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Plasma Cutter Starter

b.james

Member
I have a Cut50 cheap chinese plasma cutter .Mostly it works fine but lately the starter fails to ignite it . This happens on say the second start of cut and I leave it for 30 secs or so and it starts again then won't on the next one .

I have checked ,adjusted and played with the contact set spark gap inside the machine but it seems that as long as there is a spark across the contacts that does not affect the gun. Somrthing else is getting hot or not working properly like say a capacitor or component on the fly over transformer .

Does anyone know anything about the start circuit itself or what usually gives trouble . Thanks
 

Tony Stewart

Well-Known Member
Most Helpful Member
It takes a low current high ignition voltage to ionize the arc. If there is an initial spark but dies, then it is the higher power ,low voltage regulator or driver
 

tcmtech

Banned
Most Helpful Member
Your description is hard to follow.

So you have the ignitor spark gap firing but you don't get the pilot arc at the gun or the whole ignition circuit stops powering up the second time you trigger it?
 

Tony Stewart

Well-Known Member
Most Helpful Member
plasma to be sustained always needs igniter then power following the spark.

This is why low voltage can make a wide arc between electrodes, but only after igniter

first high voltage 3kV/mm then anything above holding current limited by supply and regulator.

so first high V
then high current.
 

b.james

Member
Hu Hu so why would it fire up the first time OK and cut then refuse to start a second time until something cools then fire up again normally . What might the component be that causes that? I have watched the spark arc at those times and it is always there.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
I worked on an arc lamp that used a spark gap type of ignitor. The part wears and has to be replaced. It looked like a small cylindical ceramic thing that flashes as part of the oscillator circuit.
 

tcmtech

Banned
Most Helpful Member
Hu Hu so why would it fire up the first time OK and cut then refuse to start a second time until something cools then fire up again normally . What might the component be that causes that? I have watched the spark arc at those times and it is always there.
Without pictures to go by your written description leaves us with near nothing of value to work with. :facepalm:
 

MaxHeadRoom78

Well-Known Member
Could be a few reasons for no pilot arc, being Chinese origin, is the value of the HF capacitor of the right HV quality?
Is the spark gap, gapped correctly?
The circuit itself is usually very simple.
Max.
 

tcmtech

Banned
Most Helpful Member
Some years ago Thermal Dynamics came out with a 'new and improved' arc start system on their plasma cutters that replaced the open tungsten electrode spark gap with a small gas discharge tube, varistor of sorts, that turned into a real problem with their machines that caused a very similar problem. The discharge tube would fire fine cold but after a few starts back to back it would act up and not work right.

They came up with a brilliant fix though. They changed the part number on the circuit boards and rain the price for them. Really, They did! I went around with the TD sales rep one time while he was visiting the store I worked at and I got after him big time when he told me they fixed the bad board issue and had came out with a new board. He even gave me the new board part number so that even an idiot service tech like me wouldn'tnt get a bad one.

I then went back in my service bay and brought out one of the old boards and one of the brand new ones and laid them side by side and asked him what the change was being they were identical right down the the circuit traces and individual component part numbers as well. I didn't get any definitive answers after that regarding what was changed since he too could not tell a old from a new other than by the part number embossed on the circuit boards. It was a new part number and cost more so it clearly had to solve the problem. :( :facepalm:

To end it I told him I had a fix for it that worked well. I pushed our customers to by the Hypertherm brand machines since their engineers apparently knew how to design machines right the first time being I rarely ever saw one come back for warranty work. :D
 

Tony Stewart

Well-Known Member
Most Helpful Member
maybe just ultralow ESR caps and low DCR air coil with high Q with hi dV/dt. Furnace
igniters use many turn air core to generate a 30kV arc for gas furnaces good for 30mm min. in a very simple cct.
 

b.james

Member
Thanks Max I can at least work with that and check/change a capacitor or two . Not sure any of the other is helpful as I don't have the circuit.
maybe just ultralow ESR caps and low DCR air coil with high Q with hi dV/dt. Furnace igniters use many turn air core to generate a 30kV arc for gas furnaces good for 30mm min. in a very simple cct.
Perhaps you could break that down a bit . So what do you think is happening?

