Valid Arc Lost, Thcad 300 config & Plasmac override

tommylight
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23 Mar 2020 20:24 #161271 by tommylight

Replied by tommylight on topic Valid Arc Lost, Thcad 300 config & Plasmac override

That is an inverter plasma, so by definition it is high frequency, usually well over 20KHz up to 200KHz.
To test the current state of the equipment, connect the thc to the mesa, DO NOT connect the plasma side, power up everything, start Linuxcnc, open halsope or halshow and monitor the encoder.0.velocity pin, now touch the outside of the upper most resistors when looking from the plasma input side. Check if the scope changes value and stays so so stable. Should be enough to move the scope a bit if everything is OK.
Report back, with screenshots.

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rodw
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23 Mar 2020 20:31 #161276 by rodw

Replied by rodw on topic Valid Arc Lost, Thcad 300 config & Plasmac override

So the manual says this:

The advantage of using the 5V and 10V THCAD models and an external resistor is that the resistor can be selected to work with high-frequency start torches. 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.

Question for PCW. What power rating do the THCAD-10 scaling resistors need to be? Could Fred just get a THCAD-10?

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CNCFred
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23 Mar 2020 20:39 #161278 by CNCFred

Replied by CNCFred on topic Valid Arc Lost, Thcad 300 config & Plasmac override

Misunderstood:
The 300V model can withstand a 2500V input overload indefinitely.

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PCW
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23 Mar 2020 21:17 - 23 Mar 2020 21:19 #161280 by PCW

Replied by PCW on topic Valid Arc Lost, Thcad 300 config & Plasmac override

The problem is this: The input resistors on the THCAD-300 total 3 M Ohms
This is made up of 10 300K 1/2W 3500V resistors. It will survive short spikes to about 5KV.
At say a continuous 3000V input, the power dissipation is 3W (less than 1/2W
per resistor) and no harm is done, But at say 20KV, you have 133W dissipation
(not to mention that the PCB/connectors etc will likely arc over)

Last edit: 23 Mar 2020 21:19 by PCW.

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tommylight
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23 Mar 2020 21:26 #161282 by tommylight

Replied by tommylight on topic Valid Arc Lost, Thcad 300 config & Plasmac override

This case:
The first arcing occurred at the cable inlet inside the box due to low isolation of the wires and or pressing them tightly together. That creates metal vapor that got stuck to the lover part of the board and resistors/capacitors, so the second round did not take much to arc over and make a mess.

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CNCFred
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24 Mar 2020 11:01 - 24 Mar 2020 11:07 #161340 by CNCFred

Replied by CNCFred on topic Valid Arc Lost, Thcad 300 config & Plasmac override

yes, using the voltage before the 20kv, 20Mhz for the pilot arc are induced like on a tesla coil ,would have been best. :whistle:

I will find that out, it is a chalenge.

Tom:
I tried to put my fingers on the resistors but no reaction...encoder.0.velocity stayed

So then I didn't care anymore and I disconnected the thcad300 and reconnected so I saw once a peak...

either way all broken or not :

My 7i76e so far works , have still to check the encoder or try firmware upgrade(change) to get another encoder working with a new tcad(

RODW secret)

Last edit: 24 Mar 2020 11:07 by CNCFred.

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tommylight
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24 Mar 2020 12:26 #161345 by tommylight

Replied by tommylight on topic Valid Arc Lost, Thcad 300 config & Plasmac override

I can not see what is going on at the bottom, but can you please do the same thing with halscope like so:
Set the THCAD to 1/128 with the jumper, connect it to 7i76 and do a halscope for encoder.0.input-a and encoder.0.input-b.

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CNCFred
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24 Mar 2020 14:10 #161350 by CNCFred

Replied by CNCFred on topic Valid Arc Lost, Thcad 300 config & Plasmac override

this is what I get, no matter touched or untouched!! unfortunately

thank you Tom, I fear all dead

thcad blinks steadily....has two leds on...

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CNCFred
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24 Mar 2020 14:30 #161351 by CNCFred

Replied by CNCFred on topic Valid Arc Lost, Thcad 300 config & Plasmac override

no I also see something on the velocity with f/128
not touching the resistors

with touching ...but does not tell me anything

input b does not show anything
thank you!!

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CNCFred
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24 Mar 2020 14:42 #161352 by CNCFred

Replied by CNCFred on topic Valid Arc Lost, Thcad 300 config & Plasmac override

touching with the resistor measurement I can produce that..
eg. I think some voltage is applied

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Valid Arc Lost, Thcad 300 config & Plasmac override

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