New Project with Mesa 7I96 and THCAD-05

phillc54 wrote:

Sorry Tom, I forgot to post this yesterday. It needs a bit of work but should give you the general idea of what I did.

 

Please do not insert any components between the THCAD, the 24V DC power supply and the ohmic probe (like the relay shown). That is something PCW was very clear about.
The THCAD circuit is deigned to be exposed to arc voltage and the use of appropriately rated components good for overvoltage of 500 volts (power supply and THCAD) makes this safe. Any relay there is likely to fail badly....

The whole intent was to remove any components from the torch side and provide a method of adjusting sensitivity.

Stick to the relay circuit or the published THCAD circuit. Do NOT mix and match!

rodw wrote:

Please do not insert any components between the THCAD, the 24V DC power supply and the ohmic probe (like the relay shown). That is something PCW was very clear about.
The THCAD circuit is deigned to be exposed to arc voltage and the use of appropriately rated components good for overvoltage of 500 volts (power supply and THCAD) makes this safe. Any relay there is likely to fail badly....

If you read the text you would see that I have no interest in the ohmic probe being active while not probing so I will isolate it if I choose to.

You would also see that the relay is rated plenty high enough for 500V as are the diodes.

There is basically no difference to the published circuit that John uses succesfully other than the relay and the THCAD.

phillc54 wrote:

rodw wrote:

Please do not insert any components between the THCAD, the 24V DC power supply and the ohmic probe (like the relay shown). That is something PCW was very clear about.
The THCAD circuit is deigned to be exposed to arc voltage and the use of appropriately rated components good for overvoltage of 500 volts (power supply and THCAD) makes this safe. Any relay there is likely to fail badly....
If you read the text you would see that I have no interest in the ohmic probe being active while not probing so I will isolate it if I choose to.

You would also see that the relay is rated plenty high enough for 500V as are the diodes.

There is basically no difference to the published circuit that John uses succesfully other than the relay and the THCAD.

Of course I understand you are not using a THCAD from what you wrote. Its fine for you to do what you want.
I was going to proceed the same way until I got this advice from PCW at Mesa:

No relays or diode should be needed when using a THCAD for OHMIC sensing
(basically its impedance protected)

We should not be publishing things that differ from Mesa's view.

rodw wrote:

No relays or diode should be needed when using a THCAD for OHMIC sensing
(basically its impedance protected)

I agree with that statement, however it does not say "relays or diodes should not be used".

phillc54 wrote:

rodw wrote:

No relays or diode should be needed when using a THCAD for OHMIC sensing
(basically its impedance protected)


I agree with that statement, however it does not say "relays or diodes should not be used".

No but this does

I would not isolate the input but rather just turn off the power supply when not
probing if you are concerned about electrolytic corrosion

Respectfully, I spent several weeks working through this with Peter Wallace from Mesa. The intention is clear as conveyed in the circuit diagrams published.

rodw wrote:

No but this does

I would not isolate the input but rather just turn off the power supply when not
probing if you are concerned about electrolytic corrosion

I don't read that as being mandatory either.

I also don't understand what the problem is, I am not intending to use a THCAD for probing.