THCAD Environmental Fluctuations

my 2 pence / cents...

Check you do not have multiple earth / ground connection points at both ends of the cable.

This is just my opinion (former electrician / consulting engineer).

Suggest connecting the THCAD "Shield" connection to the cable shield, (directly) and in turn connect to the ground pin on the CPC at the plasma cutter.

connect the THCAD (+) to the plasma cutter (+) terminal at the CPC at the plasma cutter

connect the THCAD (-) to the plasma cutter (-) terminal at the CPC at the plasma cutter

CPC = circular plastic connector {CNC interface}

if you have a THCAD300, then (+) to plasma cutter clamp and (-) to plasma cutter torch (cutting current, not start current!!!) connection.

I would NOT suggest connecting the shield to your cabinet "GND" or EARTH coonection point, and then sub feeding it somewhere else.

If you look at the circuit of the THCAD it has an RC (resistor capacitor) filter arrangment between (-) and (shield) and (+) and (shield) connections.

If you are using the THCAD for hypersensing then... connect the shield to the cable shield but DON'T terminate it at the other end.

The question is where you take the (+) and (-) connections to....

for the THCAD with a Plasma Cutting Voltage Sense application, I have bought (awaiting delivery) of some 2-core shielded cable so that the 2-cores voltage differential are given as much chance as possilble to prevent noise (I am using a ~50:1 divided input (100k + 2K) hypertherm 45 (non XP) std) fed into a THCAD5

thee shield will "hopefully" filter out all potential external noise to this very small differential voltage signal.

(I also have available a 10:1 and raw output too from previous THC devices, but want to use what hypertherm gave it as the others have RCL filter arrangements built in).

I am not going to use hypersensing (yet) as I have an adjustable ohmic device that works fine and I dont have space for a water table.

But if I was.... I would probably either get some 1 core shielded cable, or use the 2-core shielded calble and join the 2 cores together to form a single cable with a shield and route each one to the ohmic sense point and the table material clamp. I would terminate each cables shield core AT ONE END ONLY, to the THCAD and use a piece of heat shrink over the other end so that there was no chance of it connecting to the table or some other earth / ground point. That way the shield would work as intended to filter out any potential induced voltage via the THCAD (-) <> shield and (+) <> shield RC connection network.

Just a suggestion .... each to their own... if it works for you it works.... if not, try something else. (Thomas Edison didn't get the light bulb design right first time.... but then again he did end up with > 1000 patents .... I like Joseph Whitworths ingenuity more though [ hexagonal bullet].

tea break over, back on my head!
www.sjacob.org/home/humour/BackOnYourHeads.html

interesting, not used that cable before (didn't even know it existed):

cpc.farnell.com/concordia-technologies/5...Ea0eT4BoCr9cQAvD_BwE

Thanks guys, What I did was really something quite artificial on a test bench without any need for shielded cable but it showed similar behaviour to Stefan's wonky cuts so I shared it. I think the correct approach here in the absence of any shielded cables to the outside world that need terminating is to leave the THCAD ground unconnected.

The behaviour I observed is not present on my table where I have paid careful attention to noise management.

tommylight wrote:

Not sure if i mentioned this, but Stefan should monitor the situation in his shop and check what equipment is being used when the issue happens, the ground interference can be caused even by your neighbours with faulty equipment, but since it is in Australia i am assuming desert, no neighbours and lots of animals that can kill. !

I will do Tom, its an odd problem because it only happens when the temperature gets over 32 degrees, otherwise it works properly.

The best ways of getting rid oof the 50hz is either over sampling and surpressing with the square root of samples, but means you loose detection speed.
Or do it analog. Have a floating wire parallel to your measurement cables and adjust that signal in hight, plus after add it with 180 degrees to jour analog signal.
Both old fashioned but work.
Mike

Mike thanks. Actually when I think about it, when I use this signal, I will be using a moving average.