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
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 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
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
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
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
Time to strip down and repair again, I did not take a
video of the process but made something up from still
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 camera gear is basic, but hope they
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
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 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
Here is a simple circuit I am trying.
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 less than
0.1 to 1 mA and the only current the divider really
needs to deal with is when there is a cutting voltage
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
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
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