Cebora Monowelder converted to DC

[flash-bang]

I joined and posted about help on a MIG welder a little while ago. Well that died because it was the main transformer primary was blown. However - there's nothing wrong with the rectifier and choke.
I'd been reading around the forum and DC seemed a good idea.
So I lashed on the rec and choke. First I laid a bead with 1.5 rod on 0.7 sheet without any hassle.
Next I had a small frame of 20mm dia tube to put together. Well impressed, nice and smooth, easy to strike. And then I noticed - no splatter. I ran my hands around all the welds and couldn't find even one spot of splatter, none! Amazing. So I'm going to build a box underneath to hold the extras plus the fan.
The open voltage DC is 67V. I noticed compared to previous, the current needed to be higher about 20%; that could be due to the choke I guessing.
I bought the Cebora Mono Welder mod.1722 when I was 19, and apart from a brief spell with an oil-cooled AC, it's the only welder I’ve used; and it’s worked without problem for 44 years.

 

[flash-bang]

Just in case, thought I'd better highlight the safety aspects of this for anyone not well acquainted with the danger of DC electricity.
First, with an AC shock, you get a “buzz” (not a high!) because the current is switching direction 50 times a second. But DC is nasty. Ever seen the frog’s leg and 1.5v battery experiment at school? Your muscles just tense up – solid! If you are lucky you’ll go off like a spring and find yourself on the other side of the room. But if you are holding something, you can’t let go. And when they find you on the floor dead, you’ll still be holding on.

Secondly, the “earth” clamp is NOT actually earthed at all. If it were earthed, there would be a greater chance of getting a shock. Also if the “earth” clamp connection wasn’t good (or if you forgot to put it on) the welding current would be flowing through the earth wires in the building installation wiring. Not sure how an RCD would react in this situation, but I’m guessing the earth imbalance would trip it. But what if there wasn’t an RCD or it’s faulty? Smoking conduit! And every twin+earth cable would be compromised – complete building re-wire.

 

[rtbcomp]

...

Secondly, the “earth” clamp is NOT actually earthed at all. If it were earthed, there would be a greater chance of getting a shock. Also if the “earth” clamp connection wasn’t good (or if you forgot to put it on) the welding current would be flowing through the earth wires in the building installation wiring. Not sure how an RCD would react in this situation, but I’m guessing the earth imbalance would trip it. But what if there wasn’t an RCD or it’s faulty? Smoking conduit! And every twin+earth cable would be compromised – complete building re-wire.

An RCD wouldn't react because it detects imbalance between live & neutral, and the secondary of the transformer isolates the welding current from the mains. There is definitely a risk that you could burn out the earth conductors in the building wiring under the conditions you describe.

I've got one of these Mono welders, (and have had for over 40 years) and I find it's a great little machine. I've also got a SIP MIG welder with a good rectifier and choke, I wondered about combining the two so I think I'll go ahead having read this.

 

[flash-bang]

67 volts (be it AC or DC) is considered a 'safe' voltage

I can't entirely agree with that. This is one of those things that comes under the, "it all depends..."
Personally I don't suffer as badly as some, never had any sensation from loading a rod, whereas I've known others who always need to use something insulating to grip the rod. I've had a few belts over the years and happily flash a finger across "whatever" just first to confirm it really isn't live (Got caught once due to a faulty mains indicator).
But I did once get an unpleasant belt from a 24v truck battery, I felt it tense me up.
Variety of views here: http://www.candlepowerforums.com/vb/showthread.php?261353-When-does-DC-voltage-become-dangerous
So I still believe these voltage levels are potentially lethal and cannot be considered as safe.

 

[flash-bang]

pdq, you are rather jumping to conclusions. How about cutting me a bit of slack!
I don't expose myself to uv or get spatter burns. I don't like welding gloves because they're usually uncomfortable and clumsy. So I have a shield on the rod holder - that hands protected and the other is behind my back.
And I didn't say, "I test something by touching it". I said, "to confirm it really isn't live". I had assumed it was obvious that before working on wiring, it will turned off or isolated. And then checked with a meter or mains tester. But these are not 100% reliable, meter leads brake down usually intermittently before completely failing, and mains testers/screwdivers can fail at any time (as I have experienced). So I don't place complete reliance on these either. Therefore, after having done all the previous, I would not just grab something which could possibly still be live, so I flash a finger across before grabbing it.
It's a last ditch method of confirmation.

 

[rtbcomp]

Also it might be an idea to incorporate a thermal cutout on the rectifier.

 

[rtbcomp]

The Cebora does have a thermal cutout.

 

[brokenbiker]

67 volts (be it AC or DC) is considered a 'safe' voltage - you probably don't want to lick it, but for 'most' people it won't do any harm or have any effect.

My inverter arc welder has an ocv of 100vdc (less safe) and the ubiquitous cut50 plasma is around 250vdc.

lowest recorded death with DC power is 55V at 30mA...or atleast was when i was in college a few years ago

 

[flash-bang]

The thermal cutout is mounted right against the secondary winding.

It never has tripped, but the duty cycles I've put it through couldn't be described as heavy.

Unfortunately no. Not enough room to house the extras in the case. However it does mean the air flow can be controlled better.
Right behind the fan the rec will be mounted, behind that the choke, and the air will exhaust up and out past the transformer.
But the addon doesn't need to be higher than the fan housing, so shouldn't be too top heavy (getting knocked over).

Westinghouse? Never heard of them.
I'm part of the backbone of this country - the self employed.

 

[Gixxer]

The thermal cutout is mounted right against the secondary winding.
View attachment 89850
It never has tripped, but the duty cycles I've put it through couldn't be described as heavy.

Unfortunately no. Not enough room to house the extras in the case. However it does mean the air flow can be controlled better.
Right behind the fan the rec will be mounted, behind that the choke, and the air will exhaust up and out past the transformer.
But the addon doesn't need to be higher than the fan housing, so shouldn't be too top heavy (getting knocked over).

Westinghouse? Never heard of them.
I'm part of the backbone of this country - the self employed.

I know this is old thread but have just been given one of these welders by father in law, its been sitting in the garage for a few years and all paint on case has turned a nice shade of red (rust) is there any chance of a picture of the scale thats on the top of the machine please

 

[rtbcomp]

This is mine -

 

[Windy Miller]

This is mine -

View attachment 136360

I'd get you a pic of mine, but it's almost as rusty I think. I'll have a look in a bit for you.

 

[Windy Miller]

I'd get you a pic of mine, but it's almost as rusty I think. I'll have a look in a bit for you.

Actually it's not as bad as I remembered!

 

[rtbcomp]

Magic I'll print that out and stick it on my welder.