Friday, May 15, 2015

Overview of a Sunkko 788+ Welder

Power 788+ Battery Spot Welder

So, you find yourself needing to spot weld a battery pack (or something else) together.  And, spot welders are expensive, and most people end up building DIY devices, but there's this little gizmo (and it's siblings) all over the internet.  Alibaba, eBay, Amazon... it's a common item that claims to be a decent spot welder. It runs about $200 shipped to the US.  Be sure you get the 110v one - most of them run on 220v (unless you have 220v power, of course).

But what is it?  What makes it tick?  Why is it so much cheaper than every other spot welder out there except ones that are very obviously related to it?

Well, as it so happens, I've gotten very friendly with one (details in a separate post), and will happily share some of my insights with you.

Let's dive in.

The photos are mostly from a bit of repair work, so if it looks like it's not fully assembled... that's because it's not.  Good eye!

The "User Interface"

This is the unit.  The top display shows, on the left, some number related to current.  The rightmost digit shows the number of pulses (1 or 2).  You set these parameters by using the "star" button to select the position to edit, and the up/down arrows to change the number.  It goes from 00-99, and 60-70 seems to be a reasonable setting for double pulse welding of 0.2mm thick nickel sheeting (with the "set current" knob at zero).  Bigger numbers mean more power.

Below it is a "set current" knob.  I cannot find out what this does.  Turning it up appears to increase the power of the weld.  It doesn't seem to increase the number of pulses, which is what the (very poor) advertising documentation implies it does.  Each pulse is a very distinct thump, and I only get two, even with that turned up a bit.

The top right has welding and charger power switches.

The foot pedal plug is on the right side, and attaches to the foot pedal that comes with the unit.  Be aware that the foot pedal is an "or" with the normal welding microswitch, and it will happily try to weld air if you tap the pedal with nothing attached.

The main welding arms are in the center.  Under normal operation, when you push up on them with a workpiece, they hit a micro switch and apply welding current.

Below that is the charger, with voltage or current settings.  I haven't played with it, since I don't find myself charging batteries with this unit.  But, in theory, it's capable of doing it.  I think it has a voltmeter capability as well.

Finally, on top of the unit is an adjusting screw for "welding pressure."  This is not some archaic use of the term pressure to refer to welding voltage - it literally is adjusting the spring tension that holds the arms down.  Increasing it increases the pressure on the workpiece.

Going Inside

The core of the Power 788+ welder is a simple: One big transformer, and one big circuit board.

Power comes in, goes through a fuse, goes into the circuit board (which handles both the high voltage and low voltage), through a triac, into the transformer, and back.  The high amperage welding current flows through the wonderfully shielded braided copper cables and into the arms.  I intend to heat shrink those at some point, but for now, I just rely on the fact that any strand of copper that touches the other arm will vaporize on the next weld or two.  I'm pretty sure that spiral wrap isn't designed for this, but the welding current is very low voltage.

I'm fairly confident this welder isn't UL listed.  Or, really, listed by anyone.  And I wouldn't leave it plugged in unattended.  You probably shouldn't plug it in at all, but... that's no way to get a job done.  Make sure you have a good circuit breaker.  This thing is really a bit frightening on the inside, and the circuit board is worse.

You can see, on the top, the adjuster for the welding pressure.  It's just a machine screw and it pulls the spring tighter when you crank it.  It could use some oil, if you're willing to put oil inside this enclosure.

If you find yourself needing to remove the circuit board, it's doable.  The front panel comes off easily enough, and the board is screwed in.  Once you have the screws out, there are a few points that cause trouble removing it.  The first is the power switches on the top - they're just a press fit in the terminals, so a bit of pressure pops them out.  The second tricky part is the switch at the bottom, in the charging section.  I wasn't able to remove this properly, and ended up separating the two parts of the switch - the top remained in the front panel, with the bottom in the circuit board.  You could easily enough desolder the switch, though.  If you do choose to separate it, it takes a good bit of pulling, and a spring will fly out when they come apart.  Don't lose the spring!

Be careful of the voltage sense wires (the small red/black wires going to the terminals above the main power connector), and the LED light wires.  They're not very long!

