Voltage divider schematics for HF-start pilot arc machines

[alangibson]

Hello all,

I've been looking around for voltage divider schematics known to work with cheapo HF-start pilot arc machines. So far I haven't come up with anything. Does anyone have an image or link they can post?

I'm not looking to support any particular cutter specification at the moment. I'm mostly interested in the type of circuit they typically use. Low pass filters and transient voltage suppression diodes seem like options, but I'd like to know what the industry prefers before really getting into it.

Thanks!

[acourtjester]

Have you tried the search function here (upper right corner) I think that has been posted here in the past.
May not be what your are looking for but here is a guy who developed a stand alone THC using an Arduino.
https://openbuilds.com/builds/standalon ... ting.9571/

[alangibson]

Thanks for the reply. I tried a few different searches but didn't come up with anything. I have a feeling that I saw a voltage divider plan on here once but I'm not finding it.

In that project you linked, he assumes that the plasma cutter already has a 50:1 voltage divider. That's why he's able to run it directly to the Arduino's analog pin. That voltage divider in the plasma cutter is basically what I'm trying to find a schematic for. There are quite a few aftermarket ones that support external connection to an HF-start cutter. If I can't find a schematic, I might just reverse engineer one of those. For example:

https://www.poscope.com/wp-content/uplo ... smaDiv.jpg

[acourtjester]

You are correct I overlooked the connection to the hypertherm with it's internal divider. I did see the one you posted, many seller of a similar one ($45 to $75 each). I did not post those as you are looking to build your own. If you do find one please post here to help other who may be doing the same thing.

[alangibson]

I've found one questionable schematic using an op-amp filter in this thread:

https://www.everlastgenerators.com/foru ... THC-Pass-2

He mentions in the thread that there are problems with the drawing. An op-amp filter also strikes me as overly complex. I might build the circuit in a simulator and see what happens.

[alangibson]

I never found an open source design. This seems like something that should really exist, so I'm going to make one.

Right now I'm sketching out a couple of boards that'll 1) read the voltage of a plasma cutter arc then safely and accurately transfer that reading 2) to a decoder attached to a Raspberry Pi 4 housed elsewhere. I would appreciate some constructive criticism on my approach below before I get any deeper into this design.

My plan is to have a board close to the plasma cutter power supply that taps into the leads and reduces the voltage 50:1. A VFC on this board will then transmit the voltage reading over a shielded cable to another board housed in the control cabinet. This second board contains a FVC that decodes the voltage and sends it to a Raspberry Pi 4 via an ADC. A LinuxCNC component then multiplies the voltage read by 50 and sends it on to the PlasmaC component.

So The signal path is: Plasma power supply -> Voltage divider -> Low-pass filter -> VFC -> optoisolator -> shielded cable -> FVC -> ADC -> Raspberry Pi 4

Power path is: 24VDC supply -> shielded cable -> isolated dc/dc converter (24V->24V) -> VFC

Important requirements:
- Galvanic isolation between the plasma cutter and everything else.
- Accurate transfer over 2 meter shielded cable.
- Survives high frequency arc start plasma cutters
- Practical to make by hand (i.e. no SMD components)

Notes:
- Isolated DC/DC Converter is there for galvanic isolation only.
- V1 and 0 are for Spice simulation only.
- Low-pass filter is there to remove interference from the high frequency arc start.
- HF arc start is assumed to be max 5kV at 2 MHz (based on public statement from Jim Colt).
- VFC and FVC schematic is taken from the LM331 datasheet.
- ADC will be connected to a MCP3002 which is in turn connected to a Raspberry Pi 4 via SPI. I've left this part out for the sake of brevity.
- Opto-isolation will be added to the VFC for galvanic isolation. Haven't added that to the schematic yet.

Rough schematic is attached.

[weldguy]

Very interesting and I will be following this topic to see your progress on this. I hope it works great for you.

[alangibson]

Thanks weldguy.

I did a smoke test of the "voltage divider" stage. It didn't self destruct and the output voltage is in the expected range, so that's encouraging.

Parts for the rest of the attached schematic will be here next week. I still need to figure out the passive component values for the LM331. The data sheet is very detailed which should help a lot.

[robertspark]

remember the torch is negative (-) with respect to the work which is positive (+)

electrons flow from - to +

[robertspark]

instead of using a 100k plus 2k resistor is series, I would recommend that you increase that impedance and split the 100k up into a number of resistors.

250v / 102k = 0.00245098039 A

i^2xR = power

0.00245098039 ^2 X 100000 = 0.6 W

5000v / 102,000 = 0.04901960784 A

0.04901960784 ^2 X 100000 = 240.292195309 W.... for as long as the hf start is enabled..... good luck with that resistor

[robertspark]

what you are trying to replicate is a board called a thcad by Mesa

https://store.mesanet.com/index.php?rou ... path=83_89

have a read of the manuals as they may give you some ideas.

they are used a lot with linuxcnc and I have a couple of the boards

[robertspark]

the hf can get up to 25000v too.... at low current ... but high voltage

this may help you too suggest searching thcad hf

https://forum.linuxcnc.org/27-driver-bo ... sma-cutter