Rods "Spaceship" Scratch built Plasma Cutter build

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17 Apr 2019 00:13 #131021 by islander261
Rod

I think your neighbors machine uses HV/HF start like most industrial machines so yes he should have the local ground rod for the table. You may not need one depending on the amount of moisture in your slab and how well the rebar was bonded when it was poured. I don't need one because the rebar in my slab is welded together for good bonding and then has it's own stranded #4AWG copper wire connection to the buildings ground rods.

John

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17 Apr 2019 08:36 - 17 Apr 2019 08:37 #131047 by rodw
Well, I didn't get as much time today as I had hoped but I did get the Meanwell isolated 24 volt power supply (with the blue light) fitted so that mains power to it is dropped when estop is pushed.



The blue solid state relay is just sitting there for now but it is triggered when the torch shield touches the work piece. I've got two of these relays (for the Aussies, I sourced them and the power supplies from Ocean Controls).

I rigged this up on the table in a temporary way and hooked a 9 volt battery across the relay load terminals and was able to confirm everything worked as expected.

I figured that I need a couple of IN4007 1000 volt diodes to prevent the torch voltage being introduced into or circuit so I found a couple of terminal strips and soldered them on to a scrap of vero board. Then I remembered i had a current sensing board at home with a DIN rail mount so before I cut the vero board down, I decided to see if I could mount it in the DIN rail mount. five minutes later, I had it all fitted up nice.



The relays don't have any LED indicators on them, so I think I might add a bit more circuitry to this board for some diagnostics.

This looks so neat, I gotta do the same with the other dodgy circuit board that drives my LED power indicator lights. At the beginning of my build, a friend told me to add these for each power supply in the system for error checking and they saved a lot of time today. I inadvertently disconnected mains power the 48 volt power supply driving the steppers. I couldn't work out why I had problems until I check my power diagnostics LEDs and I instantly knew what I had stuffed up!

Hopefully, I'll have most stuff mounted up and working tomorrow.
Last edit: 17 Apr 2019 08:37 by rodw.

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18 Apr 2019 07:30 - 18 Apr 2019 07:31 #131114 by rodw
Well I nearly have this done. Its taken a while which might be why I've been putting it off for ages.

So I've now got the power supply and the relays mounted up


I had a few issues and had some strange voltages floating around the table frame until I consulted my friend Chris who knows about these things so we just earthed the negative power rail of the isolated ohmic power supply to one of the copper posts inside the control panel and relied on contact between the table frame and the control panel where its bolted on.

Then I realised that the solid state relays I purchased were designed to switch AC only so when they triggered, they latched on as there was no zero crossing to turn them off. Grrr. I ordered some more but the company was out of stock so fell back to good old fashioned mechanical relays I had lying around!



So the left hand relay is connected to an output and it switches power from the isolated power supply to the ohmic sensor. When the torch touches the plate, it will trigger the right hand relay which will switch field power to an input for ohmic sensing.

So in practice, we enable an output to say we are probing which enables the ohmic sensing circuit. Then when the torch touches the plate, the signal will reach the ohmic input.

So with a bit of tidying up of the wiring, plasmac here we come!

But before we do, can someone educate me about flyback diodes/? I've heard I should use them to protect the Mesa inputs and outputs but not quite sure what I need to do.
Last edit: 18 Apr 2019 07:31 by rodw.

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18 Apr 2019 08:00 #131119 by tommylight

rodw wrote: Then I realised that the solid state relays I purchased were designed to switch AC only so when they triggered, they latched on as there was no zero crossing to turn them off. Grrr.


Yea SSR have that nasty habit of remaining full on while DC current is passing through them ! :)
Been putting of due to time constraints doing something with some big SCR ( 2 pieces of double 105A ), and lack of a decent idea of what to do with them! Thought of making a plasma power source but had 4 of them, now i am left with only 2 broken ones maybe it is time to recall that idea.

rodw wrote: But before we do, can someone educate me about flyback diodes/? I've heard I should use them to protect the Mesa inputs and outputs but not quite sure what I need to do.

Flyback diodes can not fly ! I have no clue why they are called that, but they are used to suppress any back EMF induced by switching inductive loads, and also protect against reverse voltage generated by motors.
You just need to connect a diode ( made out of anode and cathode ) parallel to the relay coil. That is all there is to it, just mind the polarity, the diode side with the round mark goes to positive of the controlling output.
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18 Apr 2019 08:44 #131126 by rodw
Thanks Tommy. I just had dinner and remembered I left a soldering iron on up at the shop so no red wine for me tonight. I better go and turn it off! :(

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18 Apr 2019 08:54 #131132 by pl7i92
so i think i need to replace regulary the cable
the tables 4 Plasmas in shop are running almost the whole workday
so it is a matter of time to get them broken
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19 Apr 2019 16:09 #131243 by tommylight

rodw wrote: ........ and remembered I left a soldering iron on up at the shop so no red wine for me tonight. I better go and turn it off! :(

Done that many, many times ! :)
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