing that incredibly safe bodge wire running between the power switches. any idea what that's supposed to do. it is connected to the power resistor on the other side of the board.

[dutchy]

follow up:

i tried it with a MOV i pulled from a old 1kW enermax power supply but that refused to release. it just kept blocking current.

in my digging in the bins i found a nice big 52ohm 50W power resistor and put that in there as a replacement. (with a bypass switch) now it powers up without any transformer "omph" and runs fine.
i measued the inrush corrent with my clamp meter and it measured 1.5 amps over the resistor. so i must say i am quite pleased with this.
ps: as for good measure i put the entire filter board of the enermax in it as well. now it also has a nice standard power plug.

[eneuro]

If anybody has advice on using it, it would be much appreciated.

Maybe this transformer could be usable

It is strange that, there is no huge fans inside and airflow holse in enclosure? 

Anyway, It is interesting if this thick strand wires are... really copper wires or... plated aluminium, which I've found in one of the China MOTs 

If primary winding has desetn quality, I'd checked secondary windings and maybe replaced with custom few turns of thick  strand copper wires and replaced transformer primary controll circuit with own SCR's and small MPU, so than it souldn't be a problem to... melt thin copper wires like 2.5mm2 in my custom made spot welder, controlled by ATTiny85 and two SCR's in antiparallel on trafo primary



I've used Hall effect current sensor on this huge trafo primary to controll phase angle when running this thing at full mains wave lenghts 20ms @ 50Hz 230VAC is too much power, so MPU on two ADCs is looking for potentiometers settings of power setting potentiometer and another which allows set total pulse time between  20ms pulse up to 10seconds in 20ms (@50Hz) multimply to let transformer operate always at full wavelenghts at high power 

[Cubrilo]

i tried it with a MOV i pulled from a old 1kW enermax power supply but that refused to release. it just kept blocking current.

You mean PTC, not MOV.

[anishkgt]

An old post but this link may help somebody.
 https://georgehobby.wordpress.com/2018/01/27/maxweldv2/
 

It is a replacement PCB for Sunko Spot welder or any DIY spot welder that would use a Microwave oven Transformer.