The replacement tube is fitted and I am doing alignment / focusing now.
The mA meter goes between the cathode (negative) end of the tube and ground. So that is correct…
Maybe I don’t understand the question or I confused you previously?
I am using a mA meter on the negative lead. If I set the power in Lightburn to 50%, I get 16mA on the meter. If I set it to 25%, I get around 10mA. So a power level in Lightburn of 50% represents the highest that I can drive the laser without damage (though in practise I will use say 14mA or 12mA for some headroom).
The above scenario applied to my previous tube. Now that it is dead, I need to know if it was because of the method that I used to manage the current. Maybe this was not the right thing to do and led to damage due to overdriving the tube. I think I doubt this as there was no decrease over 310 hours in cutting power until say 3 days ago when it dropped off sharply, but am very open to correction.
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I have the new tube installed, aligned and focused. At a conservative 25% power I can cut 3mm mdf at 12mm/s, which is shocking considering I was cutting it on the previous tube at 5mm/s at 50% even on the day I got the machine.
I dropped the ammeter (don’t ask) and will have to get a new one in the morning. Then a quick call to PerfectLaser should get me some info on the max current for this tube, and hopefully a more thorough explanation of the rating sticker that they filled in. Will update here when I get a better explanation from them.
When I talk about maximum current draw I’m speaking when the tube lases during it’s on time. The pwm has a period (sometimes called the pwm base frequency) and is usually expressed in seconds… a period of 1mS is 1s/1000 or 1mS. You can set the Ruida pwm period. It’s usually 50uS (20kHz).
I think you missed what I was saying. When your tube lases, it lases at 100% power for 50% of the time, a 50% pwm cycle. Your meter reads 16mA or 1/2 the total current. For a 50% pwm reading of 16mA, it has to be drawing 32mA during it’s ‘on’ time.
Changing pwm percentage does not change how much current it draws when it lases… only the lps can control that. It affects what you read off the meter only… however, it’s the only real indicator we have available.
Does that make sense?
Did you take the time and really look over the lps for a current adjustment?
Bummer… could have been worse…like dropping the ‘new’ tube…
I think you ran this rather warm, if I recall and I’d attribute it to that above anything else. Of course it all adds up…
If it were me, I’d change out the lps for one that I could control the current…
What kind of current are you reading at 50% with your new tube?
Whatever it is, multiply it by 2 for the actual current.
Truly, the darkest art is the art of laser mirror alignment. 
Surely the current the tube is seeing is the same as what the ammeter is seeing? That is, the RMS current?
Either way, the laser supplier recommends no greater than 12mA on the meter - seems high for a 50W tube so I will de-rate that a bit too.
I found it hard to get a good explanation of the rating card. But I have a friend coming over with a scope to check the lps output and will adjust accordingly. The new ammeter is arriving Monday
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In the meantime I have been cutting - the tube just just ignites at 13% (Lightburn) power and cuts 3mm MDF comfortably at 8mm/s 18% so for now I am going no higher until I can confirm the amperage. Maybe this is risky but I have a very urgent cutting job that needs to be done, so I am taking a calculated risk.
Experiencing a brand new tube makes me really think I was taken for a ride previously (the seller promised the tube was new and was treated well, and I believed him because he seemed honest). The beam quality is night and day. The kerf width is practically nothing compared to the previous tube.
I will continue to update this thread as I progress, until I have gotten stable operating parameters dialled in.
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My controller is a Ruida 6445G and I am trying to decipher the poor English to find out how to reset the tube life counter.
My tube is 880mm in length… it is rated at 50W but reads 44W with a Mahoney power meter and with the doHICky (~30 min video) by Sadler…
The max current rating on my tube is 21mA… almost twice what you have as a maximum. I don’t have a problem cutting at 18mA and have been. So I’m wondering if you have the correct maximum or someones interpretation of what a safe maximum is… Seems awfully low to me… the manufacturer would/should know.
