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Forum Index : Electronics : DQB-120A rectifier bridge ex MIG
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| domwild Guru  Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Hi, At my favourite scrap yard I came across a 150A MIG welder with this bridge, which I quickly removed. Have read on the web that with AC welding the hot point changes from material to rod 60 times (here 50)and as my welding is "multi-point tacking" I have this dream of ever so slightly improving my welding by changing over to DC welding from AC. This bridge rectifier consists of two huge 100x100mm heat sinks and each heat sink has four circular diodes pressed in. One heat sink is marked plus and the other one minus. The two rectangular AC supply bars are marked with the swung dash (~) symbol and two cables are coming off those bars. Dr Google cannot help me with this DQB-120A model number. A circular cap (?) connects the two heat sinks. Questions: The negative heat sink has one thicker cable coming off it but there is no positive lead coming off the positive heat sink. A huge M14 hex bolt is tying the heat sinks together via isolating porcelain disks. There is a U-shaped bar coming off that hex bolt, which connects to the outside frame of the MIG welder. 1. Have just noticed a witness mark of a spring washer or lug on the heat sink opposite the negative cable. Must suspect the positive comes off there. Perhaps someone has already checked if the rectifier is OK. 2. Am ignorant of the AC voltage at the welding rods of my transformer/fan welder, but is it possible for me to divert the AC to this rectifier and make the smoke escape from the rectifier if I try hard enough? 3. The cable coming off the negative heat sink only has a dia. of four mm, how can that carry a welding current? Thanks. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Had I not been so ignorant I could have avoided contacting the forum. When I connected the 36VAC to the ~ marked bars, the fuse blew. No wonder! Learned all about diode checking from Dr Google and noticed two diodes were dead shorts. A reason for throwing that MIG welder away. DC welding compared to AC welding gets a good rap. Now I have to wait for a cheap single phase rectifier bridge of 80A or thereabouts. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Sorry i missed your post Dom. between some sneaky cacheing by telstra and a few power outages I have missed a lot of TBS posts. So... diodes are replaceable, press fit for a few dollars, only problem is they will probably blow again they are usually pretty cheap and nasty, they are the same as car alternator diodes. A complete 3 plate rectifier assembly is around $60 new have a look at this video mig welder upgrade it might provide some inspiration. I'm confused, no wait... maybe I'm not... |
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Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
You probably can better results too by adding a choke which will make it even smoother. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Thanks for that Yahoo2. The rectifier in the video is exactly the one I am working on. Looks like those MIG welders are prone to fail with those cheaper diodes. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
the other common one is 4 plates and a single bolt, it gives extra options for output  I snuck a quick peek inside a 300amp mig that is waiting for repair, that unit has a choke and 8 decent sized capacitors in there as well. Although for anything under 10mm plate steel my little 140 will do a comparable job. I just have to let it cool down every 5 minutes. I'm confused, no wait... maybe I'm not... |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
You mean eight diodes? Noticed the chap in the video has chosen 1,600V stud diodes and as I do not have a black belt in electrickery and the price goes up with voltage for those diodes I am wondering why the voltage needs to be so high? Which voltage can I get away with on the cheap? My stick welder runs at 36VAC and a MIG may not even run at a much higher voltage. Must keep looking for stud diodes at the scrap yard as I am too stingy to pay $60 for new ones. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Try looking where I found this one will save a lot of money. HERE There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Just saw this one: Vishay Siliconix VS-25F10, Thread Mount Diode, Stud Cathode, 100V 25A, 2-Pin DO-203AA for about $4 from RS. Can I get away with four of them?? Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| joebog1 Senior Member  Joined: 07/11/2015 Location: AustraliaPosts: 114 |
I would think 25 amp rating is way too small. It is possible to parallel them, but you need sharing resistors read way big losses. I know you will hate me, but its probably best to search out a cheap inverter multifunction machine. My welder is a 200 amp MIG/TIG/MMA ( stick) welder I paid $750 brand new, with all the leads and other sundries, so second hand should be about half that price, or perhaps way less if you attend an ex mine auction. Mucking around with "old" welders is cheap, untill you add up all the "try it and see" failures, not to mention the time wasted. and at the end of the day you still have an old welder, that is perhaps better than straight AC, but a far cry from a modern machine. *news* my mate just arrived and he just bought a 140 amp MIG only from Repco for $200 go figure!! Joe |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
That cheap mig from repco would be a fraction of the machine being repaired here. The old tech machines are a lot less likely to release much needed smoke and the repair bill for this job will be a fraction of buying another machine. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| joebog1 Senior Member Joined: 07/11/2015 Location: AustraliaPosts: 114 |
