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I wouldn't use the 50:1 voltage divider. a THCAD5 with a 5 volt full scale would be more suited to use with it. Instead just calibrate the THCAD to read 200 to 300 volts full scale with a resistor.

I have a Mesa 7i96s card with a THCAD - 10. Do I need a voltage divider for this? I purchased two resistors - 2.4M OHM 1/2W 1% AXIA ( digikey ) as well as a 50:1 voltage divider ( CNCpc ). Do I need both? 

it's been a good little tool for odds and ends. i have mostly used it for guards and covers. i had a thought about thread inserts and have seen the brass ones you heat and press into your parts but thought you might be able to infill around your holes and run a tap into it then use a Heli coil insert. the cheap one's work just as good as the Heli coil brand. i thought it might be a little more cost effective than the ones you press in with heat.

i might try this out on a housing for the THCAD so I can mount it to the plasma cutter when i get that far

HelloI have a plasma cutter runing on mesa 7i76e / thcad10 / linuxcnc 2.8.My Problem:As you can see on the attached picture (left cut) it starts really good but after about 100mm the THC starts to move up and down the Z Axis. The move distance of the Z Axis gets more and more up to 10mm at the end. Is there an another setting as in the Config Penel Arc - Height Per Volt? (currently set to 0.05mm)So I really don't know how to fix this and which settings could be responsible for this behaveHope that you know the reason for that and I would really appreciate a hint how to fix that.Thanks in advance for you replay.The settings:Config Panel THCDelay:1Threshold(V):1PID P GAIN(Speed):10VAD Threshold(%):90Void Sense Override(%):100PID I Gain: 0PID D Gain:0Run Panel THCAutoUse Auto Volts: (ticked)Velovity Anti Sive Enable (VAD): (ticked)Void Sense Enable: (ticked) 

Id like to reduce the amount of slag I'm getting on tight corners, I've searched the forum for settings on the THCAD2 and the 7i96s and really haven't found anything related to it. longer cuts and or curves come out perfect its just tight cutbacks it leaves a pile of slag. I'm cutting 12ga Steel and reduced my speed from 120 to 100 for the 12ga settings. So I'm not sure if I'm getting the height control locking at at the right time or its something else. my air quality is consistent and I run a refrigerated drier so I know my air is clean. I did change the Post processor to one of the Updated ones here called Plasmac local materials for setting the material from plasmac instead of sheetcam. it improved quite a few items during cuts. Maybe my motors and drivers aren't tuned correctly? open to anything to reduce the amount of cleanup I have to do after the cuts lol. and i might have left out some speciffics needed so ask away.

So, after the success of my first plasma CNC, I thought it time to move her on to a new home, and build a new one. Bigger, faster, more powah!!! 

Im starting with a MESA 7i96 with 2x THCAD10 for volts and ohmic sensing. Closed loop stepper motors will be Nema 34 running 60v. Reduction gearboxes onto helical gears. The z axis will bolt the hypertherm xp45 machine torch and an air scribe. I’ll use an air actuator to drive the scribe to the required height. Everything is running on aluminium extrusion and linear rails. I have prefer up a water bed that is 1500x3000. Giving me adequate space to load up a full sheet. 

I’ll get some pics and links to the parts used as I go along. 

lots of new things for me to learn with this build so no doubt I’ll be calling on the brains trust for guidance again and to help me when I get myself stuck. 

cheers

the cheap drives i got are only rated for 42V so i am looking to run them on 36v, this is going to be on the cheap side for builds. i don't do things as pictured by others i just use it for a starting point for planning. i plan on using double shielded wires everywhere with shield drains on everything. i have spent a lot of time reading up on grounding and a lot of it was plasma related for a source. i have seen way too many builds get sideways because of grounding issues

the Nema 23 motors i picked up and dives were only $110 for 4 so nothing very fancy. THCAD is about the cheapest height control on the market, so being able to do that makes me happy

i'm not ready to buy boards from Mesa just yet and the THCAD-300 is out of stock at the moment, and it is a little bit cheaper than the THCAD-2 so with added cost of having to buy extra stuff with shipping on top of that saving $20-$30 makes me lean towards the THCAD-300 just because i am cheap

I would stick to the thcad-2 with a scaling resistor. I can't really see a use case for the thcad-300
Please use a seperate 48v 7-10 amp power supply for motors.  24 v is waaay underpowered
Meanwell MDM-60-24 and  MDM-10-5 would be better choices than the cage power supplies shown.
All motors should have a seperate ground wire back to the star ground so emi does not attempt to pass across linear bearings. Hyrperthem have a good grounding document. Follow it.
I get frustrated when I see suboptimal designs accepted as gospel just because they have used pretty pictures but its a good start
Grab a couple of Mesa COMMX2 bus bars when you get your 7i96s. Also get some of their Din rail mounts.

Also just for me, use a multimeter and check if there is resistance between the torch tip and the work clamp. Most plasmas add a 100k resistor or similar to bring the current on the raw output down to non lethal levels. If so, you can mount the thcad in the control box if you are lazy.

machinedude wrote:

my understanding so far was the THCAD-2 can be used but you have to add the HV resistor for ir to work and the THCAD-300 does not need the external resistor in my application;

That is correct.

my understanding so far was the THCAD-2 can be used but you have to add the HV resistor for ir to work and the THCAD-300 does not need the external resistor in my application;

Means not for use with HF/HV pilot arcs.
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NOT advice:
The thing is it can be used but requires open the plasma source and wiring it at the rectifier output. Any place else risks letting out the magic smoke. I have THCAD 10 and 300 wired to HF start, probably over 10 or 15 of them.

"The 300V version of the THCAD has all input divider resistor on board and can connect directly to the plasma voltage of touch start type torches only."

this was the section of the description on the Mesa site that confused me.

machinedude wrote:

the only thing i noticed was that the THCAD-300 seems to be for only scratch start HF machines?

Scratch start has no HF nor HV pilot arc, it needs to be dragged on material to fire.
THCAD 300 would work fine for your plasma (i use them on HF/HV start despite warning all not to do that, it just needs to be wired in the right spot), and so will any THCAD, just get 2 of 1MOhm resistors rated at 2W or 5W and wire one on each arc voltage wire.

Rod  i did some digging and found out that this Machine has been done with THCAD before. the person that did this has some YouTube videos and i found this image on here. this is basically the exact same setup i was going to do minus the THCAD since i was not sure on that part; I watched the videos and in the video he said the THCAD-300 would have been a better choice with this machine. the only thing i noticed was that the THCAD-300 seems to be for only scratch start HF machines? this machine has a 1:1 ratio but has the blow back start so which would be better the THCAD-2 or the HTCAD-300 ?
 

Also, don't forget spikes on piercing will be much higher than cutting. Even at 40 amps, it is enough to exceed 300 volts when cut volts is around 120 volts. Not sure how much higher because that was the max voltage my thcad was calibrated to. Also when crossing a void, when a slug drops when cutting a hole, running off a sheet when severing all cause huge peaks. A thcad used for ohmic sensing Is exposed to the full torch voltageeven if calibrated fro 24volt full scale.