Finally, another Shenhui question

[Greg Facer]

At long, long last, I have a working tube again! Hitching a ride on another small laser a student was buying, I got my tube sea shipped for the cost of me handling the paperwork (and dropping it off too in the end).

So, I now have the situation that I used to have a 150watt V8 model RECI tube, and now I have a 150watt W6 RECI tube. I think I will need to adjust the power supply for a lower maximum output, but I didn't have an ammeter included with the machine. So, off to ebay I went. I bought the ammeter and shunt that looked correct for the amps (although low voltage, so maybe I chose wrong?), current item number 190684152582.

Anyways, hooked up as per instructions, and no reading. The wires are a little long to the meter, 16ft I haven't cut down yet...in 18 gauge wire. The laser fires fine, so that part of the "shunt" connection is fine, and the meter has the right power going to it to light up etc.

I should mention that I used the low voltage side of the laser circuit, not the high voltage side.....I don't think the shunt I have would work on the high voltage side. But, I just saw a post on CNCzone were the shunt was arcing and that doesn't happen on the low voltage side!

I'll start googling after lunch, but if anyone knows about ammeters and shunts, let me know! Even sending photos of your Shenhui or other glass tube laser with the ammeter installed and how it is wired would help.

Thanks everyone! Pretty excited to have a working machine again.
Greg

[matthew knott]

few things i notice here, the ammeter you have chosen is 0-50 AMPS, you probably needed mA meter, even 1 amp on a glass tube would be cooking it. Also you need an isolated supply to run this, make sure that this is the case. Its probably much easier to use and old school ammeter (with a needle and built in shunt) you only have 2 connections to make, probably 0-50mA would be about right, i need to check the max current of your tube.
Good luck, but i dont think that meter will work for this (unless im missing something)!
cheers

[Rich Harman]

Why oh why did you not have Shenhui toss in an ammeter with your order?

The low voltage side is correct. Current is equal everywhere in a series circuit, voltage is not.

You need a milli-ammeter. The one you bought is for up to 50 amps. That means is will have a shunt resistor capable of carrying more than 50 amps of current. You need to measure currents that are (more than) one thousand times smaller. Item 310480480916 is the kind you need. Looks like the same one is on Amazon (http://www.amazon.com/Amico-Class-Accuracy-0-50MA-Ampere/dp/B009PKE1VK/ref=sr_1_2?s=hi&ie=UTF8&qid=1350596491&sr=1-2&keywords=Amico+Current+Measuring+DC+0-50MA).

The shunt is a precisely calibrated bar of copper where it's exact resistance is known, commonly measure in milli-ohms. Whenever current travels through wires (anything) there is a voltage drop. An ammeter measures the amount of voltage that is dropped across the shunt. Using ohms law you can calculate the current because you know the voltage drop through the shunt and the resistance of the shunt. So when you pass only 30 milliamps through a shunt that can carry almost two thousand times as much current the voltage drop measured will be infinitesimal.

[Greg Facer]

oops!

Ok, for some reason I thought it WAS in amps, but thinking again.....that would probably blow my panel, wouldn't it! I'll reorder a milliamp one. Probably still digital though, I'll have to see.

Any yes, I should have had it included, I just assumed they all came with one, and you know what they say about those who assume!

Thanks!

[matthew knott]

you can use the meter you have, just use a 1.5 ohm resistor instead of the shunt, the numbers would them be reading in mA, ignore the little 'A' symbol on the meter. Use a 0.1% resistor and it will be pretty accurate. Saves buying a new meter and will work fine.

[Rich Harman]

Any yes, I should have had it included, I just assumed they all came with one, and you know what they say about those who assume!


I meant with the replacement tube, not the original order.

[Greg Facer]

Rich, since we were on the third attempt to replace the tube, I didn't want to complicate things any more than I had to. I did ship back a power supply that I considered shot and they wanted to check over. They did send the same one back, but I won't know if it's fixed or working until the installed one fails.....and power supplies are easier to ship courier.

Thanks for the resistor idea, Matthew. I've already shelled out my $10 for ebay for 2 of the proper ones, but the digital would be nice, so I'll try that when I can pick up a resistor. Any type to get, as I know nearly nothing about electronic components except the basic functioning.

