Ground Control

Another power supply question. I'm building onto an existing 240vac machine. I'm getting 120vac from one of the hot legs of the three leads (it uses a three pronged plug) and the ground lead. That gives me a hot and a ground. One of the power supplies I'm using utilizes a three prong plug. I put one hot leg 120 on one prong, the ground/neutral on the other. What do I attach the power supplys dedicated ground lead to?

I imagine the ground from the three prong plug should be connected to the chassis ground of my project. Any other ground points of components will be likewise attached to chassis ground. So if the ground/neutral coming in from the 240 lines is going to be used for neutral whereever 120v needs it does it likewise get connected to chassis ground or left as a 'floating' neutral? Grounding (obviously) has always been a source of some confusion for me.

I was told by a friend (a working electrician) that I can attach the 120v and ground from the 240v service to either of the leads on the power supply and then connect the neutral one to the chassis. In this case I'm simply bridging the dedicated ground on the power supply with the neutral coming in from the power source. Since I'm looking at AC it shouldn't matter he says. I really don't get it. There is no continuity right now (naturally) between either of the power prongs and the ground prong on the power supply. Why bridge them at all? But if not, then how do I ground the chassis of my project? Inside the 240vac machine (a welder) the ground/green wire coming in from the service outlet is attached directly to chassis. I hope this isn't too unclear. Sorry for that.

The reason for using borrowing off of the 240 inside the welder is because I need this 'project' to use the main power switch on the welder. It's only a couple of amps draw so it's no problem with the switch or breakers. It needs to power up/down along with the welder.

Yeow!
Your 240v machine has L1, L2, and GROUND coming into it. This is GROUND, not NEUTRAL.
There is a difference!
Ground is there to protect the users of the device from lethal shocks.
Neutral is there for power.
If you only have a three-prong plug, you must use a transformer with a primary of 240v, and go between L1 and L2. Otherwise, you will be putting current through the ground wire, and there will be voltage on the chassis.
Your other choice would be to install a 4-prong outlet, and replace the power cord with a 4-prong cord. This will be quite a bit more expensive than just getting the correct transformer.

In a residential electrical system, there is L1, L2 (the two ends of the power company transformer) and the NEUTRAL line (the power company transformer center tap.) There is also a GROUND line, which is earth ground. Somewhere near your meter or breaker panel is an 8' long copper-clad rod that was pounded into the ground, and has a big copper wire attached to it. It goes to the GROUND buss in the breaker panel.
There is also a NEUTRAL buss in the breaker panel.
These two busses are connected inside the breaker panel, and this is the ONLY place that ground and neutral should be connected. This is for safety, as otherwise the output from the power company's transformer might "float" to several thousand volts, which would obviously be very dangerous.

Everywhere else in the residence, the GROUND wire is just that - connected to earth ground. If there is current in the ground wire, there is a problem.

Your electrician friend is not doing you any favors.

First, a note on safety: Do not leave a floating neutral.

Next, let's define some terms...

"Neutral" is your grounded wire. "Ground" is your grounding wire.

The purpose of the neutral wire is to complete the circuit - circuit is from source through hot wire through load through neutral back to source. The purpose of the ground wire is to save lives if the neutral wire should ever become open on the way back. You see, if the neutral becomes open somewhere between you and the source, the neutral conductors will now be at the same potential as the hot conductors. The ground wire from your chassis back to the source protects everyone if/when such a thing happens.

The ideal solution to your situation would be to have your electrician friend fish in a neutral wire for your welder to run on and re-wire your welder with a four prong plug. Not only could you then wire your new gizmo up safely, but your welder would be safer than it currently is.

Failing that, I suggest powering your gizmo from a separate circuit and using an opto-isolated relay to control your gizmo from the welder's switch.

This is mostly duplicative of what's been said, but has a slightly different slant.

In an old house (circa 1950's), there was no ground. 220V appliances had three wires, as implied above. They were L1, L2, and grounded(neutral, white). A device such as a dryer or electric range that needed both 110V for lights, clocks, etc. and 220V used the neutral to "split" the 220V, just as we would do it today. There simply is not ground.

Since neutral is grounded (usually at the service entrance) getting 110V between it and one of the legs is completely normal.

One can't argue with the good advice of running a ground wire (green) for new outlets. Code does not require old outlets to be updated, however. But, in theory you cannot run a new outlet without providing a real ground . That is, one is not allowed to "extend" an old system that would not meet current code. You cannot duplicate the old wire, so any new wire you use would be a giveaway that the non-code change had been done recently. Will your fire insurance cover a non-code installation of an arc welder?