Circuit Board Shots

The circuit board is two sided.  You may notice something slightly wrong with the triac.  You may also notice a different component, detailed, that is connected to a trace that's pulling clear of the board.  High quality, this is not.  And, really, the rest of the board is a hand soldered, globby shitshow as well.

That's about it.  I haven't reverse engineered the board, but if you are interested, contact me and I can send you native resolution images.  I'd love to know more about this thing.

Does it work?

Yes.  It does.  It does a surprisingly decent job of spot welding for a $200 welder.  At least until it blows up.

I'll caution you to be very careful on the duty cycle.  The stock triac location is not "good" in terms of cooling, which is the whole reason this thing was in pieces.  Either the triac was bad, or it overheated with my welding.  Either is possible.


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  2. I use my 788+ for the first time to make a battery pack. I have to admit it worked very well. After reading your blog I was very careful not to push it very hard and wore PPE particularly safety glasses. I left the “course current adjustment knob” at zero and adjusted my current digitally at 65. I set the number of pulses to 2. That seemed to make a pretty good weld. I also ordered some spare triacs to have on hand in case mine decides to give out. I discovered that they are hard to find in the United States in low quantity so I had to order them from China. There were not very expensive. Thanks for your blog. It helped much more than their manual.

  3. I plug in my device and when I turn it on, the socket's fuse goes off. What is the problem? Could you please help me?

    1. At the bottom of I have a suggestion from someone who has the same issue to switch to a Type C breaker instead of a Type B breaker on the circuit - it helps with the inrush current (which is very high).

      I've also seen some people create bypasses with resistors to handle this, but I don't know details about that.

    2. Make sure you are using a circuit with a 20A breaker as specified on the last page in the owners manual. I haven't had any issues tripping the 20A breaker.

  4. Documentation is not very good so first question is did you get the right voltage? Comes in two versions. 240V and 120 V. What did you get and what do you need?

  5. Thank you very much for this post I am think of buying one of these and seeing the inside is very helpful.

  6. I would like to know more about this also could you send me some additional images. I would be happy to share what I learn with you!

    1. All the good photos I have are up here. You should be able to click the images for higher res versions.

  7. Hello, the welds are good. Too bad that I just tried the charger I heard a low, the charger does not work now. The disassembled and I saw that under the radiator there is a MIP0224SY, it could be broken? Thank you

    1. Under the heat sink, there should be a triac of some sort. Mine failed and I had to replace it:

      The part number you mention is not the same as the one I had, but it's a power handling device, and it's certainly worth replacing it to see if it resolves the issue.

    2. Do you think the 788H model is better built ?

    3. exactly the same thing happened to me when I turned on the charger. something "popped" and now the only thing that comes on is the welder only. not a biggie for me since I do not trust the charger that comes with it. I bought it mainly for the welder.

  8. Hi there we have purchased this weirder and every time try to use it it blows the whole house trip switches . Any ideas why. The instructions are all in Chinese .

    1. It's probably because the inrush current on this welder is insanely high.

      Are you referring to "whole house trip switches" as the individual circuit breaker it's on, or the whole house breaker? It shouldn't be tripping the whole house breaker, but it's known for tripping the breaker it's on. You may have an improvement switching from a Type C breaker instead of the more common Type B breaker.

      You can also try leaving the unit turned on and resetting the breaker. This works for some people.

      They're not particularly well designed units, sadly.

  9. Hi, After constant MCB trips I finally managed to fire this thing up by leaving the unit on and then flipping the breaker....I then did aboit 5 cell welds and now it's dead...Could this be the Triac you mention? Any way I could test this? Thanks in advance. Mark

    1. It's worth taking a look.

      The symptoms on mine when the triac died were that the unit powered on, and the LED digits would dim when I tried to weld, but nothing happened. There was also a loud bang when it let go.

      It's worth taking a look. My triac had a huge obvious hole in it.

