Question about 12v DC power supply.

[Ken from ontario]

In closing, having looked at the LRS page, it appears that it is used for soldering metal pieces together
rather than for electronic work. Maybe it has some useful application not particularly obvious to me.
I'm certainly not trying to pick apart your project but rather trying to understand it's usefulness.

Hi and thank you for your post, I went with a smps power supply because when I started to work on this project, using it seemed less complicated to work with, and was readily available ,I also just couldn't get my hands on a microwave transformer and was more focused on making the project quickly
but after looking into it more I noticed every LRS units (for industrial use or hobby), is low tech and comes with a huge AC transformer, now that I understand a bit more I know I have to go find myself a decent size transformer, there are a few how to blogs/videos on how to take them apart and rewire them.

The reason for my interest in low resistance soldering is, I usually work with thin sheets of copper and brass for my projects and I often solder parts for these projects, the problem with conventional soldering is heat. when there are a few joints that need to be soldered, by the time I solder one, the heat from the process tends to loosen up the previous soldered joints ( which are) in close proximity, low resistance soldering creates much less heat and the heat it creates is more localized / usually does not affect other soldered joints, that's my reasoning for wanting to make my own LRS , the ones available commercially are in $500-$700 range but going low tech , I can probably do it for $100.
Thanks again for your post , please keep it coming.

 

[Bi11Hudson]

There are several reasons that the fan shuts off when the PS is loaded. I concur that the fan seems to be running directly off the output. Assuming this to be the case, keep in mind that when you are soldering, the carbons form a literal short circuit. For the short duration that the PS is loaded, you are connecting a wire across the output from one pole to another. Is this situation acceptable to you? You can continue to use the supply for soldering for an indefinate period of time. But it will fail eventually. Probably at the most inconvenient time for you. And I don't believe a warranty would cover it. The "pass element(s)", transistors, will melt internally. They may or may not let the smoke out, but would be visible to anyone experienced in circuit repair.

I have built such a device for myself. Using the ubiquitious microwave transformer. . . I connected an Amprobe (an analog high current meter) to the line side of the transformer. And using a bolted short, added turns on the secondary until I reached 7-1/2 amps on the primary winding. Only 4 or 5 turns. I judged the primary could handle more, but 7-1/2 is the highest my "Variac" was rated for. I do use the contraption for other purposes as well, where the variac comes into play. I never measured the open circuit voltage output. But the current into that bolted short was several hundred amps. It works fine for soldering.

Commercial versions of resistance soldering equipment use a transformer, a "linear supply", for this reason. It must withstand that short circuit on the output. What you are using is a "switched" supply. Where there is a transformer but it is a high frequency device that is much less suited to a short circuit condition. A computer power supply runs at around 40KHz (40,000 cycles) where the normal power furnished is 50 or 60 Hz, depending on which continent it is used on. The typical switched supply may be more or less that value. I advise strongly against using it in a "resistance soldering" application. It can be done but you should expect it to fail any time.

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[tq60]

Bill hit it.

Your power supply is designed for constant voltage, period.

Most will fail when used as say a battery charger as they are not designed for current.

Switch mode are interesting, to explain how they work is simple.

Heat is highest in the part that has the highest resistance.

A. Switch mode is either on or off, like flipping light switch real fast

These pukses of current charge a capacitor.

Constant current blows them out.

Simple coil of wire limited by wire size

Sent from my SM-G781V using Tapatalk

 

[markba633csi]

Sounds like you are exceeding the rated current and the supply is "folding back" or reducing the voltage which is what they are designed to do
Not the right device for this application- it's too smart

 

[Ken from ontario]

Thank you all ,great posts so far.
Thank you Bill for another interesting post, I kind of suspected that this power supply will eventually fail, , the fan stopping in the middle of soldering never sat well with me but did raise a red flag so I packed that power supply back in it's box again and am on my way to drop it off.

