A Cheap THC

for the Cut 40

Index  Workshop  Kit-car-build  Tube-bender  Projects  CNC-Plasma  Cheap-THC  Support

November 2016 was the beginning of the start process for my cheap Torch height Control or 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:


It provides 4 channels, has positive and negative inputs, works up to 32 Volts and takes a 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, these use 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).  Mach3 can monitor the THC input quite fast at around 25,000 times a second.

At around the same time a capacitor on the Plasma Cutter went short, took some PCB 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:


Some circuits I drew up in a free online program:

C1 is 3 KV the two 2.2nF typically 400 Volt

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, and the camera 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.
  • Maybe 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 HF 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

Thanks to Asaf Raz for bringing it to my attention I originally had nH instead of uH for the four coils, the above is now corrected.

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.

A point on why I used these coils, it was not for current carrying capacity, but simply because of the pin spacing.  I did not want any possibility of the HF start jumping across them.

The HF start has very little current, probably in the region of 0.1 to 1 mA and the only current the divider really needs to deal with is when there is a cutting voltage there.
Say:  Open circuit voltage is 300 Volts, 30 K Ohm so the current is 10 mA.  When cutting and the current drops to 100 Volts we are talking around 3.3 mA.

In theory if you can insulate the terminals enough you could use the same type of coils as I use on the output and could go to 1 mH, 2.2 mH etc. 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.

Not everything goes to plan!  I thought I would pick up an old plasma cutter, parts or repair, same type if possible from Ebay that I could get going and press into service as a spare unit, for the just in case times.

Well I managed to win a Cobel Cut 40 for £10, listed as intermittent, it cost a little bit more for the postage to my place but still reasonable price.  I found a few dry joints, replaced the relays on the lower board and adjusted the spark gap and generally cleaned it up.  Fitted my original hand torch and clamp and it worked.

Then it was a case of fitting the internal Resistive divider and  seeing if i could get it to work as a CNC replacement.

So the last thing was to try again and then go back to my original Cut 40 to make sure all was still OK.

It could probably work if I got around to using all different THC settings but for now, I have a hand held unit.

End of February 2018 and another modification to the THC unit.  I have added a couple of metering points for the divided incoming arc volts, this allows me to monitor the voltages with a digital voltmeter.  I am also trying to cut 8 mm steel for the first time!

Be safe.

Contact me via email