Greg

[matthew knott]

Any type should do you, it will just replace the shunt, the meter you have acutall measure mVolts, 75mV = 50 Amps, with the new resistor you will have 75mV @ 50mA, that will be the full scale defelection meter.

[Greg Facer]

Works good! Until I look at the power supply, I'll just make sure I'm at 88-89% and I'll be good even with the 1% error allowance (best I could get locally).

Thanks a bunch!

[Greg Facer]

What's the rule on swearing here? My new W6, with ammeter hooked up and working and current within W6 limits, has crapped out with a pinhole leak at the high voltage side. Needless to say, not so sure getting a laser was a good idea at all anymore. For those keeping track, this is laser #4, and after comparing laser 1B (in the other machine we bought), it might not be working 100% either.

So, those of you who wish to know the worst case other than the whole machine destroyed or your money stolen, this is it! Some photos for your viewing pleasure. I am going to look up neon tube repair next, see if someone can't repair these things, refill with gas.

Oh, and if anyone needs parts, let me know. I'll likely get these shipped by boat, because at least it arrived in one piece, happy to share the customs costs with someone and ship out from Vancouver, BC.

Greg

[Ronald Erickson]

I'm a total laser tube newbie, so this question may come off as really dumb... but is the tube being cooled? I have zero experience with laser tubes but if it melted a hole in the glass it seems like it's getting way too hot.

Maybe someone can educate me on how the glass tubes get cooled. I know they are water cooled, but I don't know the path the water takes. I'm making an assumption that the rubber hose in the lower right of the first picture is the coolant connection, and the water would surround the glowing purple glass coil, but I may be completely wrong in my guess. Does the water completely surround the glass tube innards? Or is the glass tubing inside where the coolant flows?

Feel free to slap me around a bit and teach me a lesson about how they work with regards to cooling. :)

[Greg Facer]

Hi Ron,

The laser was getting cooled, chiller/pump working. Even so, it only ran for about 2-3 seconds of the job before failig. Admitted, the cooling starts about 3/8" further down the tube. But, that is the way they are made. I'm glad that it didn't break the water portion, that would be scary. It could still have be some kind of thermal shock, but probably a weak spot from how the glass tubes are put together.

I am VERY frustrated, but truthfully, none of the problems could be detected beforehand, the way the Chinese test (open stand, not in a machine). Shipping damage is unavoidable some of the time with glass tubes too.

Looking at ALL my options on this one.

[Rich Harman]

Wow, is that glowing molten glass in the second picture?

I think I would be asking that they send two tubes next time.

[Greg Facer]

I don't thick it was molten glass, but I'm not sure why it arced and energized the reservoir only a few seconds and then changed to flaming.

My current plan is to pursue the warranty issue again, possibly on both this tube and the older tube I thought was working ok. Regardless though, end up with two w6 tubes and use the V8 as the spare. I have also ordered a Coletech 80watt for a fill in for $700. The thing is, the downtime can be quite low on these machines if you a) aren't dealing with warranty issues and therefor not paying for the replacement and b) can live with the slower speed of an 80watt versus the 150. Because 80w are cheap and readily available everywhere including stocked in north America, i'll probably have something in a week (unbroken I hope, but I trust Coletech more in that regard).

The warranty thing will probably take weeks to sort out, although maybe not, but I can't let that dictate what I'm doing anymore. I was having a pity party that is silly when $700 gets me back up and running quick. The tube is supposed to be rated 8k hours, might be RECI without the branding, so shelf life maybe will be ok.

[Greg Facer]

Just to update my ongoing saga of the worst laser story save for a complete scam, I received laser number 5 (replacement #4) and it was a) the wrong model in that it was a v6 130w not a w6 150w and b) broken due to poor packaging that allow movement inside the box/crate.

Communication was terrible this time, found out the laser had arrived with no advanced warning from Shenhui, had to ask for paperwork to be sent and it was late.

Naturally I have documented it and sent an email to shenhui.

I have given Shenhui the option of refunding my money (for the laser portion) at this point, maybe I can drive to a NA reseller and pick one up there. Cost will be 1-1.5k more but the customs costs / local fees / taxes on broken tubes are probably getting close to 1k anyways. Or just use the working 80w for now and move on.

Bleh