Which now gets to my whole point. The older 2-wire systems were usually low capacity, such as 50 to 60A, maybe a little more. If capacity has been increased to 150 or 200A, then it most certainly is or should have been converted to 3-wire (ground and grounded).

Will your welder run on the lower capacity service without having to turn everything else off? John

This is the best diagram I could come up with quick. Could you elaborate on the idea of using a transformer across the mains? I worry that it might negate much of the work I've done to keep this project extremely lightweight and compact. Perhaps rewiring a four prong receptacle is going to be the only option.

In the diagram the chassis ground in the cooler is imagined and not made yet. This is the point of my concern. I wonder that if the welder is indeed quite safe, I called the manufacturer to discuss this and was assured that this is the case, then why can I not extend the same safety factor to any auxilliary unit beyond it? Here is a page from the operating manual. The manufacturer explained that this machine would normally never exceed 35amps draw at the receptacle during normal use.

http://img378.imageshack.us/img378/1605/jpegofmanualpageon5.jpg

If my welder is utilizing a three prong plug, then can I assume that it is using a transformer internally to to as you describe? Is it not then still energizing the ground? How else to get power to the welder?

I just downloaded the operators manual for a comparable welder in the Miller Welding lineup. Miller is one of the largest welding machine labels in the world. This years Dynasty 200dx Inverter Tig is diagrammed connected to 240 single phase in the very same manner. There are two power leads and one ground.

My very best suggestions to you still stand.
1) Get a tranformer with a 240v primary, or;
2) Have the outlet re-wired with L1, L2, Neutral and Ground, and the cord and plug replaced.

To do otherwise may result in unsafe (indeed, lethal) voltage levels on the chassis.

I'm not kidding.

Be safe, rather than foolish - and possibly dead.

With a TIG welder that has HF, you probably do want a real ground. Remember too, there is a risk of arcing to the gas cylinder from the welder frame.* Keeping everything nicely grounded helps. John

*Whenever you return an argon tank to the supply store, they will or should check for signs of such arc-over. If it is there, the cyclinder cannot be refilled = big expense for you.

Are you both saying that these machines as constructed are not within reasonable margins of safety for the operators? These are the latest models from two of the worlds top welding machine manufaturers.

If they build machines that are, evidently, considered absolutely safe by many thousands of weldors worldwide, why can I not build a device that utilizes the same power source, and meets the same factors of safety?

If these machines use three prongs to get 240 volts with evident safety, they must be using the ground to complete the circuit.. no?

anne said:

Are you both saying that these machines as constructed are not within reasonable margins of safety for the operators? These are the latest models from two of the worlds top welding machine manufaturers.

If they build machines that are, evidently, considered absolutely safe by many thousands of weldors worldwide, why can I not build a device that utilizes the same power source, and meets the same factors of safety?

If these machines use three prongs to get 240 volts with evident safety, they must be using the ground to complete the circuit.. no?

Click to expand...

You still don't understand.

The machines, as designed and built, MUST conform to safety standards, and they do.

What YOU are proposing is making UNSAFE alterations to one of these machines.

Is obtaining a transformer with an appropriately rated primary winding SO difficult where you reside?

Let me give you an example of my concern.

Normally, the 240v to high-current transformer and whatever else are running along and doing whatever they should. If something fails in a shorted mode, there is no problem - the fuse or circuit breaker blows; the power is cut off. No one is ever exposed to a problem.

With your scenario, if the transformer windings fail over time in a shorted mode, either L1 or L2 would tend to "pull" the ground line away from "real" ground. You really could kill someone in this manner, without your transformer or power supply even visible.

The equipment that exists already is safe.

What you are proposing is DANGEROUS and may very well kill someone.

Please don't attempt this.

If you can't get a transformer that has a 240v primary winding and a secondary winding in your range, you simply aren't looking hard enough.

What do I have to do to convince you of the error of your way of thinking?

I'm not trying to give you a hard time.

I'm trying VERY hard to keep YOU from killing YOURSELF and/or others.

I really have nothing to gain by this thread. So, what's my motivation for even replying to it?

I simply couldn't stand by and let someone do something extremely dangerous with high-powered circuits.

So there.

anne said:

Are you both saying that these machines as constructed are not within reasonable margins of safety for the operators?

Click to expand...

Do you mean as defined by the American tort system or by what a conscious individual would consider reasonable?

