Milliammeter for laser

[john banks]

The pot controlled laser engravers are usually pictured with a milliammeter. Is it wired in series with the laser tube? If so, how do they route the wiring safely behind the panels and connect to the ammeter to avoid arcing given the 20 to 30kV? Or do they read a proxy current from the low voltage control side of the power supply and infer the tube current?

Reason is that I have heard of lasers getting overpowered by the power supplies that come with them. Limiting to 90% power in software is one thing, but it would be like driving my car at 90% throttle opening because at 100% the engine would knock itself to pieces because I hadn't tuned it right ;) I gather some power supplies have a pot to calibrate them but when the software controlled setups don't usually have a milliammeter you'd be guessing.

When spending good money on tubes you'd want to limit yourself to the current through the tube at which they are rated for their advertised lifespan I think.

[Rodne Gold]

All the machines have a way of setting max current , they can do so from software that can read and write parameters to the control card that always stay in memory and from the lcd panel. If you have set this to lets say 90% , no matter what you do when specifiying power in engraving software , they will never override this setting. The power supplys are almost always NEVER set to deliver max tube current anyway , this is the pot thing. The pots are used mostly to balance twin tube setups so each one is exactly the same power as the other. Not really a "lets wind it up to full blast" thing.
So you can basically work on power delivered being about 85% of "rated" power if you take upper limits and losses thru lenses and mirrors into account. My RECI tube was delivered as an 80w and it was measured at 89w , so it actually delivers 76w - still cheap for the $450 and if you take a "cheap" glass tube , at 80w , lets say it only delvers a TRUE 60w, its paltry money per watt at $170 or so.
I doubt the ammeter is EVER connected to the HT side of the tube, the HT side can create arcs that can jump 5cm in air to an earth , I havent checked its exact wiring , but there are no thick red leads to it.
If you drove your car at WOT all the time , it's engine would most likely also have a very much shortened life as would some of the other systems like braking etc.

[paul mott]

John,

The milliamp meter connects in series with the thin GND return wire from the tube to the PSU.

Fitting one is really a job for a qualified electrician - Remember that Confidence is no substitute for experience when it comes to potentially high voltage stuff.

Paul.

[Rich Harman]

John,

The milliamp meter connects in series with the thin GND return wire from the tube to the PSU.



That is how mine is wired.

[john banks]

Thanks all.

Machine won't arrive until January, but I'd like to see the current actually supplied to the tube so an ammeter would be a good install. So if anyone happens to be around their laser with a camera and can see the back side of the ammeter to photograph or comment on how well they have insulated the connections and wiring that would be most interesting.

Rodney, I'm off topic, but some would say that if you can't use full throttle all the way down the straight then you don't have enough traction, and if you don't have black lines painted all the way down the straight then you don't have enough power ;) Regardless, if I'm doing the engine bit, I don't have it as the limiting factor, if the car has to change its tyres, brakes or let the transmission cool down then my stuff isn't the weak spot, but I want the driver to be able to go WOT everywhere they dare for as long as they want. Same with the laser, so I need to know where my requested power coincides with the current level that the tube is expected to last a good time. I feel that knowing the current gives me the result, setting the power on the controller is only my request. Depends really on how well Shenhui set them up, whether the power supplies I get are offical RECI ones or not etc?

[paul mott]

John,

There are no special insulation requirements for the wiring or the connections (this wire is at near GND potential in operation).

My first milliamp meter was digital (very cheap China import) but did not perform well at low current settings so I later replaced it with an analogue, moving coil milliamp meter which seems to work just fine, not only that but I like the 'retro' look. ;)

Paul.

213555

[john banks]

Thanks Paul. I like the LED panel as well. Custom?

[paul mott]

Hi John,

Yes, the whole thing is custom - scratch built from the floor up, so to speak. :D

Paul.

[Ken Shea]

How does one calculate the milliamp output to the actual high voltage going to the laser tube?


Ken

[paul mott]

Hi Ken,

With regard to the Chinese glass tubes - basically..... Power = Voltage * Current.

The Voltage from the PSU is (or should be) constant (after the initial strike) and laser output power is then varied by adjusting the current that flows through the tube up to the maximum current that the tube manufacturer has specified for his particular tube. Most laser machine manufacturers preset the PSU top end to the tube max current so that the user can safely use 100% power without risking any tube damage but some, on the Zone, have found that this is not always the case and a milliamp meter has turned out to be a worthwhile addition.

Paul.

[Rodne Gold]

The Shenui's come with ammeters , mine did. The Tube is matched with its PS , in the case of a Reci 80w it is the DY10. I'm cautious as to running at max , if you go to the reci site , you can see pictures of what will happen to the electrodes in the tube if overpowered. Not sure whether lifespan is diminished if you see slight signs of this or not. Reci recently upgraded their tubes , so also not sure what the real lifespan of the "improved" version is. Time will tell , at the moment we are insanely busy with all the lasers , and , touch wood , our chinese ones are running flawlessly on an 8 hr a day schedule.
http://www.recilaser.com/2010/en/index.html
go thru the site , look at the "download" section for tech stuff and the pics etc.

