Replacing failed psu? with 60w for 40w tube?- Solved

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Replacing failed psu? with 60w for 40w tube?- Solved

Postby dbdbd2000 » Mon Apr 22, 2013 7:29 pm

Just a few questions that i am sure that would be useful to many people building a laser cutter and of reference when searching.
My laser cutter is based on the gutted internals of one of the chinese engravers found via ebay. It has been cutting fine for at least a year making mdf models, although needed a fair amount of work to get it to work nicely via mach3. (i'll find some photos of my builds)
the original laser power supply i have doesn't support pwm and is a simple on-off trigger, with a pot for varying the current. there is also no mA meter inline with the tube return, only a 0-3A on the front connected to control board.

---------------------------------
now comes the problem
"Today whilst cutting the first half was fine and then the current just dropped away, increasing the pot made no difference to the output and now it will only occasionally fire on minimum current. I believe this is a dead flyback transformer for the power supply as it happened so quickly and the tube was definitely not overheated as cooling was fine."
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If this is the agreed consensus i will be buying another and posting the results, or if there is any easy way to check that this is what is wrong?

I was going to buy either a 40w or 60w supply
my question is with the 40w max 15kv 20ma or 60w 35kv 24ma power supplies
would buying a 60w cause any problems running a 40w tube as long as i kept the current withing the 18ma tube spec even with the higher running voltage.
would this be better in lifespan or harder for fine control (ie welding, higher current machines not always the best as lack fine control for thin material)

i'm mostly thinking if i want to upgrade to 60w in the future (not very likely) as more keen to get the pwm working for raster with a dsp.

Thanks for any input
Last edited by dbdbd2000 on Tue Apr 30, 2013 12:30 pm, edited 2 times in total.
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Re: Replacing failed psu with 60w for 40w tube

Postby bdring » Mon Apr 22, 2013 8:01 pm

I have not used a 60w power supply for a lower wattage tube, but others have. Supposedly you just need to set the current right.

BTW: The place where that pot attached, probably could take PWM too. Mach 3 has a pretty low PWM frequency and may not work well. I have had mixed results.
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Re: Replacing failed psu with 60w for 40w tube

Postby dbdbd2000 » Mon Apr 22, 2013 9:30 pm

When i started a year ago i looked into mach pwm, and decided that it is not possible due to the frequency not being fast enough of the kernal driver. It was several orders of magnitude too slow.

If i remember i did see a converter which could take analog to pwm but i decided it was too expensive almost comparable to a dsp controller which is next on my wish list.

I mostly want to know about peoples experiences of laser failure. from what i have read

laser tube= normally cracked glass or slow reduction in power
power supply=bang(magic blue smoke),nothing works
Wire=check loose connections
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Re: Replacing failed psu with 60w for 40w tube

Postby TLHarrell » Mon Apr 22, 2013 9:57 pm

You say your:

1. tube is 12+ months old
2. you have been feeding it 18mA
3. power dropped off

I'd say laser tube. But as far as the super cheap, crap power supply you have, I'd consider replacing that too. Get both. Replace power supply first. If that fixes it, you've got the tube for when the current one dies... and it's either dead or close by now.
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Re: Replacing failed psu with 60w for 40w tube

Postby dbdbd2000 » Mon Apr 22, 2013 10:15 pm

yes i just no experience of laser tube failure. Its just the way it failed i assume power supply as It cut 3 mdf sheets fine at 19C coolant temp and then went from good cutting to only pulse firing in under 30seconds-tube has no cracks and still fires with pot set at 0.0 v little A. This evening just checked the pot and its good having taken it out of the circuit as it was the first thing i suspected may have arced through.

I have seen a few people ask about uprated psu. i know most the 40 and 50W are identical, the 60w supplies seem a lot beefier with often dual flybacks and as a machine in progress it might be more economical if possible to upgrade it now and then when the tube dies stick in a 60w and enlarge the table.

But if the 60w is very easy to overload the tube and also may limit the power range if the minimum trigger raises, then sticking with 40w seems more sensible.
I'll get one or the other on order, the ebay ones all seem the same design (still rubbish i believe). as i am in the uk i can't get one from usa without exorbitant import costs
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Re: Replacing failed psu? with 60w for 40w tube?- Solved

Postby dbdbd2000 » Tue Apr 30, 2013 12:46 pm

Solved
ok Just got a new 40w power supply through the post this morning and spent the morning wiring it up properly unlike the original machine.

Pleased to say it works fine with very nice on, off and variable power even with the old tube.

whilst wiring i also put a mA meter (from china) inline with the tube neg to replace the original 3Amp power control meter!

Was cutting at 9-10mA at 500mm/min on 3mm mdf for the last year and still cuts the same, so hopefully some life left in the tube.

So just thought if anyone ever get a total loss of power in under a minute but it still fires a marking test spot at 0 on the pot and works when cold, the power supply is most likely to have failed. The other tell tail sign is a slight buzzing which i thought was normal but was apparently the transformer flyback slowly burning through over several hours of use.

Its always nice to follow up a problem with the solution as i don't often comment but get all my info from reading forums so thanks for the help.


For anyone:
the wiring i used was
WH not used
WL line to parallel port output laser pin no. line to test button
WP (microswitch open door to ground) or just WP-G to override
G ground line to parallel ground line to WP line to power pot low Line to test button
In Line to power pot middle wire (variable)
5v Line to power pot high


mA meter on laser output end of tube to neg (thin wire) of laser power supply
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Re: Replacing failed psu? with 60w for 40w tube?- Solved

Postby Greolt » Wed May 01, 2013 12:31 am

Thanks for the update.
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