Another plasma component...

yes i mean the work clamp. Sorry for my English.
I do not wait for the ArkOk I make marks with the pilot arc
when the work clamp connected, the marks are deeper but not round enough.

rodw wrote: Re relays, I've been using these for 24V which are 250 usec on and 2 usec off
RS Components Part # 888-6843

LinuxCNC only looks every 1000 usecs on the 1 ms servo thread so we can't get better than that. Some of the opto22's are much slower.

Rod, those relays are solid state using a Mosfet, normally you'll see that with plasma that the datasheet asks for "dry contact" (i.e. non-solid state).



I think it is to do with the leakage current and also maybe the minimum switching current.

To be fair those solid state relays seem fine, but some optoisolators probably are not a good idea. Hence why I switched to reed relays.

Just something to consider or be aware of .... if it works, it works.... but may not work for everyone... if it ain't broke don't fix it etc.

I agree 100% about the 1kHz (1mSec) loop, anything being faster than this is fine. It was actually Keith (aka Beefy . mancavedweller) who pointed it out a couple of years ago now to me, about looking for relays which are consistent. i.e. a 10% variance on a 1mSec relay is not much.... but 10% variance on a 500mSec relay is a lot more of a consideration... hence again I try to find a branded product with a good believable datasheet if my aim is consistency in something.

Rod,

Also the thing to probably watch is the switching frequency at 50Hz, i.e. you can't (or shouldn't) switch it faster than 20mSec, even though it makes in 250uSec, and breaks in 2uSec

also the RS datasheet does not really tell you the switch on and off voltage (it says mnimum switching voltage 10, maximum 30V)
docs-apac.rs-online.com/webdocs/1696/0900766b816969f5.pdf

compared to the ones I posted say for a 24V one, it says the switch on voltage is 18 v and the switch off voltage is 2V
cotorelay.com/wp-content/uploads/2014/09..._relay_datasheet.pdf
(i.e. a clearly defined wide window of high and low so noise is less likely to be an issue)..... but magnetism / magnetic field effect may be a consideration with a reed switch relay I admit.....

again If it works ... it works.... (maybe someone else reading this may want to be aware of trying to understand all considerations)

It often comes back down to what you can find in your market. Initially, I was using ordinary electro magnetic panel relays but purchased these as I wanted opto isolated relays for the Ohmic circuit and because I had to buy a pack of 5, I upgraded the mechanical ones to these as well. I don't think using opto relays is an issue. People are using the built in relay outputs on the 7i96 and even the spindle relay on the 7i76e to trigger their torches.

One thing to be aware of if you use a magnetic relay with a Mesa card, be sure to install a flyback diode between the relay input terminals (with the band on the diode pointing towards the Mesa output).

rodw wrote:
One thing to be aware of if you use a magnetic relay with a Mesa card, be sure to install a flyback diode between the relay input terminals (with the band on the diode pointing towards the Mesa output).

good point

but does the diode direction not depend if you are running a 7i76E (sourcing) or a 7i76ED (sinking)

and the manual does also state

VOLTAGE CLAMPS
The output driver chips used on the 7I76E have built in Zener diode clamps to
clamp inductive turn-off (fly-back) spikes. This means that flyback diodes are not normally
required on small (less than 60 mA) inductive loads. If high current inductive loads are
switched or inductive loads are switched at high frequencies, they must have
flyback diodes to limit power dissipation in the 7I76E's driver chips.


www.mesanet.com/pdf/parallel/7i76eman.pdf

Possibly. I only added the diodes after PCW from Mesa discussed them. I always forget about the D version as its a new product. It is usually only need for a retrofit that has sinking inputs. In many ways, the 7i96 is a better card for plasma as it does both ways and has the relays on board. I just wish it had a few more inputs. A fully configured gantry machine consumes 12 inputs for min limit, max limit and home switch. But I guess you can use one input for all limit switches instead of the seven I have on my machine.