Converting a THCAD-300 to a THCAD-10?

robertspark wrote:

PCW wrote: The 10 through-hole resistors on the THCAD-10 and THCAD-5
are 10K .1% . The 10 through hole resistors on a THCAD-300
are 300K 1% HVR (3500v rated)

The THCAD-5 input current to voltage converter feedback resistor is 49.9k
0.1% The THCAD-10 and THCAD-300 feedback resistor is 24.9K 0.1%

may I ask which is the current to voltage converter feedback resistor?

thanks

R14

May I know what was wrong with THCAD-300 so you decided to convert it? Just curious.

There was nothing wrong with my THCAD-300, I just switched to a divided voltage source from the new plasma cutter instead of the raw voltage from the older plasma source. By changing the input voltage for the THCAD I would get better resolution with my divided voltage source signal.

I was asking as I was wondering what is the practical resolution. Everyone talks about better resolution, but do we practically need this "better" resolution? From mesa website: "10 bits at a 1 KHz sample rate and 12 bits at a 250 Hz sample rate"
is 250 Hz update slow? is 10-bits @ 1 kHz not enough? Heard somewhere that 0.01" THC control accuracy today is pretty good and pairing it with other info that 1 V arc voltage change relates to ca 0.004" (varies but can be used as rule) gives 2.5V arc voltage change. 300 V range @ 10-bit gives 300mV resolution. If 250 Hz is enough then 12-bit gives 73 mV.

I know 0.1 nV is better, but do we need it?
It's just because i have two plasma sources: Powermax 45XP and CUT 70P from Stahlwerk. I'm not going to buy CPC kit for 45XP so THCAD-300 will be more universal. I don't know if my 45XP has any divider installed inside. I don't know if CUT 70P has divider as well. I'm away so have no access to machines to open them and check internals. But I have now option to buy THCAD cheaper (shipping to US instead of to europe to avoid unnecessary and significant costs).

My graph in the plasma primer in the linuxcnc docs is the only real data I have seen about volts over distance.
linuxcnc.org/docs/stable/html/plasma/plasma-cnc-primer.html
It showed for the specific machine, it was 7.53 volts per mm with 99.4% confidence over 16000 readings. So a 2 volt range is a good target to get your 0.01" which is realistic
You voltage will be around 130-150 volts when cutting. I have run a 300 v full scale and also 170 to 200v an I never saw any difference. I would just use your thcad 300, seeing you have it, set it to 1/32 frequency and make some parts.

Thank you rodw.

I don't have THCAD yet and i just saw on Mesa website only THCAD-2 is available:( The rest is out of stock - looks like i'll have to go with THCAD-2 anyway and deal with voltage dividing somehow. My Hypertherm 45 XP is not equipped with CPC apparently, the second source as well. Buying CPC now is not and option so i will need to look at THCAD-2 if i can add some resistors to extend the range.