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PostPosted: October 25, 2011, 4:34 pm 
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Trochu wrote:
There are about 150 units on e-bay for less than $300.00.
So, are those sub-$300 units, the ones for $250+$150 shipping?? Those the ones you talking about? That's a bit far from $250 or even my $100!

Or are you talking about the $0.99 units that even with free shipping always end up being bid to almost exactly what the BIN units go for? I spent a month trying to win one of those for under $300 and it never failed that if the auction ended at 2:30AM or Sunday afternoon the price was almost the same as the BIN! Lots of human bidders or robo bidders.. jury's out, but I got tired of waiting to get mine at those amazing prices!

So, I hear your opinion, but this was not about how to spend money you don't have. It was a small view into the scams that are out there on the web. Into how it is possible to build one for next to nothing that is entirely capable of doing all that the other guys can do, and if you are so inclined.. the WORKING plans to do so.

Are their cheap Plasma Cutters out there? Yes. Do you want them? Jury is out. Your trying to show how much better they are without ever having used one, right. I have yet to buy a HF hand tool.. why? Because I like my hands! Craftsman all the way! Same here.. do you want to buy a 3-in-1 unit that is OK at all things and good at none? Same reason all the folks here gave me to stay away from a Smithy 3-in-1 Lathe, your telling us to buy these units, right?

I built my Plasma Cutter with very high end parts. Will they die.. yes, eventually all parts die. So when they do I can just toddle off and get a similar part and adapt it in. Can you do that with boards? I hope their warranty is as painless as they state, but we all know that there are often little got-yas along lifes paths. Does anyone have 1st hand knowledge of the units and their warranty practices?

I do know a gentleman who bought one. It was an OK TIG unit, but liked to cut out after any lengthy runs. He tried the Cutter. Sheet steel cut great, but anything over .125 the cut tended to wander and there was no way to up the pressure. Sure you could add an external tap, but then there goes your 3 year warranty, huh?

They are all give and takes. Build your own, or buy it. Have fun either way!

KS

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PostPosted: October 25, 2011, 4:45 pm 
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I am with you Botbasher.
We are facing hard financial times, and DIY equipment that is very simple, robust, and easily fixed is not to be scorned.


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PostPosted: October 25, 2011, 4:52 pm 
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Warpspeed wrote:
This is all pretty fascinating stuff, and as an EE myself, the whole thing seems pretty doable.
But it does look potentially rather lethal the way it is, without any mains isolation........
Just as a reminder, I never stated that folks shouldn't use an iso on the mains. I simply stated that I didn't.

OK, not being adversarial... but from an EE point of perspective... Just what are you protecting from? There is no electrical path back to the mains, correct? I SAY THIS WITH ALL SERIOUSNESS... I tried everything I could, short of putting the clamp on my earlobe and tongue on the tip and could find nothing harmful (and I wouldn't suggest doing that with a store bought one either!!). Maybe I am not holding my tongue right when I tested it, but I found no frequency of any sort at any point in the system, save the 60Hz into the rectifier.

I have never experienced nor heard report of a rectifier diode failing in the conductive state, so I don't know how it'd conduct mains voltage and freq to the work. Hell, quite honestly, I am far more scared of the caps and the damage that they are capable of than getting hit by 208V! By the time it hits the caps, your already looking at near 300VDC! It makes a hell of a bang (louder than my 22-250!) when shorted at full charge!

Seriously.. tell me what your training says should be dangerous and how to test it and I will test it. I want to be able to prove (or disprove) the fears surrounding this topic!

Cheers!

KS

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PostPosted: October 25, 2011, 5:20 pm 
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botbasher wrote:

OK, not being adversarial... but from an EE point of perspective... Just what are you protecting from? There is no electrical path back to the mains, correct?
KS


Huh ?

Trust me, neither side of the output will be at safe ground potential with a bridge rectifier straight off the mains.

If you don't believe me, try connecting a 110v light bulb between your exposed metal work piece and a solidly grounded metal water pipe, and see what happens.

If it lights the bulb (and it will) it can also light YOU UP if you are grounded sufficiently well then touch whatever metal you are plasma cutting.

I will also add, that the US power system is unique in the entire world, in that either side is only 110 volts above ground.
In every other country, there is 230 volts between one side of the mains and ground, and that is far more lethal with this particular circuit.

So while you might just get a tingle in the US, it will be far more deadly for anyone outside the US building this circuit where the entire mains distribution system is all three phase referenced to ground.

In summary, in the US you will only get a 110 volt zap, even when connected across 220v.
Everywhere else you will get a 230 to 240volt zap with the same identical circuit, which is far more dangerous.


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PostPosted: October 25, 2011, 7:11 pm 
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In every other country, there is 230 volts between one side of the mains and ground, and that is far more lethal with this particular circuit.


