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Saturday, February 24, 2018

Will a 40 Watt LPS Drive a 60W Laser?

Under-powering a Laser Tube

Caution: these are preliminary musings .....

Recently +Chuck Comito presented me with a question regarding how his 60W laser would perform with his stock 40W Laser Power Supply (LPS).
I hadn't spent much time looking at LPS specs matched against Laser Tube specs. I just assumed that if you got a bigger tube you had to get a bigger supply. 
Although to get to full power you will need a LPS that will output enough current to match the tubes specification, you can under power your laser ... theoretically. 

That said, +Chuck Comito is running a 60W K40 with a 40W LPS somewhat successfully, still testing. 


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Why do we care?

Like most users when my tube goes bad I plan to upgrade to a higher power tube. We all know that our K40 really is a K30-35 in that these tubes don't actually output 40 watts.
30 watts is marginal for cutting thick materials and experience has shown us 50-60 watts is a better range for us makers.
The cost of upgrade would logically include the cost of a new tube and a higher wattage supply. This post suggests that you can upgrade your tube using your current K40 40 watt supply. That't a $100-150 cost savings and it allows us to use the stock supply to end of life.

The short theoretical analysis

1.) You can run your 60 watt laser with your 40 watt supply if you keep the current <=16MA
2.) Although at 16ma you will not reach 60 watts you will achieve a marked increase over what you had before at the same current.
3.) You can reach 60 watts if you run the supply at full capacity (22ma) but this will likely reduce its life.

My learning: you can upgrade your machine to 60 watts without getting a new supply unless you need run at the full 60 watts.

The long theoretical analysis...............

We know what the voltage vs current curve looks like, see the ref. at the bottom of this post.
I do not know what laser output vs current curve looks like when the tube is in the Normal Glow region [update: I added an attempt to model current vs optical power in the model below].
I assume that as long as the tube ionizes and the voltage is held at the operating voltage it will output light proportional to its current.

...We know that the needed trigger voltage relates to the length of the tube
...We know that the supply will current limit based on its pot setting up to close to 100% DF where is will be at max current.

Can you use a 40W supply to power a 60W laser?

This question of LPS wattage vs laser tube wattage is one I never did any research on. a I never compared the specs of supplies and laser tubes. So in the model below I captured some specs for comparison. I used similar supplies and tubes just different wattage's.
If you look at the specs for tubes and supplies in the model you will notice that the HV specs for a 60W tube can be met with a 40W supply! This means the supply will provide enough trigger to ionize and hold that ionization level.
There is a small mismatch between the current specs when using a 40w supply with a 60w tube. Note that the 40W supply must run at MAX current to provide enough current for the 60W tube, whereas the 60W supply provides enough current running below its max output i.e. its operating curent.

Assuming your tube and LPS have specs like those below, I would theorize that you can operate your 60W tube with a 40W LPS operating in the range <=16 ma and not >22.

The 40W LPS current will not likely get you to a full 60W. If things were linear, which they never are, you could estimate that with a safe LPS current of 16ma you will run 16/20 = 73% of rated power or 43.6W.

This may seem like a hugh loss of power but consider that your 40W was probably putting out 30W (as some high current). You now have a 13.6 watt (45%) increase over your previous setup. This means you have more power at a lower current. Not bad since you saved $100-200 in your 60W upgrade.

A Model

Using a purely specification based comparison of LPS's and laser tubes we can build a model to better understand how matching/mismatching LPS and laser tubes might perform.  Here is the analysis that led me to the conclusions I summarized above:

Comments and corrections expected

1 comment:

  1. This answered my question perfectly, thinking of swapping out the 40w tube on the k40d for either a 50w or 60w but wanted to keep the psu for cost saving, my thought was that the lower power psu would lengthen the life of the larger tube as its not over powered, like the 40w has been, ? this blog would have been really good with a link to some tubes that would have crossed over, great effort and thanks