Isolated Ohmic Sensor Switch for CandCNC Bladerunner

[Voodoo Vintage]

I'm hoping that maybe Tom and Jim might jump in here...

I'm looking for an "electro guru" out there that may be able to help me with a project.

I would like to isolate a circuit switch that is triggered by the ohmic state of my hypertherm torch head and the workpiece (basically a "touch-n-go" system) that doesn't fry my micro electronics on my CandCNC I/O board!

I'm NO electrical engineer... This is why I draw instead of talk....



Anyone want to put a RADIOSHACK shopping list together for me???

[Voodoo Vintage]

I just ran from the bathroom to post this additional thought that just came to me!!!

Would this make sense...



What you see in the photograph is an photo-coupled 9v dc relay. COMPLETELY isolates the I/O board components from the plasma operation...

[Voodoo Vintage]

By the way... Here's the link to the spec sheet on this photo-coupled relay switch:

http://www.crouzet-ssr.com/english/p...oad/dc_ssr.pdf

[Torchhead]

Originally Posted by Voodoo Vintage I just ran from the bathroom to post this additional thought that just came to me!!! Would this make sense... What you see in the photograph is an photo-coupled 9v dc relay. COMPLETELY isolates the I/O board components from the plasma operation...

Better stock up on several. The 300V spike will take out the input on the SSR as sure as it did the input opto. At the least you need a 24 V TVS across the inputs. Maybe an inline small fuse before the TVS to blow to protect the SSR.

The other thing that would worry me is how much surface oil, or dirt you need to stop the 10 ma of current needed. At 9 V you only need maybe 100 ohms of resistance for the relay to not fire. I may be just fine but it needs to be tested in a wide range of conditions.

You could do the same thing with a small FET (like a 2n7000) a small relay and an isolated power source. There is virtually no current required so a 10A rated anything is way overkill. The 9 V battery is not going to last very long. A Lithium Ion would run it for weeks if not months and a lot of them can be recharged. Even a low cost wallwart (DC) would work.

Relays are old technology but they do offer total isolation. You still need to protect the input FET but that can be done pretty easy with a series resistor and a zener clamp on the Gate. The amount of current to trigger the FET is small compared to turning on an opto LED so series resistance (bad contact) becomes less of an issue. You could probably make it sensitive enough to use the surrounding 60hz AC pollution in the air to trigger it as soon as the metal was touched!

Then there is the mechanical approach and just provide up bias with a spring on the Z to make the touch off "light".......

I would think the floating switch would need to be a LIMIT rather than in parallel with the Z Home to stop motion because you would need to factor in an offset (that was NOT used if the ohmic touch was done) so it would not work off the same input.

TOM Caudle
www.CandCNC.com

[Voodoo Vintage]

I already have a pre-load on my floating head. Problem is, often times the shaft trigger binds and the head raises WAY above ICH. I've tried dry carbon- spray lithium- WD40- Machine Oil... I've even considered slightly sanding down the block! (Don't want to mess with that one!) If I could get a functional Touch-N-Go system working, I don't think I would ever look back!

Since I posted the last diagram, I opted to hook up the floating head switch as a limit! (thinking along the same lines... ) I also hooked up a 9v AC wall adapter instead of a 9v battery. I've been using the system for a couple days now with some pretty oily, dirty, scaly material just to see if I experienced failure. None so far...

I had yet another thought in the past couple days... I've considered using two AC-input / DC-output Optical Relays. I would use a coolant command to trigger the relay through one of the Bladerunner outlets. I could modify my posts to include a "coolant on" before every Z-REF and then a "coolant off" directly afterwords. This would completely isolate the plasma ignition from the I/O board.

Seems like a lot of work when you can just buy an OPTO-ISOLATED I/O board for around $35? But I'm not sure I'm technically "savvy" enough to tackle that one!

[Voodoo Vintage]

Originally Posted by Voodoo Vintage I had yet another thought in the past couple days... I've considered using two AC-input / DC-output Optical Relays. I would use a coolant command to trigger the relay through one of the Bladerunner outlets. I could modify my posts to include a "coolant on" before every Z-REF and then a "coolant off" directly afterwords. This would completely isolate the plasma ignition from the I/O board.

These opto-isolated SSRs are triggered by as little as 3vdc (LPT 5v signal will trigger it!) Using a dedicated LPT pin, I could close/open the Z-HOME circuit using a "coolant on/off" command before and after my Z-Ref post. This should provide total isolation from the plasma circuit... Shouldn't it?