[paul mott]

Some interesting stuff on the RECI site who, I must say, give far more details about their products than most others in China.

Although they expect their tubes to last 10,000 hours they are only warranted 300 days with free replacement within 180 days if it is shown that it has been used below the 85% power setting.
Even the 10,000 hours life expectancy has a maximum current limitation so this perhaps reinforces the need to fit a milliamp meter to any laser that doesn't already have one fitted.

Paul.

[Rodne Gold]

There is a wiring diagram on their site as to where the ammeter is to go.

[matthew knott]

You can fit an ammeter in series with the earthy side of the tube as its very low voltage BUT
Its only low voltage as long as the ammeter is working correctly, the ammeter is a very low resistance coil, if it where to go open circuit or the ground wire was to become disconected (from the meter) two things would happen.
1) your tube wont light as it has no ground path (not so bad)
2) The treminal on the meter that is connected to the tube still is going to be in series with the tube (could be bad)

I suspect if you touched it you would be ok if no plasma had developed in the tube but i would not want to chance it you could be putting yourself in series with the tube.
Basicaly what im saying is its all safe if conected correctly but it only needs a lose terminal on what effectivley looks like low voltage wiring to become high voltage. Its not an ideal setup, but it works and you would have to be very unlucky to get a shock.

[Ken Shea]

Thank Paul,
Still not sure though.
Can you give a calculation example for a laser operating on 240v ac

Ken

[paul mott]

Thank Paul,
Still not sure though.
Can you give a calculation example for a laser operating on 240v ac

Ken

Hi Ken,

Not quite sure what you want to see or the purpose for it.

Paul.

[Ken Shea]

Hi Ken,

Not quite sure what you want to see or the purpose for it.

Paul.

That's because I don't know what I am asking for myself :D

Doesn't the laser tube operate with KV?
Trying to figure how to calculate the KV laser power supply output from a milliamp reading?

If that still does not make sense then don't waste any more time on this.

Oh, I have no intentions of getting nearer to the PS then walking by it, just curious.

Thanks
Ken

[john banks]

Well I'm a laser newbie but I can explain this bit (and hopefully anyone can correct me on the laser specifics):

P=VI

So multiply volts and amps to get watts.

30kV x 25mA = 30000*0.025 = 750W

So the laser is quite inefficient, the rest being lost as heat to the coolant (water/air).

If you take the 230V power supply and check the current there, you could work out the efficiency of the power supply. Many transformers can be around 90%, some better, some worse, but I'm not familiar with those in lasers. If the current draw from the mains is 3.5A, then P=VI again, = 230 * 3.5 = 805W

Efficiency of power supply = 750/805 = 0.93 or 93% (I fudged the figures to give an illustration).

There is much more to get into as there are AC to DC conversions, AC voltage is quoted as RMS (root mean square) to allow the simple conversion I've done here.

[matthew knott]

the voltage and current draw of the PSU are not really relevent, unless you know the efficency of the unit its all guess work. The Amp meter goes in series with the tube, giving the actual current being drawn from the tube.

+25'000V ----[Laser tube]-------(A Meter)-----------GND(0v)

< 25kv > < less than 1 volt >

All the volts are dropped across the tube so the meter is at earth potential, its actually quite hard to get a digital meter to work well due to interference that comes from the high voltage that can cause false readings, a moving coil meter works much better.

[Rodne Gold]

Oddly enough, we had the GCC guys come look at our machines for general maintenance , our 30 W machines are producing well in excess of 30W (37 average actually) using their power wizard meter , our 60w chinese machine we measured at 9 watts.!!!!
We measured using the "laser" button on the lcd panel which Im not sure is full power firing , I messed a bit with the power setting on the lcd panel , but couldnt get it higher than 9. Thing is , we know the 60w machine is NOT firing at 9w , as we reference it to our mainstreams in terms of cutting 1/8th acrylic and its almost 2x faster and the ammeter was measuring 20+ milliamps......
My 80w Reci was busy so we couldnt measure that but when firing at full power its ammeter measures 25+ milliamps.

[Dan Hintz]

We measured using the "laser" button on the lcd panel which Im not sure is full power firing , I messed a bit with the power setting on the lcd panel , but couldnt get it higher than 9.
Run a file at 100P and 0.0000000001S ;) See if you can't beat that 9W figure. I don't know how that button is configured (never took the time to look at it when I had a Chinese system), but like you, I suspect the button is significantly less than full power.

[Bob Curwood]

It is wired in series, but in the negative side of the tube, not the high-voltage side

[Alexa Ristow]

Hi Creekers, specifically John Banks and Rodney

Time has come for me to replace the tube and power supply on my Rabbit 6090 60W. Got about 2500hrs running mostly at 100% cutting mdf.