    2. Yes, once I watched your video I realised that the Triac was the fault....It turned out that although mine hadnt blown, one of the legs on the Triac had actually melted and there was a gap - Some solder later I finally bridged the gap and it's up and running again. I suggest to anyone new to these (as somebody before me already has) is turn the course current dial to zero and use the led adjustment - I am using 60 - twin pulse and all seems fine for now using 0.15mm Nickel. I wonder if upgrading the Triac would help? One thing for sure this thing isn't for the faint hearted or a PAT test :) - Thanks for your help & video Russell!

    3. Well, that sounds about like Chinese QC.

      When mine blew up, I was playing with the current adjustment knob as well. I leave it set to 0, and weld on "70 2" for 0.15mm nickel strip with great success. I still don't know what that knob does other than seem to be correlated with blowing up triacs.

      When I replaced the traic in my unit, I actually hung it off the back of the board. The stock location is folded against the front of the board, pressed against the front casing. On the theory that this was less than optimal for cooling, I installed the replacement one hanging out the back, into the open air around the transformer. I don't know if this helped or not, or if I just had a bad triac to start with, but I've welded up a few packs now with no trouble. I'm also quite gentle on the unit now - I let it cool every few batteries worth of welding.

      At least on my unit, I think many of the problems are related to it being the 110v version. I suspect the engineering used to go from a 220v unit to a 110v unit involved changing the power converter circuitry to the board, and possibly changing the transformer. Possibly not. But, in either case, I expect the 110v unit to be handling twice the current of the 220v unit with no change in anything other than the bare minimum needed to make this work.

  10. Hello all, my welder which I purchased last year is still yet to weld its first cell. I had similar symptoms as Russell mentioned above, where initially the display would dim when attempting to weld. After a lengthy and challenging email conversation with a lady from China, I decided to investigate further with her help... Long story short is that it too did blow up and resulted in the electrics tripping. Since then I've had no further support.

    Could you please post a link of a suitable triad replacement? For the sake of a few quid it's worth it trying to repair it oppose to having a $170 lump of metal.

    I also have a video of it blowing up with the covers off, if it's of interest to people I can forward it on.


    1. azman -

      Just grab the model number off your blown triac and see what you can find. eBay is a good source.

      Mine has a BCR30AM triac - I believe they may be different between units, but this one will probably work if yours has blown all it's identifying numbers off.

      I bought a 5 pack of replacement triacs on eBay, since I wasn't sure at the time if the triac blowing is a regular occurrence or just a one time event. I've not blown another one yet, though I'm rather gentle with the unit now and the triac is hanging out the back of the board into the open air near the transformer.

      Details of my repair process are documented elsewhere on my blog - has photos of the repair.

      When you do get it repaired, I suggest leaving the current knob alone. I don't know what it does, but I was messing around with it shortly before things blew up. I weld with the current knob firmly at 0 and "70 2" as my digital display settings. It nicely welds 0.15mm nickel strip onto cells without any signs of excess energy.

    2. Mr Russell,

      Just wanted to say thanks for taking the time to post your blog. If I could, I'll buy you lunch. Thanks for sharing your findings.

    3. Rob - if you run across any interesting cheap gizmos you'd like me to tear down, I'm happy to do some analysis. Contact form is on the right if you want to ship me something.

      I'm glad you find it useful. It's an esoteric corner of the internet I lurk in.

  11. Hi Russel,

    I just try the same unit.
    At first, the unit works well for some weld. But after that, it causing a trip on the mcb everytime I turn on the welding switch. For the charger part, it is ok when I turn on the charger switch.
    Do you have idea that causing this problem?

    Another thing, it is ok if the two soldering tip accidentally connected when we install that two welding tip? Because, seem that I just unaware that two tips connected after installation.

    Thanks and Regards,

  12. Hi,

    My unit just arrived. I have the 220V version (live in EU/NL) and have the same Circuit Breaker problem (16A trips). Not sure if the unit is defective or if it's just the inrush current. One other thing I noticed, is when I switch on the charger, I see some blinking (for about 10 sec) an 3.Eo 00.00 on the Amps and Voltage display before it shows 0.00 00.00 Is that considered normal?