There are just a few places on our globe where line voltage is not ground referenced. Many places are 230V but some are 110 or 120V. In places that have no ground reference, a circuit like this would be (relatively) safe. With the WWW being so easy to pass on designs and schematics, one must be carefull just where circuits such as these began their life. But as you suggest, "most" locations have ground referenced line voltages. Any ground referenced line voltages can be LETHAL!!!! Make no mistake about it. I have been bitten many times from line connected tv sets and audio amplifiers. I would NEVER build something like this without an isolation xfmr, a ground fault circuit interrupter, and proper fusing.

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PostPosted: October 25, 2011, 7:35 pm 
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Warpspeed wrote:
botbasher wrote:

OK, not being adversarial... but from an EE point of perspective... Just what are you protecting from? There is no electrical path back to the mains, correct?
KS


Huh ?

Trust me, neither side of the output will be at safe ground potential with a bridge rectifier straight off the mains.

If you don't believe me, try connecting a 110v light bulb between your exposed metal work piece and a solidly grounded metal water pipe, and see what happens.

If it lights the bulb (and it will) it can also light YOU UP if you are grounded sufficiently well then touch whatever metal you are plasma cutting.

I will also add, that the US power system is unique in the entire world, in that either side is only 110 volts above ground.
In every other country, there is 230 volts between one side of the mains and ground, and that is far more lethal with this particular circuit.

So while you might just get a tingle in the US, it will be far more deadly for anyone outside the US building this circuit where the entire mains distribution system is all three phase referenced to ground.

In summary, in the US you will only get a 110 volt zap, even when connected across 220v.
Everywhere else you will get a 230 to 240volt zap with the same identical circuit, which is far more dangerous.


ummmm...... you sure?

I exported GE motors for about 10 years to Germany and was sending units spec'd at 1740 rpm 240v60Hz/1530 rpm 230v50Hz. .. worked just fine except they cogged a little under heavy loads.
US voltage would, iirc, be better described as "split phase" ie 120v between either leg and ground with each leg 180 out of phase. .. giving 240v RMS between the pair of legs. Boy, that didn't come out the way I thought it would :shock: :lol:

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PostPosted: October 25, 2011, 7:42 pm 
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Haven't really "brained" the circuit through fully yet, but what I seem to be seeing is high amps at low volts and low amps at high volts, which is what seems reasonably safe.... unless you're gonna play with it in the bath of course.

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PostPosted: October 25, 2011, 8:42 pm 
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I has been a decade or more since I had to design a SELV (safety extra low voltage) power supply. I seem to remember something like 42.5 V peak as the maximum safe voltage for a human to accidentally contact. That would equate to 30Vac. Even that had to be isolated from the mains thru several layers of insulation/isolation. The current (amps) part of the equation is mainly for fire protection, it has nothing to do with human safety. I have been thru safety engineering classes and been intentionaly subjected to (safely) all kinds of voltage and current AC and DC. Believe me when I say that virtually NOBODY wants to feel more than about 3.5 mA. That is 3.5 thousandths of an amp. Micro amps (milionths of an amp) will kill you if applied directly across the heart.

These are not safe!

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PostPosted: October 25, 2011, 9:35 pm 
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oldejack wrote:

ummmm...... you sure?

US voltage would, iirc, be better described as "split phase" ie 120v between either leg and ground with each leg 180 out of phase. .. giving 240v RMS between the pair of legs. Boy, that didn't come out the way I thought it would :shock: :lol:


Yes I am absolutely certain Jack.
And you are correct about the US being split centre tapped two 180 degree phase, a VERY unusual system compared to the rest of the world.

And in every other country that I know of, there are three phases arranged 120 degrees, and each phase is voltage with respect to neutral that is nominally at ground potential.

Here in oZ every domestic house is supplied with either one, two, or three phases, and each phase is 240 volts to neutral which is also ground, and 415 volts between any two phases.
That is very different to the US system.

I have all three phases here at home, and am very familiar with the system.


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PostPosted: October 25, 2011, 9:57 pm 
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This describes it far better than I could. ..

http://en.wikipedia.org/wiki/Split-phase_electric_power

Quote:
A split-phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications. It is the AC equivalent of the original Edison 3-wire direct current system. Its primary advantage is that it saves conductor material over a single ended single phase system while only requiring single phase on the supply side of the distribution transformer.[1] Since there are two live conductors in the system, it is sometimes incorrectly referred to as "two-phase". The two live or "hot" conductors waveforms are offset by a half-cycle, or 180 degrees offset, when measured against the neutral wire. To avoid confusion with split-phase motor start applications, it is appropriate to call this power distribution system a 3-wire, single-phase, mid-point neutral system.
In North America, the high-leg delta system allows single-phase 120 V loads and 240 V three-phase loads both to be served by the same three-phase, four-wire distribution system.
...
A transformer supplying a 3-wire distribution system has a single-phase input (primary) winding. The output (secondary) winding is center-tapped and the center tap connected to a grounded neutral. This 3-wire system is common in countries with a standard phase-neutral voltage of 120 V. In this case, the transformer voltage is 120 V on either side of the center tap, giving 240 V between the two live conductors, shown as V1 and V2 in Fig. 1. The two outputs are properly called "legs", not "phases".

