Paul is @pauldg123
The IN and LO inputs are basically different in nature. IN feeds the driver IC UA491 that drives the H bridge to generate the High Voltage based on the input signal while LO enables or disables the UA491. If you drive the LO via a controller with PWM, no real conversion happens and the IN takes the required voltage from the analog signal present at IN. Itās like running an engine on high speed and using the ignition to control the running speed. Now this seems to work but my engineering back ground sees this as a no go since it is sort of uncontrolled and you have the additional start up threshold of the circuit (which can be small). Also LO is entirely controlled by the Mini Gerbils M3,M4 and M5 commands which provides additional safety. Ideally you should hook up a scope at the low voltage side (e.g. h bridge transistors, not the HT sideā¦dangerous) and see the different patterns when using LO or IN to control the tube. Also using IN can help to extend the life of the tube by setting the minimum PWM at 2mA (below engraving output) so the tube has a lower startup threshold during engraving (itās āonā at all the times during the scanning engraving action). Hope this explanation does help.
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This may end up to be an endless debate.
With the utmost respect I see it this way.
The laser should be made safe by the manual enable (laser switch) not a program. I do not see LO being programmatic as āsaferā since it can be turned on without the operator manually initiating it.
Your engine analogy does not apply, in my view, because no matter what you switch using PWM the engine (HVT driver) is cycling fully on and off. A cars engines power in no way works that way because its power is not digitally controlled. The throttle is an analog control of power.
The ignition key is more like the Laser Switch on the panel not the LO and the IN is a REV limiter. The LO is the throttle. 
The engineer in me sees it the opposite as you. The IN is a power level set-point control and the LO decides if it should be on or not. The IN is like the setpoint control on your heater and the LO is what cycles it based on actual temperature. The IN sets the lasers base power level and the program decides how much and how often to use it.
When the LO is not asserted (no PWM) the laser is OFF and just as safe as any other approach.
The engineer in me does not like having programmatic control of a lasers ON-STATE thatās why I do not like enabling the laser from inside a job. I think that M commands should not be used for enable. I doubt that one could get laser safety certification this way.
Certainly different controlling methods will look different at the HVT drive but its drive is still a composite of them both and it is still on-off? I do not yet see that as a reason for a particular approach? I have for some time had it on my list to study the composite characteristics of IN + LO control.
āAlso using IN can help to extend the life of the tube by setting the minimum PWM at 2mA (below engraving output) so the tube has a lower startup threshold during engraving (itās āonā at all the times during the scanning engraving action)ā
Are you saying that if the IN is always on (at 2ma), like a keep-alive, that tube life is extended? What mechanism in the tubes failure mode would be noticeably improved if the tubes current is kept below ionization levels vs going from 0 through 4ma. I donāt knowā¦ but my understanding is that the tube life is mostly affected by disassociation as the result of ionization. In the pre-ionization case the tube is not ionizing?
That said I have for some time wondered if āpre-ionizingā speeds up the tubes firing time? I think the light output response time in these tubes is much longer than we think and that has an effect on high resolution marking.
There is still an important aspect of choosing a power control method that I think is overlooked. These tubes āwearā in that the optical power output vs current function changes over time. This infers that if a jobs power is set up programatically the program will need to be changed as the tube wears.
The operator really has no way to easily tell what the power vs current function is when a job is run. This means that at the running of any job the operator has to readjust the programs values to get a good result. This means generating new Gcode.
Leaving the pot in place and controlling the LO lets the program remain unchanged while allowing adjustment for tube wear. The IN as controlled by the pot adjusts the base power level.The pot now functions like an āintensityā control.
I prefer developing a jobs Gcode as independent as possible from the machines changing characteristics.
Cheersā¦
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An additional problem with using IN for PWM is, that the voltage of the TTL PWM signal metters!
If your board is generating a PWM with 4.8V you get less power than if it is generating 5.0V!
Interesting ā¦
All the DSP systems I have seen use PWM on the IN terminal, with L being switched as On/Off Exactly the way Paul has the miniGerbil set up.
Likely those are all Chinese designed machines not a standard of design to my thinking.
BTW: These approaches are not WRONG just different.
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Thank you for your help. I have read all your post. Most of it above my head. I did check the wiring per your drawings. Everything looks good. I checked values for LightBurn. Thatās ok. I am wondering about grounding issues ? I took grounding lug off back of case. Ground around the until I got bare metal on both sides. Have a # 10 wire going to 4 foot long 3/4 ā copper rod in ground. My home has (1960) wiring. 100 amp service. No ground system. Is my grounding wrong and how to make good ?
The best way to confirm the best operating method is by measuring it and compare the dead/threshold times. Just hook a trace scope (if you happen to have one availableā¦ smile) and measure the current/voltage across the mini current transformer (low power side) that feedback the signal to the UA494.
Also the datasheet gives some good insight in the operation of the UA494 (see http://www.ti.com/lit/an/slva001e/slva001e.pdf)
Enjoy!
You donāt have ground at the pole or where ever the power comes from the electric company to your main junction box? Usually neutral wire is connected to ground at the main junction box. If you create your own ground rod, do it near the main junction and connect ground and neutral at that point. Also you should use galvanized steel or special copper alloy intended for ground rod. Typical copper oxidizes and becomes ineffective unless you have a special copper alloy specifically used for grounding.
I reread the thread.
Boy we sure went down a rabbit hole, an interesting one nonetheless 
Can you describe in more more about the problem and its symptoms?
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