    Cheers, Henk

    1. What you are seeing is confirmed. I see something like that too. But mine didn't go as long as 10 seconds. Maybe 3 to 5 seconds. However, I blew up my 788H when I inadvertently switched the weld (green) switch on before the display had time to change to '0.00 00.00' (after switching on the main red switch). I've ordered a second 788H from ebay (in case I can't fix my currently dead one). The dead one has a blown surface mount rectifier IC near the molex-like 4-pin connector, and on the other side of the circuit board is a blown 5D-9 thermistor.... busted in half. They blew up after all LEDs went 'off' (disappeared), and when I switched the MAIN switch from ON to OFF..... yes... on to off, and not the other way around. In fact, I had though that the fuse had blown, so was very surprised to see a big flash when I switched the main switch from on to off.

  13. I just received my 788H (eBay, AUD $203 delivered to Australia), connected it in the kitchen (240VAC, 20Amp breaker), there is a thump when it turns on and the UPS's in the office on another circuit squeal in annoyance! I welded a few tabs, seems to work fine (on steel, not on the tinned copper protection circuit boards on the back on some 18650!).

    I then opened it up and added one of these:
    (Not bad, $3.92 and free delivery, ordered online Wednesday, delivered 9am Thursday)

    I installed it internally in line with the active, just after the fuse.

    Turned it on, no thump or squeals at all, smooth as can be!
    Thermal camera showed it at 80C just after turn on, rapidly dropping to 70C

    I then did a few more welds, didn't appear to be any change in quality.

    Early days, but looking good.

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    3. i put an 22ohm NTC after fuse. now it works perfekt

  14. David Varley,

    Can you post some pics of what exactly you did please? I am a novice when it comes to messing with electronics.

    Cheers mate!


  15. Hi guys,
    Installed the thermistor that David Varley proposed (RS Pro Thermistor 50Ω) in my 788H 230v since I have the same problem with it blowing my 16Amps fuse.
    After installation, the first start went nice and smooth, no thumps, no blown fuse..
    Sencond start, the thermistor blew :(
    Any ides? Wrong thermistor, bad thermistor or what?



  16. Hi Peter,

    I am not sure about your thermistor but to overcome the fuse tripping I would suggest you try and find a Type D 16A breaker for your consumer unit. These are designed to withstand the short surges of higher inductance that the spot welders produce. It's not so much a fault with the welders but more that the Type B and C breakers are designed for domestic appliance only surges.



  17. i: Great article(s)

    What triac did you use?

    Do you know a replacement source for the electrodes?

    Have Ryobi 40 v lawnmower. Battery is dying. All UTUBE dis-assemblies are not helpful. I have supplies of the excellent new Panasonic NCR18650GA
    ... and appropriate new bms bds.

    The ryobi battery uses 4 contacts...but the bms seems to sense only when the ON button AND the safety "kill lever" are activated.

    I do have a schematic from the net (below)....but not sure I understand it. I would just like to build a new battery with bms. But B+ and B- do not go directly to the lawnmower wiring. But rather a M+ and M- go to the motor. It looks like when the “kill lever is released…the motor is shorted to provide a brake. The “key” is shown as connected to the li-pos inside the battery case. It is rather wired within the lawn mower. I wish folks would provide actual schematics.

    Any help is appreciated.


  18. Hi friend... I have 788 and works fine at this momment. But I have a question... I would like to install the "remote" welder (it´s on other sunkko models, how 709). I found this and I was thinking to connect, when I need, to the main poles (removeing the screws and attaching there... and after that it´s supposed to I can use pedal for force spot). What do you think? will it work? Thanks

  19. Hi,
    Can i ask question?
    788+ has microswitch(Circuit Board Shots second picture)
    Do you know switch spec? (Ex: 120V 1A)
    My spot welder broken microswitch, but can't find.
    If you can find, Please reply. Thank you.

  20. Hello I got my device today and I wanted to charge a 36 v battery, but there is an SMd exploded and I can't see the number of it, divce is sunkko 788h, can you pls send me a picture with good quality of the circuit board, so I know the number of it ?

    1. I'm not going to disassemble mine again, sorry. Unless it blows up. You should talk to the seller if yours is defective.


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