AC vs. DC

Hello, and thanks for taking the time.
What is the advantage of a DC stick welder over an AC type? thanks, John

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I am not an expert welder but did convert an AC buzz box to DC for a bit of fun and since DC seemed to be preferred or at least more popular than AC. I found that using DC made it easier to get the arc started. I can pull the rod further away from the material and still maintain the arc but that is not really useful. You can also use electrodes that are DC only although for general purpose work on mild steel I doubt if that is a big benefit since the common ones are available in either. If you want to have the electrode positive or negative for penetration reasons, with DC it is always positive, not switching back and forth 60 times a second.
If you are welding typically in a corner and the steel is magnetized then you will get arc blow using DC (the arc is deflected by the magnetic field) so AC is required.
Since I built my rectifier I haven't used AC but I only weld steel.
Billh
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Interested in telling how you did it? I have a 230Amp AC buzzbox I wouldn't mind converting.
Paul
On Fri, 28 Nov 2003 14:17:09 -0500, billh wrote:

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Probably not the easiest way but I used 12 35Amp 600V bridge rectifiers in parallel. To use diodes in parallel you have to be careful to ensure the load is balanced or else one takes most of the load, fails, then the next one takes most of the load, fails, and so on until they all fail.
I brought the AC leads from the welder into an old desktop computer box. I was fortunate to have some old electric furnace lugs which made connecting to them easy. I wired the 2 input terminals of the bridge rectifiers to the lugs using 18" of #18 wire all cut to the same length. These leads are more than just connecting wires they are actually 0.01 ohm resistors to balance the load. I wired the outputs of the bridge rectifiers to the output terminals using the same length and size of wire thus adding another 0.02 ohms for balancing. So each bridge rectifier has in effect 0.04 ohms in series with it to balance the load.
The open-circuit voltage of the welder is about 85V AC but when the arc is broken collapsing magnetic fields can cause much larger spikes. That is the reason I went with 600V diodes and the extra cost over 200V is little. Each diode in each bridge is parallelled with a 0.01uf 1kv capacitor for transient suppression and each diode bridge has a 200V surge protector across the dc terminals. Personally, I think it is overkill and if I were to do it again I would leave out the 0.01uf capacitors since they were relatively expensive because I needed 48 and a pain to solder in place.
I mounted the bridge rectifiers on a large aluminum finned heatsink I happened to have. I also used 2 small fans (muffin fans) to blow air across the heatsink. I don't think it really gets very hot but better safe than sorry. What you do want is to keep the diodes at close to the same temperature so their characteristics stay similar. Also I bought all the diode bridges at once so they have the same manufacturer and date code. That way their characteristics are more likely to be similar than ones from different batches. Makes the load sharing less likely to be any problem.
Note that for my normal welding only 3-4 of the bridges are required to ge about 100-140A. The others are there for higher power and to handle current if the rod sticks down when the machine and also a safety factor.
Now we have pulsating dc but we don't want to current to go to zero when the voltage goes to zero. I did this by building 2 inductors which I placed in series in the positive output lead. Could be in the negative lead if you want. I got the cores out of the biggest junked microwaves I found at the local dump. Microwave transformers are cheaply made thank goodness. All of the E shaped laminations are stacked up and all of the I shaped laminations are stacked up, the coils placed on the Es and then the Is are welded on. A better way is alternating the Es and Is but they are hell to get apart. So just hack saw the weld and you can pry the E and I stacks apart. Pull or cutoff the existing coils. I used #4 welding cable and put something like about 9 turns on each core. You can't put the I part back on tightly since the high current will cause the steel to saturate and you will lose the inductance. I put something like about 1/8" masonite between the E and the I pieces to create a gap.
I couldn't easily find any connectors for the rectifier box that were compatible with the Miller connectors and weren't worth a fortune. I bought 4 sets of generic welding connectors and made a set of adapters with the miller plugs on one end. I put the other ends (males) on the leads which are the input to the rectifier box. I mounted the female parts of the last 2 sets in the box and put the males on my welding cables. If I want to switch to AC welding I can disconnect the welding cables from the rectifier box and the box from the welder and connect the welding cables to the welder.
Other ways of doing this, like I said this wasn't the easiest way. Ideally, you can find 4 big 400A diodes and you only have to deal with 4 pieces not 12. You also don't have any load balancing issues. I don't have ready access to surplus places so I didn't pursue this route but it is the best way to go if you can. You still require the inductors. Note that long welding cable leads will provide some inductance as well and so just how critical it is to have the inductors - I don't know. I often think of measuring what the current is when the voltage is zero but have never done it. I did buy some 6010 which is a DC only rod and it worked just fine so I am happy.
Another cheap source of diodes is from car alternators. These diodes are probably rated at around 100A so you would need about 3 sets. Problem is that they are pressed into plates and are I believe a 3 phase full wave rectifier configuration with 6 diodes per alternator. You have to be able to mount them to create a bridge.
HTH, Billh

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The car alternators I have seen use 9 diodes in the pack. 6 provide 3 phase full wave rectification and the remaining 3 provide field current for excitation. They are high current devices but may not have a high enough voltage.
John
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Thanks for the info. My knowledge about using alternator diodes for welders came from some posts by others using them. I would guess that the voltage is high enough because of a lack of information to the contrary. Years ago it was not uncommon to see rectifier diodes with voltage ratings of 50V but now you rarely see less than 200V or perhaps 100V sometimes.
Billh
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