Attachment:
Split_phase2.png


In Australia and New Zealand, remote loads are connected to the grid using SWER (Single Wire Earth Return) transmission lines (it is cheaper to run one wire than two). The primary of the transformer is connected between the high voltage line and earth, the secondary is a 3-wire single-phase system as described here, the secondary voltage being 230/460 V. Single phase loads are split between the two circuits. Hot water services use both circuits.

In countries whose standard phase to neutral voltage is 120 V, lighting and small appliances are connected between a live wire and the neutral. Large appliances, such as cooking equipment, space heating, water pumps, clothes dryers, and air conditioners are connected across the two live conductors and operate at 240 V, requiring less current and smaller conductors than would be needed if the appliances were designed for 120 V operation.
No individual conductor will be at more than 120 V potential with respect to ground (earth), reducing the earth fault current when compared to a 240 V, 2-wire system that has one leg (the neutral) earthed.


It does explain the difficulties with importing into the US since things is 230v baseline.


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PostPosted: October 25, 2011, 10:07 pm 
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Jack, the SWER (single wire earth return) system is only used here in very remote areas.
A single steel wire, usually at 11Kv to ground can go tens or hundreds of miles and be a very very low cost system to install.

But city and suburbs in every Australian state, all use 240/415 volt three phase, available to every home and commercial business.


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PostPosted: October 25, 2011, 10:44 pm 
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Warpspeed wrote:
But city and suburbs in every Australian state, all use 240/415 volt three phase, available to every home and commercial business.


Wowsers!!

And to think that it was only last year that we upgraded our rental FROM 120v 50amp service. ..
I thought I was being a go getter putting in 200amp service (240v single phase) and you get to have 3 phase right there at home.
The strange thing is, they had soooo many questions about why I wanted 200 amp service with a special, extra expensive permit needed to go any higher.

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PostPosted: October 26, 2011, 12:14 am 
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oldejack wrote:
The strange thing is, they had soooo many questions about why I wanted 200 amp service with a special, extra expensive permit needed to go any higher.


Horticulture. "Tomato" plants.

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PostPosted: October 26, 2011, 8:22 am 
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SkinnyG wrote:
oldejack wrote:
The strange thing is, they had soooo many questions about why I wanted 200 amp service with a special, extra expensive permit needed to go any higher.


Horticulture. "Tomato" plants.


Gotta be. .. and I just wanted to make sure that the new hot tub kicking on wouldn't blow the mains if the dryer was on.
Funny thing is, the house had been upgraded to 50 amp from the original 30 amp service just after the Korean Conflict according to the permit stickers. .. but not 240v. :?

Warpspeed, THANK YOU!! Your explaining Oz power pointed out the issues I've been having with getting approval for a machine I'm trying to import from Italy in a way that makes sense to me.
The exporter does a good job with sales support and paperwork but. .. He's a salesman not an engineer, no wonder the NEMA 14-30 plug requirement caused confusion.
He was thinking of it as a 3 phase plug and I didn't comprehend the issue.
(If anyone's interested I should have commercial grade "dry steam" pressure washers, 116 psi/376f, available in another month or so. .. but that's another story)


Boy talk about a thread hijack. ... :mrgreen:

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PostPosted: October 27, 2011, 10:16 am 
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oldejack wrote:
Boy talk about a thread hijack. ... :mrgreen:
Ah.. no worries.. I've done it enough in other threads, this is only fair!

While reading the material on the differences on 230V and other countries (which is accurate as I deal with imported cooking equipment and know about how they incorporate neutrals into the power supply!) I think I may have hit upon why I may not be having the "issues" and unable to find the deadly portion of this experiment!

Just to say, I have, as part of my testing, clamped the work clamp to a fence pole sunk into the ground and used the plasma cutter on the pole. All OK. I have put the clamp in a puddle of water on the ground and hit the button. All OK. All of my tests have resulted in a total "0" in the special effects department! I think I finally hit on why I am not seeing the issues with voltages in places they shouldn't be.

Anyone care to guess what voltage I'm running this on? Until I read that post about the Neutral line, I totally forgot that when I was initially testing this at 208VAC, I was seeing voltages at the caps in excess of 480V! The caps are only rated to 400V, so I decided to run it off 120v as a test one time to see if the cap voltages changed and it worked perfectly. It cut fine and the tips even lasted longer. Cap voltages are in the high 280V now and since I was sourcing all my own voltages for controls, I didn't have to change anything else except the plug.

I totally forgot about the 120V and never mentioned at the time because I didn't want to confuse folks who were going to build it if they followed the plans. Sorry for clouding the waters a bit, but could that explain why I have never gotten as much as a tingle from anything except when the HF start bites me?

KS

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