So that's that but now I'm thinking, where to find a large enough transformer to use, I'm also wondering if there is any other options like a power supply that's suitable for this project, I can't give up so easily but if I don't find a used microwave with a good transformer, what type of power supply could I find on eBay or Amazon .

If you can direct me to a suitable alternative, please let me know, BTW, I could buy a used American Beauty(105-03) LRS powering station without the probe/handpiece for a high price but still it is an option.

 

[Bi11Hudson]

Most linear type supplies have gone the way of the Dodo bird. There are still a few uses for large/high current transformers. But not many. . . A microwave is about your best bet to find one. You might find an old CRT type TV in some dusty corner of a TV shop. But a transformer from there would have a stout metal case and would still be marginal. Short of a heavy industrial drive, one is not likely to be found short of a microwave. I have found several just by word of mouth, perhaps a repair center has one "beyond economical repair". The biggest failure is not the transformer but the "klystron tube", a rather expensive part. In many (most?) cases, salvage by the shop does not include salvaging the electrics. They are hazardous. . .

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[Ken from ontario]

I could get a $10 used microwave close to where I live but ideally I would like a seller near by with a few of these transformers so I can choose one tall enough for at least 3-5 turns of thicker wire, is there any way to guess what type or size of transformer is inside a microwave?

 

[Bi11Hudson]

When I "rewound" the one I am using for soldering, it was a fairly "squat" casing. I used a wire that was about 1/2 inch (12mm) over the insulation for the transformer. And some (salvaged) AWG-6 or so rubber insulated from a piece of SO type insulation for extension leads. I don't remember the wire size of the heavy stuff, AWG 1 or 2 maybe. Pretty heavy insulation. . .

The determining factor for the windings is the "turns ratio". The exact length of the wire is widely variable. I used a piece of closet rod, split down the middle to make spacers at the ends. This in addition of "fische paper" to insulate any leakage to the core. Fische paper isn't an absolute, you could just as well use shoe box cardstock. Something, anything, to keep the soft wire away from the sharp metal edges. It will vibrate when you apply a short to the output. Lay the wire in loosly and tighten everything up after you have determined the number of turns you want. Keeping everything loose will ease passing the wire through a small opening.

To my knowledge, there isn't any way to determine the size or shape of the transformer short of "looking" at it. It really shouldn't matter much, you're only looking at a very few turns. The wire size is a sort of hit or miss. Just use the largest that you can fit. My transformer puts out several hundred amps into a short. For continuous use, I would need a 4/0 or larger wire. 250 to 500 MCM would be code.

But normally in this application the usage is far below 15%. So far below that things only start heating up on mine after several minutes of experimenting at full current. I'm sure there is a limit to how much you can get away with. But it doesn't seem to be a hard and fast limit. I would say to use a starter wire for a 12V car as the secondary. That is good for several hundred amps for a short time. And there is already a lug on one end. You can splice the other end easy enough.

One thing that does come into play here is that you are in a different country than I am. In the States, electrical codes, the NFPA, limit wire sizes on transformers differently than in conduit. And do not apply to such things as welders and soldering devices. They fall under a different set of rules, one of those revolves around the line cord and plug. I do not know if there is a similar code in your case, you would need to research that yourself to know. I suspect they are similar, but cannot say for sure.

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[Ken from ontario]

Thank you Bill for taking the time to reply and make things clearer.
I found out my next door neighbor has a couple of older microwave and I could have them both , the only thing I'm missing is heavier gauge welding wire, I do have several feet of 10AWG primary wire for the winding but I'm not sure if it'll give me the output I need .
Reading your last post twice already and I keep finding more info, I sincerely appreciate your help , also everyone else for that matter ,god knows I need all the help I can get.

 

[Ken from ontario]

I just bit the bullet and went with a used American Beauty (105-A3 ) power unit only. I had to look deep to find a couple of good excuses to convince myself that it was the right move LOL.