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Converting an Lincoln AC welder to DC

R

rlp in Co.

Well-known Member
Well, I'm very disappointed today. I got the welder wired up and tried it out, but it won't hold an arc, even at 225 amps. I was using 7018 1/8 inch Forney rod.
I also tried some 3/32 7018 AC rod that I couldn't get to work very good on AC, and it welded that good, on DC.
I double checked the voltage polarity, and it is positive DC on the ground wire. So, I'm guessing that I have to add some filtering to smooth the pulsating DC to pure DC. What kind of filter do they use? Maybe someone has a picture of what is inside of a DC welder.
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Those are bridge rectifiers. Each one is made up of 4 diodes in a bridge. You use enough to get the amperage you need.
 
You probably have too much voltage drop. Welder use SCRs instead of straight diodes. I tried the same thing years ago with little success.
 
You have a way to go.No choke coil or filter cap.Most DC welding is done electrode positive.Too much guessing not enough book time
 
How much cooling airflow is planned? What is the input terminal voltage while striking an arc? I"m wondering if the 240V supply system has vd.
 
How much cooling airflow is planned? What is the input terminal voltage while striking an arc? I"m wondering if the 240V supply system has vd.
 
I would bet you could find a good used AC/DC welder on Craigslist for not too much. I personally would not attempt to rewire a Lincoln AC buzz box.
 
All diodes and in particular diode bridges have a small yet significant difference. Of the threshold voltage where the diode begins to conduct in forward bias. Difference in temperature from diode to diode will also vary it's internal resistance.
End result is that some diodes will conduct more than their share of current while others loaf along. End result is the current hogs will burn out or fail prematurely.
As previously stated an inductive choke coil and some capacitors will be required. Otherwise the DC arc will stop 120 times per second.
 
Is your machine just an AC 225 or an AC225,DC 125? If it's only an AC225, it's not worth the trouble to add a rectifier and make it DC.
 
Ì have been looking on craigslist for a long time. They either want almost new price or they are snapped up within minutes if they have a reasonable price on a DC welder.
 
A bridge rectifier only has a 1.4 voltage drop. I have 48 volts at 225 amps. It welds good with the AC rod, so the voltage drop is not hurting much.
 
Well, I will change the voltage that I had backwards and build an inductor tomorrow. Thanks to the ones that gave constructive criticism.
 
You may be right about the diodes, but I have seen bridge diodes paralleled in power supplies and battery chargers before. I guess I'll see what happens.
 
It's hard to tell how you have it wired, but I think you have it wired as a half-wave rectifier rather than full-wave. If so, no amount of filtering is going to help because the voltage is pulsing at 60 pulses per second. It's actually much worse than ac because you're only getting current half the time.

If you have a center tap on the transformer secondary winding, you'll need 10 each 50 amp diodes to handle 225 amps. If you don't have a center tap (more likely), you'll need 20 diodes to make a full-wave rectifier bridge.
 

I know what you mean about Craig's List. I search my local Craig's List tool section 3 times a day. I see those little AC, or AC /DC welders on there all the time. Some of the ads I feel like calling the person and just saying, seriously! What have you been smoking?

When or if you get this completed, what are you really going to have? On another site I belong to, there are a couple guys who have the Everlast PowerArc 140ST, they love them! I have the PowerArc 200, I challenge anybody to show me a better arc for the money! You can join TractorByNet.com in the welding section Everlast is a sponsor there. Ask Mark what kind of a deal he can make you on an Everlast machine.

http://www.tractorbynet.com/forums/general-forums/
 
Paralleled diodes have problems.My converter has 4 300 amp diodes in it.You cant take a bunch of 35 amp bridges and draw 225 amps from them.My diodes cost 40 bucks.The converter cost 100 bucks.I had some of the parts on hand.Making the inductor took a while.
 
electrode positive is much stronger welding than electrode negative in DC welding.With out a choke coil and filter capacitor the DC output is dirty and no better than AC.I built my converter in a seperate box with its own welding leads.It has its own fan.The diodes have 3/4 inch mounting studs.built mine 5 years ago after using a friends DC welder.
 

There are only two places that repair welders in my area. They are both about an hours drive one way. They are both right at $100.00 an hour for labor. Do you really believe I'm going to take a $200.00 to $300.00 welder in for repair? The Everlast machines come with a 5-year warranty, all you have to do is pay for shipping back to them.

I've had my Dynasty 300 for about 7-years, not one minute of trouble. Inverters are here to stay, might as well get use to it. :wink:
 
I agree with some of the others. Diodes will NOT share current when paralleled due to the minute differences in forward voltage drop.

Normally one will take all the current and burn out, then another will do the same thing, leading to a catostrophic failure.