I try to set mine up with what makes me comfortable… 90% or more of the people using these are totally ignoring many of these ‘conservative’ steps such as a very low mA. Many run for a long time, so don’t worry yourself too much about losing a tube. If you use it, it will get ‘dull’ just like a knife, the difference is a tube will get ‘dull’ just sitting there with non-use…
If you need to use it, I’d say use it…
The most simple way to set the lps current is with an installed mA meter.
If you don’t have a mA meter installed, then you need something to measure current with… a current probe for a scope is usually pretty expensive.
I measure it with the scope across the mA meter, but it needs to be installed…
Good luck …
How long is your tube ?
My tube is 1020mm long but my lps is either underrated for the current tube or overrated for the old tube (more likely).
Will report back when my friend brings his scope.
The writer says there are 3xx hours on his tube. I had about 50 hours on my K40 tube, ran it with an ammeter, never going over 15 mA. Then I started getting two beams in my work and traced it all the way back to the tube itself. Had to replace it. My beam spot is almost imperceptibly small, maybe 0.2 mm. Cuts like a knife. I attribute this to a making my laser head adjustable up and down (Z axis) and better focused. Check the beam at the first mirror, should only be one, and be very, very fine.
Just reading this thread and i think i have my head around it but there’s seems to be people here that are missing the point that @jkwilborn is trying to make.
Correct me if I’m wrong but i believe you need to ensure that you calculate the current your running using the power percentage and the ammeter reading. Not just the ammeter.
As the pwm signal is made up of 5v pulses no matter what your power percentage setting is the laser is getting 100% of the current the power supply will allow, just for less time.
So from the table jkwilborn posted if you’re ammeter is reading 16mA at 50% that equates to 32mA that the laser is seeing for every 5v pulse. The max on the table above is 20mA (50w). The graph animation on the webpage below might help.
Www dot - - quora.com/How-do-we-calculate-the-PWM-time-period
I think jkwilborn said it very clearly but noone has seemed to taken it in. It sounds to me like your current was too high and thats why the tube died prematurely and the new one will do the same if you don’t adjust the current.
Am i understanding this correctly, maybe there is something i am missing??
Update time!
First I contacted the supplier of the tube and confirmed that the ratings card is devised by setting a limit on the PSU current (in the case of my tube, 12mA) and then running it at 20, 50 and 99% duty cycle, and capturing the laser energy output with a laser power meter.
I spent about 2 hours with a scope and got readings that did not make sense. We put a 1KOhm resistor in series with the tube and put the probe across that. Dividing the measured voltage by 1000 gave a current-time graph. Across a 100ms pulse, the first 10ms or so would spike to 32mA then quickly settle to 4-5mA no matter the duty cycle. I will parse the CSV data and supply a graph here later.
The tube actually lases at 12% duty cycle, so I am cutting at 20% (just cuts 3mm MDF at 10mm/s). I feel that the safety margin is sufficient, but I do not know this for sure. I may very well still be overdriving the tube.
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In testing the tube, I found a point at about 30% where the nature of the discharge visible through the tube changes. Between 12-25% there is a purplish transparent beam inside the tube. Around 28% a more ‘solid’ ‘core’ forms within that beam and the laser makes a louder hissing/tearing noise at the point of cut - this ‘core’ is unstable and lasts about 5-8 seconds from ignition. At about 30% and above the ‘core’ is much more stable. It’s an interesting phenomenon that I found mention of a while ago but can’t seem to find where. And I can’t formulate a search term to find out more about it. What exactly is going on here?
Which agrees with what I measured some time ago. Basically, the power supply acts as an analog current limiter controlled by the low-pass filtered PWM input, so that the tube current is not full-on during the high part of the PWM cycle.
For example, the 40% PWM input is the cyan square wave at 20 kHz and the laser tube current is the green trace at 10 mA/div:
The current eventually settles down to about 8 mA, roughly 40% of the supply’s maximum 25-ish mA.
The hash in the beginning of the pulse seems to be the tube starting to conduct and, even though it looks horrible, the average current is just about 8 mA.