What should this simple circuit look like please?
 
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b.james

Member
Well here I am in January 2018 with that machine still sitting on my bench and that starter circuit still not working . Been right over it and can find nothing wrong but despite sparks at the contact air gap no spark gets to the torch end. If I touch the open end of the torch without tip to ground the main power boost sparks so it is ready to go , just needs that initial spark to get it up and going .

Any people come across this now ? Figured there would be a lot more real experience out there now not just armchair oldies spouting.
 

tcmtech

Banned
Most Helpful Member
Any people come across this now ? Figured there would be a lot more real experience out there now not just armchair oldies spouting.
Nothing of value given, nothing of value returned. (As I stated in post 7.) :rolleyes:

Its cheap, it's chinese and nobody here owns one to use as a reference point to go off of so what exactly are you expecting? :confused:

Something greater than undefinative word descriptions has to come from your end before things will move forward.

Pictures, schematics that sort of stuff.
 

dr pepper

Well-Known Member
Most Helpful Member
The circuit probably uses a marconi oscillator, the simplest likely cause would be the HF being lost in the weld cables or torch assy, have you checked them?
Theres also usually a HF coupling transformer that couples the HF into the weld current output, it could be flashing over, then theres the resonant caps in parallel with the HF transformer primary, one or more could be leaky worsening when hot.
If the spark gap flashes over theres not a lot more to check than this I can think of, can you borrow another torch to try?
 

OlPhart

Member
Been a while, but... if this is true:
"despite sparks at the contact air gap no spark gets to the torch end"
Then wouldn't the HV be fine, but something's amiss in the "whip" (cable)?
Wouldn't be first time a bottom cost object reveals a quality issue.
I'm inferring this .. OP isn't being very analytic on isolating the issue.
The combination of low voltage / high current with high voltage / low current challenges the design & quality of the whip.
Dr Pepper is Spot On... Good Hunting <<<)))
 

dr pepper

Well-Known Member
Most Helpful Member
Srating voltage is high voltage and high frequency, the HV we know works, the HF not, Hf comes from an oscillator which in the cut 40 comprises of just 3 parts, a cap, a spark gap and a transformer, we know the gap is Ok and thechances are the others are Ok, so the HF is likely to be damped by the cable or crud in the unit.
Actually I just remembered, to complete the circuit for the HF theres usually a cap across the output of the weld set, this is worth looking at too as it gets a bit of stress.
A HF start box appeared on this forum not so long back, post #21 shows one I made for a mobile tig weld set. https://www.electro-tech-online.com/threads/tig-hf-coupling-transformer.149802/page-2
 
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tcmtech

Banned
Most Helpful Member
On a lot of plasma cutters they use a pilot return circuit that draws a amp or two off the main high current supply to produce the hot strike over flame that shoots out of the gun. If that's not working properly then the gun won't strike a sustainable arc inside the gun but it will strike one between the exposed electrode and the work if the nozzle cup is removed and any safety interlocks bypassed.

My guess would be that either that return circuit is broken some place or the control circuit that limits the strike over flame current and connects it to the ground internally is bad somehow.
 

b.james

Member
OK thanks . There is a lot more to go on there .I bought a new unit cause I had to have one working . Better to buy cheap and replace when broken than buy one at 10 times the price with most of it made in china anyway.

Swapped the output unit complete and still no change. Its in the unit so thats why I seek a circuit . Have checked most of that over many times, and at night, looking for arcing but will press on. The parallel caps are next to replace. Your material is very helpful dr pepper but the link did not work and I would like to read that
http://www.casano.com/projects/hfstart/
 
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