EE? I am a little over my head with these electronics, SSR's

I am trying to put together one of these isolators, Plasma Cutting Primer for LinuxCNC Users

I have two questions. Is there any practical difference between these two SSR’s if they are only going to operate both sides at 24V and the worst case scenario is 300-400V coming in from the plasma torch. Big, https://amzn.to/3q0YqAb or small, https://amzn.to/3Ix55KR. I would really like to reduce the form factor if possible.

Question two what is the third relay for that is going to the field power and input. Is it just so an isolated power supply can be used, what harm is there not using an isolated PS?

I’m not an EE, and certainly not a plasma cutting expert, but from the schematic it looks to me as if the ohmic sensing uses an existing “signal path” where power normally flows “to” the torch as the sensor, but to work the signal flow needs to be reversed/redirected in order to “read” the torch as a sensor. That third “ohmic sensing relay” is doing that redirection by disconnecting the torch from the field power and connecting it to the digital input. I’m guessing the digital input wouldn’t react well to the full 24V field power while the torch was running.

(edit to revise my initial interpretation of the lower power supply purpose) Then it looks like the other two relays are disconnecting the normal torch power and supplying just enough to the torch and table for the ohmic sensing to work. They may also be reversing the normal polarity (if I’m reading the description of the process correctly - when cutting the table is positive, but for ohmic the torch should be positive.)

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Thanks for putting some eyes on this for me.

Yeah it works like the V1 Tiny touch plate. While the arc is off, the relays make the circuit, then it touches the work to zero, before the arc strikes the relays disconnect both wires again, leaving an air gap just in case of some stray power from water and such interacting with the arc.

The inputs are 24V tolerant (they are all actually 24v). That is why I can not understand why not just put the relays and diode on the 24v and input lines and just skip the isolated power supply. I can only image it is to kill a cheaper power supply than the control board if a SSR fails open??

I think they have the third to isolate the two circuits. It is not uncommon for power supplies to not actually be tied to ground. You could have 0-24V on the cnc controller side, but the 0V might be 2000V above earth ground. Or maybe the probe psu has the same issue. My guess is they try not to just tie everything to ground with a plasma cutter because of the high voltages around. They would want to be very intentional where the grounds connect. I may be completely wrong.

But regardless, if you are isolating the power switches, you might as well isolate the feedback. Whatever demons they are trying to protect against need full isolation for it to work.

About the size of the relays. The difference (to my eye) is the amperage rating. The two psu relays are only powering the third. The third needs to be big enough for whatever current the input is sinking.

The small ones say 2A. The big ones are 25A. I would suspect they all worl fine. 2A is a lot for 24V. But I don’t use SSRs much.

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Ok,
The main difference is the operating voltage. The small relays are rated to 60V. Perfect for your circuit in normal operation. Should however the torch be left live (300-400V) then you have exceeded the voltage rating for the small relay (5x the voltage) and they could fail (big bang, smoke leaks out etc. )
The bigger relay is rated to 220V DC though the specs are confusing as it implies is also AC rated. 220V AC would handled 310V DC and be less lightly to explode (1-2x the voltage) if the torch is accidently “live”.
So will work fine so long as the torch is never left on.
You could use the small relays but expect sparks, fire and bodily injury if it ever “sees” 300-400V, or it may just stop working, or may be unharmed, can’t say for sure.

The third relay is your “Digital Input” indicating when the connection has been made.
Isolated power supply is used in case you have left the torch live again. 24V PSU would float to 300V with no problem. If it was non-isolated it could “see” the 300-400V and fail (big bang, smoke leaks out etc. )

So in normal operation the isolated 24V power supply is feeding 24V.
Your control system activated the digital output which enables the 2 “Ohmic enable” relays.
These now feed 24V to your table and torch.
Once the torch contacts the table current flows tripping the “Ohmic sensing” relay.
This closes switching the field power to your Digital Input and your control box knows the torch has contact.

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Gosh, I love having you guys around. Thank you. I understand it a bit better now.

I thought the 2500Vrms dielectric strength meant I would be safe from stray torch voltage, both are listed the same.

This is why I didn’t want it. I didn’t really want the “digital” input. The next sensor on that list actually reads the voltage as it is making contact and you can set at what point it triggers. I was hoping to do the same. Just seems more versatile and possibly more accurate, I was also trying to shrink and simplify.

I figured these guys know exactly why they designed it that way.

I bought a spare, should I fire the torch while it is connected to test it… :cowboy_hat_face:

I believe a relay for 25 amps DC can also handle 25A AC but a SSR for 25A AC might not handle 25A DC.

Which sounds weird but when switching off AC power the device can wait until the current is zero and then open the circuit, which is how old-school dimmer switches work.

When cutting a DC curcuit, you have to tolerate the voltage spike that it causes, which can be large depending on the load.

I don’t know much about the rest but it could have to do with failure modes. I would expect the input and output sides to remain isolated from each other even if the SSR fails closed (or human error), so like you said, it just blows the power supply and not everything.

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Thanks, Feels like I am learning something today!

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Me too! Can’t wait to start playing with some of this next year.

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2500Vrms is the isolation on the optical isolator.
Keeps your isolated circuit safe but the 60V contacts maybe not so much.

The next sensor on the list will looks a bit smarter.
Protection via the isolated 24v supply and the 390k resistor - 400V / 390k ohm = about 1ma.

Seems it also does THC with a little fiddling?

The Plasma CNC I built for a friend just uses gravity for the torch. (Simple design with a RAMPS1.4 controller)
Should probably look at THC at some point but we’re only cutting thin 2-3mm so we just wind the current up :wink:

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From what I gather, the thin stuff is where the THC comes in most handy?

Yeah, I do like it but it takes a second $70 card plus some components, programming the card, and extra linuxcnc stuff that I am pretty afraid of at this point in my linux career. What that takes is a second THC card, I am currently running one, he uses a second one just for touch sensing. I had hoped one would do the trick but I was mistaken.

Test cuts on 3mm galvanized steel plate (top and bottom)


Good enough. May try THC one day.

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I can’t wait I am so close to some testing.