Hey guys,
I was going to install an analog ammeter on my 80w Omtech until I noticed it came with a digital one on the power supply. I think it would not be worth installing the analog one. Thoughts?
Opinions differ, but once you convince yourself it’s working fine, the actual tube current isn’t much of an issue.
An analog meter’s needle has the advantage of being more-or-less steady during small current variations, where the digits on a digital meter just flicker.
However, when you’re testing & setting the current, you’ll use manual pulses lasting long enough to settle either meter for easy reading.
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It’s the very first thing I did after the 30 day return period expired, on both my lasers. If yours is like mine, that display is inside the cabinet (maybe not). The analog meters are great for monitoring current draw.
Totally worth it. I installed a digital one on my 100 watt DIY laser. I would recommend a digital meter so you can get an exact meter reading. I mounted it near the ground end of the tube to keep my HV wires nice and short. It is nice to know I am not exceeding the specifications of my tube. I set my current to 23.5mA when the laser is at 100% which is just under the max current of 24mA.
I will add my 2¢ worth. First thing I did was pitch the honeycomb bed for some reasonable air flow…
The next thing was add an analog meter for the cathode.
The cathode side is, people wise at ground level. No need to put it elsewhere because of high voltage worries… although it really doesn’t matter where you put it in the cathode side of the tube.
The mA meters in the lps seem to have a very short sample period, mine was all over the place with anything other than a straight line of the set power.
If you want to know what the peak is, you pretty much need a scope to view the signal … what I’m saying is that a normal meter will show you an RMS voltage, not the peak voltage. How a digital meters samples the voltage/current, most of the time is unknown… Unless you know the sample rate and how to apply it, I don’t use them for some applications, including this.
What you read on the meter, may or may not tell you what is going on with the tube current. Really depends on how the controller is wired to the lps.
With most DSP types, it is properly connected (dc excited tubes) with a pair of control signals. PWM or IN on the lps sets the current, then L-On of the controller tells it when to lase. With a Ruida the pwm is active as long as that layer is running.
A good safe way to set this up…
- use an analog meter
a. your lps meter should work for this as the current is stable long enough - set the power to 50% - generating a pwm of 50%
- set the lps current limit to 50% of the desired maximum.
This will set your percentage power setting in Lightburn (or any other software) to be in sync with the lps generated current. 60% power will get you the 60% current or power you have requested.
50% ensures no over current draw during the setup procedure. Although tube current draw isn’t linear, it’s relatively close…
Make sense?
This is my console…
I have a template for the meters. Spaces them above and next to the emergency switch.
51mm-15mm-meter-cutout-1.4-1.post.lbrn (37.4 KB)
I have an analog meter as well, but my machine is set up and running I don’t really need to mess with it. Yes the analog meter will give you a much more consistent reading when engraving etc. The digital meter does not work well for that. I have compared my cheap china digital mA meter with my much more accurate and expensive Fluke true RMS meter and surprisingly it is dead accurate. I may also install the analog meter but probably save it for my next build. I want to make a smaller machine eventually as this beast takes up so much floor space.
There isn’t an argument about which is more accurate.
It’s about what is most useful for this application.
There are some of these machines wired where the PWM is toggling the laser enable on and off, with a pot setting the maximum current as a voltage divider to the IN pin.
When these lase, they do so at the set maximum current every time they lase…
If you set it for 50% power, it lases 50% of the time, this reads as 50% power, but the actual current is 100% power for half the time…
It is exactly the same thing happening as checking pwm operation. A ttl pwm is 0 or 5V, not 2.5V. But at 50% pwm the meter will read 2.5V when we know it is 0 or 5V. It’s the RMS value based on the input signal.
I think, if the meter is zeroed properly, the accuracy of these meters is very good.
Mechanical meters were and are used worldwide, not from lack of accuracy.
What exactly you are measuring, really depends on how the lps is being controlled.
Thanks for all the input guys.
I read all the comments and went ahead with installing the meter. Regardless of how helpful it will end up being - it will be nice to see the display during operation.