The machines are safe. I have had mine (Miller 350 TIG) since 1984 and never had a single problem or tank arc. On the other hand, I don't cook and eat a frozen pizza without removing it from the plastic cover first.

John

I might have a clue on this, maybe not ....

My old Miller 35 Mig welder uses 4 prongs but that's because there are 110V requirements inside the machine. Fan. Feed motor. etc. 2 legs are hot. 1 leg is the return and that's the one that gets used to split the 110 off from one of the hot legs. The 4th and last wire is the earth gnd.

On the other hand my air compressor is a 220 only machine and only needs 3 wires. It has no 110 requirements so it uses just the 2 hot legs and the safety related earth gnd only. No prongs on it cuz I got it hardwired, but you get the point. If you don't need 110 for something inside the machine 3 wires will do.

As another example I have a lathe with a 3/4HP 220V 3 phase motor that runs on a VFD. (Variable Frequency Drive). The VFD miraculously makes the 3 phase at freqeuncies between 0 and 400hz from 220 single, and because it does NOT need 110, it only needs 3 wires.

Looking at your schematic your welder seems to be a 220 only machine like my air compressor and VFD. Your cooler needs 120 unfortunately and unless you get a return line in there to split the 120 offa one of the hot legs it won't be happening.

In simple terms, the 2 hot legs each measure 110 to the return. Together they make 220. Without a return in the middle you can't get 110. Mixing the earth gnd with your chassis and 220 volts is an absolute recipe for disaster.

I see SgtWookie has posted again and I think I understand his proposed Xfmer solution. With the proper center tap on the secondary of the Xfmer you could get a safe way to return the two legs? That seems to make sense. In any event I'd listen to the Sarge. You're gonna have to get a Xfmer in there or go 4 prong and drag a return line into your welder to get 110.

Cool on the Puch BTW.

SP

I'm not about to do anything dangerous. I won't take one step in any direction unless I'm 100percent certain of it. That's why I'm here, there, and practically everywhere trying to get the right answers to this. If it turns out that it is not possible to do safely, then I will not do it. I am extemely grateful for the contributions of others knowledge.

Turns out that I'm getting some very good direction at this. Yours Sgt. led me on what might end up being the right way, despite your "please step away from the machinery little one, and go play elsewhere" tone to my inquiries. I also drive a large pickup truck on superhighways. I use a chainsaw in the woods. I've had my private pilots license for fifteen years. Thank goodness noone ever disuaded me from such simply because of the (comparable to household electricity or worse) dangers associated with such activity. A lesson I appreciated early was on the grave danger in not asking 'ignorant' questions simply for the sake of pride. Many people are killed for only that. It's the idiots who rush into danger, and your tone would seem to presume too much of a stranger.

And that said, and you're perfectly welcome to sit this out Sgt., though I hope you won't, my primary objection to the transformer option, rather than rewiring for a four prong outlet, is the addition of a great deal of bulk and weight to this project. I've gone to great length to keep things extremely compact. I do, however, have a few of this type of transformer about the workshop...
http://www.newkunst.com/catdrill.asp?pID=77097&model=M1600&manfact=Franzus&mCat=Small%20Appliance&sCat=Transformer
It's about 1/3 smaller and 1/6 of the weight of a stacked core xformer.
...any reason why this would not suffice to step down to 120vac for my uses?

A quick peek inside reveals a rather nice aluminum heatsink body around a Q383 transistor and a couple of other components with fuse. Claims to be rated to 1600w. I can post an image of the inside if necessary.

I am not sure what a Q383 transistor is. Are you sure it is a transistor and not, for example, a triac?

Those inexpensive, wall plug converters are sold for non-electronic heating applications. You apparently propose using one for a SMPS. That application is not within its intended design purpose.

John

http://img178.imageshack.us/my.php?image=tmp6360tn7.jpg

Am I getting closer?


The switch mode power supply is being used solely for reasons of weight savings. Same would apply to whatever means I apply to transforming 240vac. I'm working with commonly available items, and good finds, in order to keep cost reasonable. Otherwise this project would be upwards of three thousand dollars by now. I got very lucky, for instance, finding a seven hundred dollar lab-grade miniature gear pump on ebay in excellent condition for thirty dollars. Is there any other option to a stacked core xformer to get me to 120vac? If lightweight component designed 240vac to 120vac power supplies are available, I don't even want to guess at what cost. I can get 120vac to XXvdc SMPSs by the armful at thrift stores for a dollar each.

Why do you need to use a 120V SMPS to generate 12 VDC? What's wrong with a 220V SMPS?

