I have been running a k40 with a mini gerbil 3 from awsometech. It turns on when it traverses to the cut and the ma meter pegs out so I assume its full power and doesn’t shut off until it’s done. in my troubleshooting, I disconnected the PWM wire entirely and it did the same thing but it did throttle the ma down to 20.
Did this used to work correctly and this is a new symptom? If not, can you provide some history of the laser and this problem?
How is the LPS being commanded on if you don’t have PWM connected? Either you have a another path from controller to LPS or the LPS has failed ON.
This may warrant a comprehensive description of how your controller has been wired with photos.
That would explain all the symptoms, particularly the initial “It turns on when it traverses to the cut” problem so common to CO₂ power supply failures.
That it continues to fire the laser with the PWM input disconnected shows something is badly wrong, because the PWM input should “float low” when disconnected.
Is that the typical failure condition? That’s not comforting.
Doesn’t most K40’s fire on low signal to begin with? If that be the case a disconnected input or faulty input stuck low would do the same? Not 100% so just questioning.
That would be for the L-ON signal, the active low enable, which should “float high” in the inactive state.
The value of the PWM / analog power input should not matter when the enable input is inactive, but that doesn’t seem to be the case here.
Which is how I felt when mine failed that way. 
I’ve lost count of the number of CO₂ laser complaints about “firing while framing” and suchlike around here. Some come from running GRBL controllers in CNC/spindle mode with $32=0, but that doesn’t cover the lot.
On my my China Blue, I can unplug the connectors, with the exception of power. It should not fire except with the fire button…
What kind of lps do you have…? Picture?
If you go to the L input it takes an active low to fire and the H takes an active high to fire.
Most of these seem to wire the IN to the pots sweeper pin, and the others to 5V and ground giving a dc input to the IN terminal for power control.
So far, doesn’t sound good for the lps…
I’m sorry I don’t have any real good data I have used light burn in the past with my K40 it came with a mini gerbil two board it quit working and I couldn’t get it going. So I swap back to you using a K40 Whisperer which worked great, of course it has a lot of limited functionality so I bought a mini gerbil 3. This has been ongoing for a couple of years now but as far as I can remember I never really was able to get the mini gerbil three to work consistently. Thus I switched back to the K4 whisper for a Time. I just got a new laptop and thought I would try to use the mini gerbil 3 with lightburn again in this is the results. I thought it odd that the with the pwm unhooked it should still be firing the laser.
So you’re saying this laser has been working correctly with the original controller right up until you swapped to the MG3?
You have a wire connected to K-. What is that? Can you remove it to see if it no longer fires when not prompted?
Yes I’ve never had a problem with the stock board and k40 whisperer.
I unplugged the K…I have no idea what it is, But it did the same constant 20ma burn on the transverse.
Photo looks like the A type lps … this is from Don’s blog.
Type “A” LPS Control Schema
This is a common K40 supply and is usually physical configuration GWWR or GWWG.
This schema uses 3 separate white connectors in the order below from left to right.
Laser Switch (enable)
P+ Laser Switch [**4.28VDC]
P- gnd return for Laser Switch [0 ohms to frame]
Test Switch (“FIRE”)
K+: Test switch [**4.28 VDC]
K-: gnd return for laser [0 ohms to frame]
Current regulation
Ground: signal ground [0V to frame]
IN: laser current control 0-5VDC [**not sure, likely floating]***
5VDC: 5V power [** 5.02 VDC]
Note: the “Current Regulation” pot is connected across these three signals with the center tap connected to IN.
*** you can partially test “IN” by measuring the resistance across IN to gnd while turning the Current Regulation POT on the panel. The resistance should vary with pot position.
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So I’m clear… are you getting 20ma as soon as you turn on the laser, as soon as you start a job, or literally only between shapes?
Since you have something connected to P+ and P-, I assume you have a laser enable switch or something. Can you confirm?
When I hit the play button in light burn the laser starts to burn.
Yesterday I hooked the Nano board up and ran K40 whisper again just to make sure that I was telling the truth and it worked fine with that.
If you’re completely changing the controller with all its connections to the power supply, this strongly suggests the new controller is incorrectly wired to the power supply.
The photos do not have enough detail to figure out what the wiring is.
Sketch what you’ve done as a simple schematic and provide links to the source you used for that wiring, which may help figure this out.
Thanks for the input. I’m smoked today but I will try to get to the shop after work tomorrow night and get some better details.
Maybe it’s what you are smoking… 
I want some of what the man on the floor had.
That’s probably alcohol if they are on the floor 
LOL! (I’ll be busy with church services all day)