Here’s a pulse at 80% PWM with the current at 5 mA/div:
The pulse settles down to 22 mA, suggesting the power supply max is 27 mA. OMTech sent it as a replacement for the failed supply in my 60 W laser, but it’s definitely running higher than I expected.
I have a bunch of blog posts going into far more detail than might be reasonable, but the effect remains consistent: there is no trace of the digital PWM input in the power supply’s output current.
Yes my results are exactly consistent with yours - waveform is the same shape.
I get a weirdly spiking current that then settles down after a short while, except that it doesn’t in my case as I have a 40W power supply driving a 50W tube apparently (and I only tested up to about 50% PWM)
I’ll do a full power pulse a few times and check what the maximum current the power supply is set to - will be looking out for a waveform similar to the last image of yours
Then my plan is to test increasing PWM percentage until I get one that corresponds to say 10mA and use that as my maximum value (as the tube was tested at 12mA)
The power supply here has a digital milliammeter that (pretty much) agrees with the current measured at the tail of long manual pulses with my Tek current probe:
I think Sadler used “preionization” to describe the tube’s behavior at low currents. That might account for the hash at the start of each pulse, as well as the continuous hash below about 25% PWM.
If you have a mA meter you can run it at 50% pwm, continuous and it will read 50% of your lps current setting… no need to run 100% unless you are curious…
@ednisley first current trace shows the current going off the screen ?
Yes, those initial peaks are breathtakingly high.
The scale is 10 mA/div, so they’re well in excess of 80 mA. The power supply controls the average current to whatever the PWM calls for, but doesn’t limit the peak current.
If the pulse is long enough for the tube to fully ignite / ionize / lase / whatever the term might be, then the current settles down to a nice-looking line. Pulses less than a few milliseconds never settle down and consist of those huge, irregularly spaced peaks from beginning to end.
Please explain… average based on what? An average has to be over a time period, what period are we expecting?
It’s measured over the active time of the laser pulse during the test pattern.
My first thought was to capture those huge peaks, but the 100 mA/div scale pushed the rest of the pulse down into the noise:
The A and B cursors mark the RMS calculation gate time when the Enable / L-ON signal (magenta) is low.
So I cranked up the gain enough to see the tube current controlled by the PWM:
Plotting the measured RMS value against the PWM duty cycle input:
Because the tube current is mostly spikes below about 25% PWM, the lower RMS values don’t match that part of the plot from the power supply’s meter:
But everything matches closely enough to say that, yeah, that’s pretty much how the power supply behaves.
For completeness, a description of the test setup and scope traces:
That’s the Tek current probe clipped around the tube’s cathode wire near the power supply.
From the look of the first ‘pencil’ graph, you can’t really lase below about 8% pwm … ?
Is this the same lps with that the other screenshots came from?
Sure like that probe… Their cost, makes the scope itself a low cost part 
I’ve got one I’m working on, but it will never be up to the level of a commercial probe. I think I’ll just build it to be a permanent addition to the machine with it’s own output.
It’s working now, but is really susceptible to surrounding magnetic fields, especially the earths… 
I also need to limit it’s bandwidth…
Thanks for the screenshots…
73’s
IIRC, the Ruida controller arrived with a lower cutoff at 10%, I lowered it to 5% to see what really low powers would be like. Turns out the tube didn’t light up until around 7%, so I set the minimum at 8% and left it there.
OMTech sent this supply as a replacement for the one that failed, but I also bought a Cloudray. Some quick measurements showed both behaved pretty much the same way, so the Cloudray sits on the top shelf as a cold backup.
I got the Tek current probes & amps long before they became eBay collectibles, so they’re not quite so bling as they seem. I should’a bought more: they’re definitely outperforming some other “investments”! 
The probe clamps a mirror-polished split ferrite core around the conductor, with a transformer winding boosting the signal. The amp senses the current, then feeds a fraction back into the transformer to buck the input current and prevent core saturation, all with a 20 MHz bandwidth. I’ve spent a long time pondering the schematics and they’re a wonder to behold.