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SirDick Diodenob esquire
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Its the choke , gives you a more stable output on dc , there are some real cheap mig welders which dont even have a choke or one thats aluminium wound as an appendage on the main transformer . I would bin your diodes and get four big ones which could be mounted on the existing heatsink or make your own up using some peices of copper busbar , you could easily make something far better than originally fitted for little cost that will become the strongest link in the circuit instead of the weakest .
dai
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SirDick Diodenob esquire
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Similar to the main transformer in construction but just has one coil which when the current is steady through it does nothing , when it rises or falls due to current ripples or other causes the back emf generated tries to prevent change so you get a more steady dc output than without it . In practice the bigger the choke the smoother the output but there are limits so capacitors help . I have a 450amp TIG machine with a choke weighing 75kG and a 100amp MIG with choke weighing 3kg but the small one dosnt do a great job , I suppose this is one of the reasons why transformer based machines are rapidly being replaced by invertors when you need such a large choke to get a reasonably smooth output . Wiki is your best bet for a good explanantion on chokes and goes on for as much detail as you can stand . Your capacitor bank looks quite substantial in relation to the choke and main transformer .
shenion
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The coil (reactor) is needed for short arc welding. Without it, the wire will hit the metal and a huge current will blast the wire away. Welds are messy and average current is low. Then the arc will have a low current. The choke ramps up the current until the droplet is pinched off the wire. It then maintains the current during the arc and causes the droplet to fan out.
My Miller had no choke, it was for spray transfer only (175 Amp and up.) Back in those days, that is all there was. Welds were cold, more spatter than weld and a crater at the end. Added choke and it welds nice down to 20A.
As for the diodes, it looks like one is shorting out as it gets hot. Probably best to replace the whole pack. Weldequip may be able to source a similar pack.
Otherwise it would mean getting 4 or more diodes and making one.
My Miller had no choke, it was for spray transfer only (175 Amp and up.) Back in those days, that is all there was. Welds were cold, more spatter than weld and a crater at the end. Added choke and it welds nice down to 20A.
As for the diodes, it looks like one is shorting out as it gets hot. Probably best to replace the whole pack. Weldequip may be able to source a similar pack.
Otherwise it would mean getting 4 or more diodes and making one.
dai
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what rating diodes would i need for the job??
and im assuming i cannot alter the final output of the machine by makeing/fitting a higher amperage diode pack
the snooper
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no you need big enough diodes to handle what amperage the machice produces
the snooper
getting older by the day
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you can go bigger and i would if i were you better to have some head room then run to the limits
the snooper
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there are various formations that i have seen so you could just copy it.
dont forget you will need to be able to mount them properly on the heat sink so they dont self destruct due to heat.
it might be easier to buy a ready built rectifier
dont forget you will need to be able to mount them properly on the heat sink so they dont self destruct due to heat.
it might be easier to buy a ready built rectifier
dai
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thats what im starting to think
the snooper
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get in touch with weldequip he will give you some advice as to what he can get hold of
SirDick Diodenob esquire
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The diode on the left is a good one and I got 8 of these for £10 from a scrapyard , if you are lucky you may find they have not begun stripping the equipment they came out of so you could get the heatsink as well but if not then use large bore copper water pipe squashed in a vice and drilled then bent in a U shape , I prefer to use scrap copper busbar , make your own insulators again using material from the scrapyard , you can make your own rectifier pack for £20 that will far exceed the performance of anything you can buy commercially ( unless you are willing to pay hundreds of pounds). These diodes will run at 250 amp continuous without generating a lot of heat and are complete overkill but if its costing so little then why not and you only need to use four of these . The diode in the middle is 85amp 200V and got 4 off from Farnell for approx £12 delivered , they will do but they get quite hot and need a good heatsink and they wont last forever . The button type on the right is claimed to be 75amp and these are fitted to a lot of battery chargers and welders etc ( the versions with a lead attached ) , they get too hot even carrying 40amps continuous and are mainly not very reliable , it was seeing crappy equipment fitted with these that got me interested in welding equipment , am not saying you cant get good ones but I only have experience of them failing and along with poor quality protective devices that dont protect causing transformers and other items to burn out .
Good question!
For some reason I don't understand, welding machine manufacturers buy-in rectifier modules like your existing one, which split the current between 8, 12 or 16 diodes, or even more. A basic rectifier circuit uses only 4 diodes.
If you search on the Internet, you'll find that nearly everyone says it's very bad practice to share the load between sets of diodes. You should always use just 4 diodes but they are very expensive for high currents, which is why the manufacturers must go for the cheaper (shared) option. The reason it's bad practice is that the diodes need to be perfectly matched, so the current is split equally. This is very hard to guarantee but the firm who produce those plate rectifiers (in Italy?) must know how to do it.
If the diodes are not perfectly matched, the current will all go through the diode with least resistance, and blow it. It's highly unlikely you could achieve DIY diode matching.
Dai, Congratulations on your success with the troubleshooting, tracking down the fault and fixing it !
Even when new, those diodes were rather wimpy for a 200A machine. The wires leading to them are very thin, and now look damaged by overheating. If all 8 diodes still work, you could try resoldering again, using copper strip in place of the wires. Diodes can be damaged by heat during soldering, so you may be able to rig up a "sandwich" arrangement where contact is provided by pressure instead.
Even if you fix or replace the rectifier, it may also be worth checking other connections where you may be losing maximum amps - by resistance - such as on the earth return lead.
Even when new, those diodes were rather wimpy for a 200A machine. The wires leading to them are very thin, and now look damaged by overheating. If all 8 diodes still work, you could try resoldering again, using copper strip in place of the wires. Diodes can be damaged by heat during soldering, so you may be able to rig up a "sandwich" arrangement where contact is provided by pressure instead.
Even if you fix or replace the rectifier, it may also be worth checking other connections where you may be losing maximum amps - by resistance - such as on the earth return lead.
dai
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intriuging sir dick
im waiting for a price for a rectifier at the mo,but im very interested in what you've said
what did you find those big diodes on??
if i can find some would the old plates do,obviously remove the old diodes first then fit the larger ones in place??
dai
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once i twigged what was what inside it wasnt to bad
there are 1 or 2 diodes that are now scrap(1 was when i soldered, then another blew)
knew it was somthing simple but i didnt know where it was and the joints lookeed ok untill i prodded them
cheers for all the help lads
dai