I would love to upgrade to a RECI W2 with DY 10 power supply ( Rodney??, ) but would like to fit an ammeter to my machine at the same time. in an earlier post, John mentioned that he was keen to do so to his new machine and I wonder if he has done so in the last 6 months?

Regards,

Alexa.

[john banks]

It actually came with one installed.

[Alexa Ristow]

damn pity that,
good for you but bad for a photo session

[Robert Silvers]

http://www.sawmillcreek.org/showthread.php?209026-My-LASER-current-meter-for-DC-tubes&p=2168023#post2168023

[Robert Silvers]

The LASER button puts out whatever power the LCD is set to. If it put out 9 watts you had it set to a low power. You can change it from the LCD.

[Bruce Dorworth]

Mine (GWeike) has a Ma. meter on the front of the machine. Can this be accurately used? I have an 80 watt Reci4 tube. Is the max current listed some where or is that a number that is calculated? Is this max current or should this be a steady current?

Thanks,
Bruce

[Robert Silvers]

Mine (GWeike) has a Ma. meter on the front of the machine. Can this be accurately used? I have an 80 watt Reci4 tube. Is the max current listed some where or is that a number that is calculated? Is this max current or should this be a steady current?

Thanks,
Bruce

Yes, you can use that.

http://www.recilaser.com/en/productInfo/fc9181e93b448cac013b44f792720e31.htm

"The running current must be kept below 28mA. The life span can reach 10,000 hours if the current is kept below 26mA."

[Dave Sheldrake]

Ergo the reason I use Beijing EFR ZX tubes, at 32mA my 1850 is pumping 177 watts (measured with an Ophir IRX20 monitor), the warranty on the tube is the same 10 months BUT it's based on power output and not by restricting the supplied current.(if it drops below 80% of the rated power I get a new tube) the ZX also have adjustable resonator cavities to fine tune the output closer to TEM00 (if you have a DWC)

What your saying is you paid for a 100 watts, got a maximum to keep your warranty of 80 watts that will degrade with use to around 65 after 9 months or so? of course not forgetting the resonator will already show signs of overpumping making the 300 day warranty invalid anyways. All nicely packaged together by a company that doesn't manufacture any of it's own parts and chose to supply a chiller from a series that have a history of early failures while overdriving the tube at a level that could result in a life of 50% of the expected time and just for giggles managing to fit the final lens backwards.

I love Chinese lasers but I also love having somebody less than 6,000 miles away I can yell at when things go wrong :)

cheers

Dave

[Robert Silvers]

Ergo the reason I use Beijing EFR ZX tubes, at 32mA my 1850 is pumping 177 watts (measured with an Ophir IRX20 monitor), the warranty on the tube is the same 10 months BUT it's based on power output and not by restricting the supplied current.(if it drops below 80% of the rated power I get a new tube) the ZX also have adjustable resonator cavities to fine tune the output closer to TEM00 (if you have a DWC)

What your saying is you paid for a 100 watts, got a maximum to keep your warranty of 80 watts that will degrade with use to around 65 after 9 months or so? of course not forgetting the resonator will already show signs of overpumping making the 300 day warranty invalid anyways.

Maybe - it is worth looking in to. My Z4 tube is rated at 130 peak and 100 nominal. Maybe that means that you get more than 100 watts at 30 mA and the 100 watts at something less than 30 mA?

I am not sure that you have to "over drive" the tube to reach the rated 100 watts.

[Dave Sheldrake]

The peak figures are usually the strike power (piercing power on a flow system or industrial) the peak pulse usually also is of low quality (it's not stable) as it's just used to fire the first strike series to get "into or through" the work. It's also the reason when engraving with a DC tube that the direction change is a deeper penetrative point compared to the running area (run some lines across clear acrylic to see an example then view the lines from the side).

A basic calorimeter can be made for around 20 bucks that will give a wide ballpark figure of what your tube is pumping.

Opinions based on experience are useful but often need to be examined using empirical data as the fundamental scientific method places little or no weight on the opinions / beliefs of the observer. If the opinion stands alone and is not testable then, while useful, it doesn't make the grade of theory. Logical fallacies are the bane of research, simply because the new party of interest doesn't know that what they are being told is false or miss-represented.They will accept the statement as truth AND more often than not, following statements as truth when in reality the entire thing is total tosh.

A false statement left unchallenged becomes truth when in reality it is belief, truth/fact cannot be changed, it is not subject to interpretation no matter how many do or don't believe it.

cheers

Dave

[Robert Silvers]

What your saying is you paid for a 100 watts, got a maximum to keep your warranty of 80 watts that will degrade with use to around 65 after 9 months or so?

Yes, RECI discourages use at full rated power and EFR chooses to not give the customer the information about at what power level the lifetime soars. Does that mean the EFR is better or that RECI is over sharing?

8 hours a day at 5 days a week for 10 months is just 1600 hours. So really, they are pretty much saying "We guarantee our tube will last 1600 hours at full rated power." A RECI will probably also last more than 1600 hours at full rated power.