Cheap THC

for the Cut 40




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November 2016 was the beginning of the start process for my cheap THC.  Are you getting the trend here, Cheap plasma cutter, CNC on the cheap etc.  Again a lot of quips about wasting my time, should be buying this, that or the other.  I had been looking at voltage comparators on the Internet and settled on a 4 Channel Voltage comparator found on Ebay, it came in at less than £10 (UKP).  So I ordered two.

Here is a picture of the board I decided to use:

4chancomp.jpg


It provides 4 channels, has positive and negative inputs, works up to 32 Volts and take 12 Volts DC power supply input.  Watch out you do not get the 5 Volt units as they seem to be made to be only to monitor up to 5 volts.  The downside is these uses relays, so may be a bit slow on switching. (actually found the data sheet on the relays and they act in a maximum time of 10 mS). But as I believe Mach3 monitors the THC inputs every 100mS, (hard to find any true information on this), then relays seem to be good enough until they fail.


At around the same time a capacitor on the Plasma Cutter went short, took some track out and stopped the unit from working, at this time the plasma cutter was well out of warranty, so I stripped it down for the repair and planned where I could  take the arc cutting voltage from:

PlasmaPSU.jpg

Some circuits I drew up in a free online program:

circuit.jpg
C1 is 3 KV the two 2.2nF typically 400 Volt

circuit2.jpg
The capacitor shown is 35 Volts working for a max of 30 volts input, dead zone around 3.9KOhm

The plasma was repaired and a resistive divider wired into the cut 40.  The board was wired up and placed into a plastic box to keep it and me safe.
Then on to testing it out and another you tube video.



I reworked a shroud to go around the drag tip to allow for my pilot arc and get air through for cooling and removing dross from the workpiece and we have anoher video.



It was now December 2016.

By mid January 2017 I had upgraded my Cheap THC by fitting a Cheap Voltage meter from Ebay (there is that word again) and a multi turn potentiometer so I had more control over the unit.



By the end of February I had updated the frame by added a shallow water tray.  ( Do water and plasma really go together ?)  But it helps to keep the dust levels down, stop thinner metal from distorting as much, but could cause a few issues.



Water does get sprayed/splashed up by the air jet and can come back at the head, try to keep the water a couple of inches below the metal being cut, I do not have this option!

May 2017 and the Cut40 has once again failed.  Flames from the PSU board, another small capacitor had failed same as last time, but this time a hole in the board.
Time to strip down and repair again, I did not take a video of the process but made something up from still images.




I got the unit working again and cut out my peacock planter.  I was asked in comments if I could provide more information on wiring up the THC, so I produced 3 videos on the subject, all very basic as all my videos are, the camaera gear is basic, but hope they help.











October & September 2017

I have had a few requests to see if there is any way not to have to open up a plasma machine to sense the arc voltage,  I believe this was to solve a few issues:

  • I have done this based on the Cut 40.
  • It means invalidating any warranty, so if it does go pop the supplier would know the cutter had been modified.
  • May be unsure about soldering onto the main boards in the plasma if different.
  • Other reasons, many I guess.
The concern I have is as follows, even with a cutter with pilot arc, there is a very good chance it still uses HF start to get the arc going.  This pilot arc is virtually the last thing that is injected on to the torch lead prior to exiting the machine.  Therefore once this high voltage (enough volts to jump several mm) high frequency  waveform is on the torch leads it is very difficult to protect against it.  Hence why I fitted the resistive divider to  the output of the inverter stage and prior to the HF connection.

BUT, I thought I would have a go at seeing if it could be done.

Here is a simple circuit I am trying.

Res Dev mk2.jpg

Again sorry for the diagram, but I do not have any proper design software, so gets done in my head and then what ever program I can draw it with.

Looking at the circuit you will notice it is very similar to the original resistive divider, plus the addition of 4 coils and two caps.
The extra coils are larger then used before and can take a current of a few amps for a few seconds, the 10nF caps are around 3000 Volts working and the 2.2 caps around 400 to 1000 Volts working, if all is OK they should only have a low voltage on them.

The input should in my mind stop the high voltage HF waveform from getting to the resistor divider with it all being across the 4 coils.

Here is the board

Extrevdiv 1.jpg

Extrevdiv 2.jpg

The board has been sprayed with lacquer, this done in the hope to stop arcing across tracks on the board, time will tell.  The wires will be, Red to work clamp connector, White to plasma connector and the brown to the Chassis of the Plasma cutter, probably under one of the screws.

Here are two videos that  go with the above board.







Cheers



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