I will also point out that with a forward voltage drop of 1.4V and a current of 225A you are looking at 315W of heat coming from the diodes.

Cliff (VA)
 
Gents, Mother and wife bought me a Lincoln buzz box back in the 70"s cost $ 99.00. Welded with it for a few years.
Long about 1975 or so got a new boss who was a master electriction who suggested that I make a "full bridge rectifier" for it. He also said that the shovel shop had had "grate big diodes"
that would work. I promply forgot about the whole idea. Several weeks later he came into my office with a paper bag with 6 diodes 3 each + and -. drew me a diagram and explaned what else I needed and how to build a full bridge rectifier. I did it and WHAT A DIFFERENCE! Still welding grate after 36 years
 
I do believe you need a large inductor in the DC circuit. I remember seeing a magazine article years ago about converting an alternator into a welder that described this. It was simply a coil of wire on an iron core, not difficult at all to make. The inductor would act to pass DC and block DC. Remember that the diodes give pulsating DC, not a smooth, even voltage. The inductor helps to smooth it out,

Josh
 
Bridge rectifiers have 4 diodes in each bridge. That makes a full wave without a center tapped transformer. It is wired as a full wave. The AC tap on the right is a little hard to see. You can see the other tap coming from the amp selector switch.
 
You may be right about the diodes, however I have built another power supply with paralleled diodes and it still works great. I knew about paralleled diodes but had forgotten about it. I wish now that I had ordered 10 instead of 5. They seem to work ok if they don't get hot. It has a good fan on it already, so that will help, I hope. I will probably never use the full 225 amps anyway, only for testing. I usually only use at the most, 115 amps. I don't know how much amperage 7018 1/8 rods take. The diodes were 2 bucks each, and the heat sinks were 1.84 each. So we're not talking about a lot of money here, only my time which I'm willing to spend. I has been an interesting project, even if it doesn't work.
 
We are not too concerned about spelling on this forum. There are a lot of people that can't spell perfectly that have a lot of intelligence and common sense. By the way, if you right click on your message after you type it, you can then click on spell check. I use it a LOT!
 
I am not a professional welder, but i do have a Lincoln 225 with 4 diodes in a bridge ofr the output, no filter or choke and it works great. I have a friend who used to do a lot of welding and he showed me that the 7018 rods have to get hot before they will work with it. He said it was the same with the DC welder they had at work.
 
Personally I hate welding grate and couldn't imagine doing it for 36 years! The notched anti-slip kind is the worst cause it's really hard on the knee's. I much prefer welding checker plate. LoL
 
The early inverters (like XMT 300's) had some
problems because they used a different type of
rectifier or something and were expensive to fix.
The newer inverters are pretty reliable though and
save big bucks on your power bill. I worked in a
shop that switched to Miller XMT 304's and they
claimed they saved about $1000 a month per machine
in power and with about 18 machines, that's a lot of
money.
 
Normally, as an electrical engineer, when I do try to parallel diodes, I add a very small resistance in series with each diode leg. You need just enough to mitigate the effect of the variation in the forward voltage drop of the diodes. Of course the current passing through the resistor is just wasted heat.

If you figure you need 0.2V drop @50A, R=V/I=0.004 ohms dissapaiting W=VI=10w each. Your wiring may be providing enough drop for this.

You still need to add the filtering as mentioned by others.

Cliff(VA)
 
1/8" 7018 usually burn about 130 amps but can be turned hotter. Of course the indicator isn't all that accurate but 115 amps is usually a little low unless you're welding vertical.
 
Without the choke coil and filter caps you will be using dirty dc.The capactitor increases the strike voltage and makes sterting the arc easier,
 
Power use dosent bother the home welder.I did a lot of welding last summer.Our bills show monthly use for a full year.My Lincoln 225 cost 93 bucks in 1965 so my yearly cost to owm the welder is about 2 bucks per year.
 
I can spell words several different ways.After age 70 you will see some strange words, mispelling, letters left out.Your welding will suffer.Not an English Major but I can build a DC welder converter that works.I still wonder why Lincoln limited the DC output of the 225 welder.Cheap rectifier probably.
 
If you have limited power service, having a machine that uses lower amps makes it a lot easier to hook a machine up. If people do bigger projects they will notice it on their power bill.
 
I am on the end of a 3 phase line that was put in about 15 years ago.I have less than 200 bucks in my welders so I dont need improvements.At 75 years I dont fuss about the Future.
 
When you get through fooling around theres a 30 page booklet that sells for 3 bucks.Plenty of information on home brew welders plus info on a DC converter.Its out of print but I found 4 copies in my stock.I used that booklet and a diagram of an Airco 160 Dip Stick welder when I built my DC converter.
 
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