Understood Madness  I have actually made new air cooled rectifiers for old MIG's along with their multiple 10,000 uF 100 volt caps. ( The value of which is also $100) I have "assumed" that domwild was just seeking an "occasional" welder. I apologise for that. My Rossi I bought when the dollar was over ( I think it was $1.12 ) from fleabay. The same welder is still available from the gas suppliers for a touch under three grand. It is an industrial machine with 60% duty cycle. Joe |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
No I meant capacitors. it was a repy to Madness's post about adding a choke and extra smoothing. regarding the diodes voltage rating. My guess is that to handle all sorts of harmonics or transient spikes the reverse voltage ideally should be 600+ volts. Most cheaper original equipment diodes are 300 volt rated, they are serviceable but not bulletproof. p.s. a lot of big name brands use 300 volt diodes on their consumer lines too! I'm confused, no wait... maybe I'm not... |
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| joebog1 Senior Member Joined: 07/11/2015 Location: AustraliaPosts: 114 |
The voltage rating I would "think" is not of so much importance as is forward voltage drop. I cant remember exactly but I bought diodes from RS components that were 150 volts and 150 amps rated. They were not Schottky, but a very small forward voltage drop. Remember if you add an effective resistor, you have to cater for the heat it develops so the smaller losses are more important than voltage rating I mean as long as its well above the working voltage. And unlike geranium ( Germanium) devices silicon STILL exhibits thermal run away. Since IGBT bridges were released this is not so much of a problem. Early MIG welders used to have BIG capacitors ( usually made by Siemens ) and were very large to keep them cool, and had super low ESR. I have never tried chokes, but I can see no reason why they would not add significantly to welding performance. HHmmm I just had a loony idea, BUT ill let it out anyway  I have an old solid copper core stick welder of the variety that used to remove or add the centre leg of the lams to control welding current.maybe chuck one of those into the circuit, but only using the secondary to control current. CUT OFF THE ORIGIONAL MAINS LEAD so you dont end up doing the 100 Hz tap dance   Just an idea. My experience with early MIG's was they had adequate current but little control over it. ( remember Im a dumb electrical eng NOT a boily!! ) Anyway I stand to be corrected, as its hard to understand all subjects. BUT I stand by what I said re forward resistance of the diodes, I tried cheapies with forward resistances of about 1 or 1.25 ohms and they fried. The ones I ended up using were about .55 ohms, and as the boily I did the repairs for hasnt come back, I expect still working Joe |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
You can buy 10 X 10,000 uf 100V caps for under $46 so long as you are prepared to wait a few weeks for them to arrive. Here They are what I am using for my Inverter build. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| joebog1 Senior Member Joined: 07/11/2015 Location: AustraliaPosts: 114 |
Yep I know about them. As Im an auld farte I call them digital caps. The ones I am talking about are true aluminium foil electrolytics, not some silver spray paint on cooking plastic ( sorry  )The Siemens ones I remember are about the size of a 1 1/2 litre Thermos with 6mm bolts as terminals. I do use Rubycon caps !!! dont get me wrong, but they are more suited to 100 kHz switchmode supply filtering, than for delivering 100 amps for a 3 minute welding arc. I have forgotten ( Im a newbie) who has as his avatar a saying : if ya dont put nothin in, ya wont get nuthin out The man is quite wise !! and I wish you to show me how to fasten 2 B&S welding cable to a 1mm X .25 mm connector on your caps ( Joe ducks Im sorry sir )Joe |
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| joebog1 Senior Member Joined: 07/11/2015 Location: AustraliaPosts: 114 |
HHMM just as a follow on I have been reading ( yours ??) posts re a whopping big inverter with mosfets screwed down to heatsinks with the buffer caps in a line in between the mosfets. I make good money from replacing those ( inevitably overheated ) caps in a well known blue aussie made inverter !!! I have a couple on soak test now with the caps replaced by big foil caps, as opposed to film caps. with THICK braid and a short distance from the heatsinks. I live in north queensland and today its 34 in the loungeroom, I dont know what the temp is in my nearest battery shed 5 kliks away ( alas I am on mains power) but even with my insulation its gotta be 5 on top of that. Heat is what kills electronics!!! even valves, and I do LOTS more work with them than with inverters. The Rubycons are probably rated at 105 C as long as they are within the stream of cold air from the computer room air conditioning. I really am trying to be constructive, but caps on heatsinks is like storing bullets on the stove  Mods please bare with me  best regards Joe |
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| joebog1 Senior Member Joined: 07/11/2015 Location: AustraliaPosts: 114 |
hhmmm to give you an idea, these are the caps Im using. ex raaf long life even if they were made almost 30 years ago. They are brand new Joe The LP cover is to give you an idea of scale |
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| joebog1 Senior Member Joined: 07/11/2015 Location: AustraliaPosts: 114 |
 er sorry 1meg size limit |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Great price, Madness! Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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