Sunko 709 Modifications: due to bad planning

barry_rawn

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Folks, have to confess that I ordered a 110V version of the 709A, even though I'm living in a country with a 220V 50Hz supply.

I opened the unit to have a look at the transformer, and I saw that the circuit board that probably implements the single phase triac


is labelled 110/220V.

Question is: does anyone know if I have a hope of modifying this PCB (assuming I could get it out) to be happy with 220V? I would be thrilled and willing to put in the work, because the alternative is dropping 200-300USD for a high-rating step down transformer, or stubbornly spending 100 USD to build one, which probably won't be as good.

I have also submitted a question to Sunko itself but I don't know how responsive they are.

sheepishly,
Barry
 

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I don't have a direct answer to your question, but I've seen several people point out this on the transformer now. What I don't understand is, if it's as simple as soldering to a different terminal to use a different winding... why would they not simply put a 120/240 selector switch on the device rather than making two separate versions? :/
 
I don't have a direct answer to your question, but I've seen several people point out this on the transformer now. What I don't understand is, if it's as simple as soldering to a different terminal to use a different winding... why would they not simply put a 120/240 selector switch on the device rather than making two separate versions? :/
Seriously that would be a big help. It's possible they weighed this, and decided the cost of the switch wasn't worth the number users who would care. For them, this is just a slight variation in assembly process I guess. ..

..but facing the mod, I do tend to think, better the factory person who does this all day, than me, making this adjustment! It's really not idea, but it's a huge relief. Will let the forum know how it goes.
 
I have the 240V version. I will open it tomorrow and take a picture of how it's connected to compare to yours (assuming nobody beats me to it by then).
Wow, fantastic! Thanks, that would give me more confidence to make the modification, and it would be helpful to others in the future.
 
Looks like the transformer has 2 windings already. Does that transformer step up or step down to the secondary coil? You could measure the resistance of the coil which is used now, and find the other one, which should have around double the ohms if stepping down, as the 220v needs more windings to do that, or it should have half the ohms if the secondary side is stepping up...
 
Looks like the transformer has 2 windings already. Does that transformer step up or step down to the secondary coil? You could measure the resistance of the coil which is used now, and find the other one, which should have around double the ohms if stepping down, as the 220v needs more windings to do that, or it should have half the ohms if the secondary side is stepping up...
It seems like we can reason from the schematic Jon of "jonshobbies.com" provided https://www.jonshobbies.com/modify-sunkko-737g-from-120v-to-240v-operation.html (also see attachment) There are two input windings and one output winding. The circuit board decides whether the input windings are connected in series or parallel. It's kind of elegant: we know the power coming across the magnetic interface is the same either way, but their design means you either have twice the voltage step down, or you have a 110V appropriate step-down with twice the current rating. Pretty cool.
 

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Damn although I have the 709, and not the 737. The SUNKKO people wrote back saying this:

"Thanks for contacting us.
I'm afraid no. The mainboard and transformer for 110V and 220V are different."

So since they're responsive I sent them Jon's site and my photos and asked them if they're sure. :LOL: Let's see

I guess the rational thing would be to maybe disconnect some of the wires and perform some tests to check whether or not the transformer has those multiple windings or not.
 
I have the sunko 737G which I have modified to run on 240V from the pictures from johns hobbies.

I also added my own spot welding leads. I made them out of 1AWG MTW. I stripped back about 1/2" of the cable, stuck the sunko tips in the wire, and soldered them in. I turned the transformer around then added a terminal strip and a circuit breaker in the machine. In my case this was needed because my 240V plug is on a 50A breaker. Changed the plug on the unit out for an L6-50P. Works quite well now.

No magic smoke yet. I would only recommend doing this if you are knowledgeable on and comfortable with mains voltages. Do not work live and modify at your own risk!
 

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I have the sunko 737G which I have modified to run on 240V from the pictures from johns hobbies.

I also added my own spot welding leads. I made them out of 1AWG MTW. I stripped back about 1/2" of the cable, stuck the sunko tips in the wire, and soldered them in. I turned the transformer around then added a terminal strip and a circuit breaker in the machine. In my case this was needed because my 240V plug is on a 50A breaker. Changed the plug on the unit out for an L6-50P. Works quite well now.

No magic smoke yet. I would only recommend doing this if you are knowledgeable on and comfortable with mains voltages. Do not work live and modify at your own risk!
Agreed @Cypher618 : there's always a better alternative to working live and taking your time to stay alive!

Thanks for sharing these close-ups. Adding some breakers is a nice idea.

Those 1 AWG leads are substantial. I'm curious, on the breaker side, what kind of terminator for the conductor is there, did you crimp something on? Also, in the first picture I can see tie-wraps and metal bar on the outside/top of the unit. Do you find that provides sufficient strain relief so that you don't get movement of the cable inside the unit where the conductor enters the breaker?
 
Seeing people's experience with the 737G, I'm going to try this out once I can secure access to our lab's soldering equipment. Despite what the SUNKKO rep told me over e-mail, this looks to me like exactly the same component found in the 709A. What I'll try to do after making the mod, but before energizing, is verify the reality of the coils implied by the schematic on Jonshobbies..

I can imagine that the SUNKKO rep wouldn't encourage mods over a paper trail, so I'm not disuaded. Thanks to everyone for the pictures. @LithiumSolar still curious about that photo if you can manage it. Will keep you all posted!
 
The 1AWG was just because that's what I had available. Somewhat overkill. With a wire this size and type I almost don't need strain relief because there is a lot of stiffness in the cable and its going into a terminal block. I mostly have the zip ties there to stop the cable from rubbing on the edge of the hole and getting cut, but it would work well enough even If i were using a more flexible cable like a welding cable.

The wires into the breaker are hex crimp ferrules. Completely addictive once you start using them. No more strands!
 
whoa that looks really good. Going on my list..

Ok folks. I have performed the mod. I see that the switching of the AC waveform is actually done on the high side (the mains side) On the low side, it should look the same. So I'm hoping that those FETS or whatever is in series with the AC can handle 240V as well as 120.. I had a good look at the circuit board, and every cap I could see was rated to 250V, 400V, or (hopefully only on the secondary side) 25V. Let's give this a try, soldering iron first..
 
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Happy to hear it worked out for you. I used mine to building another 7s10p pack today, still holding up well for me.

It takes a bit of practice to get the pressure just right, at least it did for me.

Enjoy!
 
Thanks for the tip @Cypher618. I noticed it's also important to ensure the electrodes are perpendicular to the surface. Overall, it's way easier than soldering in my opinion.

Here are some pictures of the before and after. It seems like the triac on the circuit board can gate either 240V or 120V equally hapily, and the electronics probably just run off the secondary. So it seems the 709A can also accept this mod, like the Sunkko 737.

I have a picture here of before and after. As luck would have it, I didn't need any solder: just flowed each wire to free up the solder around it, and encouraged it to migrate to just a single pad (instead of spanning two pads, as you can see was the case in the before picture) and dragged excess solder across from a freed up pad to make the series connection of the coils (centre of the photo).
 

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