My Thermaltake 430W, 18A @12VDC supply cost about $25 new at a local computer store. It can be switched to either 115 or 220 input. So-called computer surplus and discount stores have SMPS for even less.

What we are all stressing is not to sacrifice the GROUND by using it for anything else. You haven't commented whether your TIG will have HF for starting.

John

Yes, tig uses HF for starting.
And if that means I cannot hope to get an SMPS to function anywhere within twenty feet of it I'll probably have a nervous breakdown at this point. I've already had to work around so many other limitations.

The water pump uses a 1/20hp 120vac 3.5 amp motor. The two muffin fans are 12vdc @.5amps each. The panel gauges and panel light I'm using are 12vdc. I'm using a variety of 12vdc and 120vac relays within. I can use a 240/12 SMPS but I still need 120 to the pump anyway so I'm not ahead. I can replace the pump motor with a 220v but that is waaaay back to the drawing board since what I have right now has been painstakingly shoehorned into the space I have as it is.

I could have used 120vac components throughout. I could have used 12vdc components throughout. Heck, I'm sure I could have mortgaged the house for industrial level 220 components throughout. I'm trying to assemble a something for which there are no off the shelf "manufactured for extremely compact tig water cooler" items available, so I have to constantly compromise.

With an open expense account, I could find and assemble all of this for five grand or more as a prototype, hire an EE, and have it together in a few weeks time. Otherwise, it's been a worthwile winter project for me. For instance no automotive heater core would fit my requirements so I had to spring for a computer destined/designed water cooling radiator from DangerDen. Muffin fans from Surplus Center in Lincoln, NE. Relays and asst from Skycraft Surplus. All aluminum chassis material, angles, and SS grille from Padnos Scrap Metals down the street for fifteen dollars. All power supplies (the fans idle at 6vac while the pump is off since the motor generates quite a bit of heat where I have it confined), SMPS castoffs from assorted consumer electronics - 50 cents to a dollar each at Goodwill.

It's actually been great fun finding this stuff.

Don't take me the wrong way. My reply was not intended as an insult, or a "step away from the bright lights little girl" kind of thing - but you didn't seem to be getting the message that what you were intending on doing was going to result in an unsafe situation. You forced me to be annoyingly blunt.

Skycraft Surplus is an amazing store. It's one of my favorite haunts They do have some 240 to 120 step-down transformers at the very back of the store, on one of the higher shelves. Same row as the relays, but all the way towards the back. I don't remember all of the secondary ratings offhand, but I think they had a couple rated for 6A. You're right though, they'll add several pounds to your project. It would be nice to find a 240v to 120v inverter rather than a transformer, but...

Marlin P. Jones & Assoc has these:
http://www.mpja.com/prodinfo.asp?number=14833+PS
$45, and larger than you want. But, your welder is probably going to be putting some rather large surges on the 240v lines; something like this would take care of that problem.

Congrats on your PPL. What do you fly?

You can get a shiny brand new ten pound 500Watt 240/120 transformer for about $60.00. Used ones should be less. Try eBay or Craig's List or your favorite local surplus house.

This cooler that I'm building is almost completed, with the exception (evidently) of wiring. I thought I was planning with sufficient foresight but I'm coming dangerously close to being mistaken.

Fully assembled it measures 6.75wide x 5.50high x 19.50long. It weighs approx 24 lbs filled with coolant. I could have taken another six to eight lbs off of this if I had been willing to look farther for an alum radiator instead of brass (tho I was too concerned with potential dielectric corrosion since most water contact in the torch and fittings is brass and my pump is SS) and tried to manage with plastic plumbing instead of brass and copper. Plastic plumbing at these dimensions isn't as commonly available in as wide a variety of fittings as brass. Brass also seemed a reasonable compromise of bulk and rigidity where it could actually be used as a structural section. I had to modify most of the brass fittings to shave off mms where I could.

The dimensions are such that it bolts directly to the bottom of my suitcase sized inverter welder. I've bench tested the entire thing separate of the welder using a 120v outlet. Perfect psi and gph through my torch. I'm getting about .5gpm @ 15psi and have a needle valve bypass that can trim it up or down if ever needed for different torches. The pump itself has an adjustable internal bypass regulator in case of emergencies.

At this point there is an approx 4 x 2 x 2.5 inch space left available within for any additions.
http://www.mpja.com/prodinfo.asp?number=14833+PS
altho perfectly suited, would just about dimensionally kill the entire reason for undertaking this project.

Is it looking like managing 240 down to 120 within this box is going to finally be insurmoutable?