Here’s the result
I noticed it was easier to read while the laser was operating vs the digital display which seemed to fluctuate more. When holding the pulse for a couple seconds it was within 0.2-1.0mA of the digital display.
The readings are a little higher than what OmTech had sent me in this chart below, but seem within reason. OmTech has listed on their website for their 80W tube, operating current is <=25mA
Nice! Good one. Having that right up front to see is just reassuring.
I’m still new, so still learning. From reading online - it seems I should tune the power supply now to be 100% at the 25mA or something like that.
That’s what I stated in the previous post…
That will make it work like it’s supposed to…
My bad. You did say that. From what I read is this accomplished via a little screw on the power supply box? I do see a suspicious small hole on the bottom that has no label
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Yes, then you will never over drive the tube. As long as you keep it cool and don’t try to jam all sorts of current through it, the laser tube should last a good long while. All good things permitting 
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Mine requires you be a contortionist. Far end is the front of the machine.
Do not set it for 100% power (pwm) for setting the adjustment. Set the power (pwm) at 50% and adjust for half the maximum current.
If your lps can produce 50mA, then you will do damage to the gases. Doubt that’s the case, but adjusting at 50% is safe, 100% is not recommended. If you set it up for 50% will will translate properly with Lightburn.
What I’ve found while doing this is manufacturers should give you two maximums for tube current.
- working current
- maximum or do not exceed current
Number one should be usable as it’s withing the working area of the tube. Number 2 means any lasing at this current will damage the gasses…
It’s similar to a light bulb… if you force enough current through it, it will be very powerful for a short time, much like a flash bulb.
If you purchase a good 40W tube, you should be able to use the 40W to do work. This is working current…
This is supposed to be on the low voltage side, but I’m hinky about sticking a metal rod blindly into a device that normally works in excess of 25kV. I use ceramic screwdrivers for this and RF type of work…
Many do not…
Good luck
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My screw was in the same spot 
.
Settings are adjusted good now. Was way to high at the start
OmTech advised me not to operate normally over about 70% or 17.5mA, so that will be my limit then
Thanks guys!
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Your tube should have a working current and a don’t exceed current.
If you are within the current specifications of the tube, you should be good.
17mA for an 80W tube seems ridiculous to me… I’ve gone by this chart for ever since I started messing with these and it’s pretty accurate.
You have an 80W tube, so 0.7 (70%) * 80W is only 56W… what happened to the available 80W tube you paid for?
Kind of like selling you a supercar and advising you not to drive over 60.
Glad you’re up and running anyway…
Good luck
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That’s my thoughts exactly.
Thanks guys. Last night I noted a chart in the manual with estimated settings for cutting plywood running at 25mA, so I’m not going to feel bad running up to that
Friends, don’t let it be too academic or too random, test your machines to find out the effect curve of your tube.
The best way for ME is to put an thick acrylic block in the machine and shoot a test series with 5 or 10% interval power variation. The pulses can be one or 2 seconds, it is actually inferior.
The depth of penetration peaks at approx. 65-70% (at my new 60 watt tube) and from there it no longer rises.
There is no sense to run the tube with 90% power when the effect tops at 70%. The numbers will vary between machines and tubes but the characteristic of the power curve is the same.
The picture shows a test from today, curve starts with 10% and has an interval of 5%. At 55% - the tube is at its top, it is somewhat smaller than when the tube was new, but still fine for my daily work.
55% correspond here to 14.5 mA It is also 2ma smaller than the new tube has present.
At 75% I get about 18ma.
The poent is that anything over about 14+/-0.5 mA / 55% is without more effect. (on my machine)
PS. I haven’t “downgraded” calibrated my HV supply and all parameter for the test is set in ruida and not in LightBurn.
I did the same thing with a watt meter… Haven’t looked at it since I put the new tube in it…
Unfortunately you can’t tell wattage out with this method, although sound for it’s application. Not that it really matters, but I like to know if I got the shaft.
Excellently point to bring up…