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Old 10-29-2012, 04:49 PM   #46
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Originally Posted by knc1 View Post
Those large, red font, printed stickers inside the microwave that say: "Danger High Voltage" should be a big hint about being careful.

Despite that, posts on the 'net can be found about people being electrocuted by those high voltage transformers.
Evidently they either couldn't read, or thought that warning label did not apply to them.

The key thing to remember, is that high voltage secondary is going to be cut away and replaced by the DIY person.
WITHOUT ANY "lets just plug it in to see if it still works" adventures. ...
I used to hook a vacuum tube audio transformer in reverse (for high-voltage step up), in series with a flashlight battery D-cell and a small buzzer, and connect the high voltage wires to my temples (next to my eyes) to see colorful "sparkles". Perhaps that explains my behavior.

Of course, that was nothing like when I woke up on the garage floor after an unknown amount of time after getting zapped by my 15KV neon-sign transformer "jacob's ladder", completely blind, with colorful pinhole "static" in the all-white visual field appearing and spreading, as vision slowly returned.

And those are extremely tame compared to many of my "near death" adventures.

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Old 10-29-2012, 05:25 PM   #47
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An interesting side note:
I have been viewing a lot of 3D, DIY, CNC devices recently.
Many of them are using CO2 lasers -
Which ignite at 18Kv, run at 28Kv (and up).

Haven't read anything about "beware of high voltage".
I also haven't read about anybody DIY'ing the power supply for that gas tube.
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Old 10-29-2012, 07:24 PM   #48
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I have not tried firing up my 40W CO2 tube yet. I was thinking of testing it with something from my collection of neon sign transformers (per Sam's Laser FAQ):
http://www.repairfaq.org/sam/laserco2.htm
Quote:
A Variac may be used for voltage control. Even many commercial CO2 lasers have used a manual or motorized Variac and neon sign transformer or phase control of a neon sign transformer for this.
Sadly, the variac I had since a young child has been shorted from copper vapor plating where not wanted, since I incorrectly hooked the input AC to the variable side some years back (which vaporized the winding on one end).


The chocolate printer seems a lot safer though...

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Old 10-29-2012, 07:37 PM   #49
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The chocolate printer is alive once again, with the author working on a "version 2".

Myself, I like the serious challenge - I want to print in colored glass.
NOT **on** but **with** colored glass.
I.E: 3D printed art glass objects.
There are some serious engineering challenges involved when printing with glass.
(Which might be why I haven't found anyone with DIY directions.)
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Old 10-29-2012, 08:02 PM   #50
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Quote:
Originally Posted by knc1 View Post
The chocolate printer is alive once again, with the author working on a "version 2".

Myself, I like the serious challenge - I want to print in colored glass.
NOT **on** but **with** colored glass.
I.E: 3D printed art glass objects.
There are some serious engineering challenges involved when printing with glass.
(Which might be why I haven't found anyone with DIY directions.)
A couple of those "engineering challenges" are that the glass must cool slowly to prevent cracking or crazing, and you need high-temperature lubricants to operate your ball-screws inside a kiln.
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Old 10-29-2012, 08:28 PM   #51
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What kiln?

The plastics used in DIY 3D printers generally have a melting point around 200c
Glass melts around 1400c
That can have a very negative impact on electronics and mechanicals.

And it has a very wide temperature / viscosity range.
If you've ever formed glass by hand (and I suppose you have), you know what that means - it will "slump" on you at any moment, without warning.
It sort-of gets tired of being handled and lays down on the job.

The art of hand formed glass takes decades of apprenticeship for a good reason.
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Old 10-29-2012, 08:35 PM   #52
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Large glass objects (such as a glass sphere/crystal ball over 6 inches diameter) need to be cooled in a kiln slowly over many days. MineMy crystal ball is larger than that, which is why it cost $600 thirty years ago.

In the case of printed glass, uneven shrinkage during cooling could be problematic at best, and slow cooling in a kiln is the "usual" approach. Unfortunately, my electric kiln got its nichrome wire elements broken last time it was transported (about 35 years ago) and I never got around to fixing it.

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Old 10-29-2012, 08:44 PM   #53
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Yup, like I posted, plenty of interesting technical challenges in this project.

I did not consider printing it inside of a kiln, but I do recognize the need to 'cool' it in a kiln -
a general anneal and stress relief stage/step.
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Old 10-29-2012, 09:10 PM   #54
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Yup, like I posted, plenty of interesting technical challenges in this project.

I did not consider printing it inside of a kiln, but I do recognize the need to 'cool' it in a kiln -
a general anneal and stress relief stage/step.
Cooling during construction would fracture bonds between layers if the printed glass is not kept pliable during the entire print operation. And then moving the printed object to a separate kiln would cause extra stress fractures. Which is why I would actually print inside the kiln.

Perhaps an internally cooled "robot arm" extruder that reaches through a port in the side of the kiln could suffice. The arm may need internal heat-pipes routed to an external heat exchanger.

I am sure other complications will arise on the iterative path of discovery. But first, to google it... Somebody else may be thinking along those lines and could provide useful advice for such a project.

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Old 10-30-2012, 12:36 AM   #55
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I wanted to toss in a brief suggestion to use Kapton (or similar) tape if you are messing about with transformer windings. It is an excellent insulator and can withstand fairly high temperatures that often are seen at either high amperage or voltage. Also very good for masking off PCBs when soldering/desoldering in a confined area.
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Old 10-30-2012, 06:23 AM   #56
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Originally Posted by cavorite View Post
I wanted to toss in a brief suggestion to use Kapton (or similar) tape if you are messing about with transformer windings. It is an excellent insulator and can withstand fairly high temperatures that often are seen at either high amperage or voltage. Also very good for masking off PCBs when soldering/desoldering in a confined area.
The link in the post above is to Kapton (tm) tape, only without the Dupont registered tradename.
That particular product uses a silicon adhesive good to 260c.
See datasheet attached.

Which brings to mind another safety note -
The primary winding (the 'mains', or the 'line') is going to be good old fashioned enameled copper.
Probably intended for a 40c rise in a 20c environment. I.E: 60c maximum.
So consider that (60c) your maximum transformer temperature, not the maximum working temperature of the hand wound secondary's insulation tape (260c).

Cooling:
Your wrapping the secondary with copper strip + Kapton tape on one side, only the "outside" end of the 'coil' is easily accessible.
When you reach your 'turns count' mechanically anchor the strip+tape (so it does not 'buzz' too much), drop the tape cover, bring the strip out an appropriate distance to the other (supported) wire terminal connection.
Now arrange a fan to blow air across the copper strip - I.E: the secondary coil's copper strip becomes its own cooling fin.

Losses:
Running a laser diode is, well, one diode drop (about 2.2volts - in general, think of the range 2v .. 3v) ;
The supply needs to be a constant current supply ;
A bridge rectifier circuit is 2 diode drops ;
An 'on' SCR has a significantly lower diode drop than even a Schottyt rectifier diode ;
So 'secondary control' is the low loss way to go at these high currents rather than the 'primary control' ideas you find in the on-net articles.

And you are talking high currents for laser diodes.
A typical 1W (output power) laser diode has a 2.2volt drop.
It would be nice if it only took 1/2 amp to drive it -
But laser diodes are only 10% .. 18% efficient -
In the case that I am looking at, it takes 5 amps (5 x 2.2 == 11 watts) to drive it to 1 watt output. The other 10 watts is wasted as heat that you have to remove from your laser diode.

A bit more:
So you decided on secondary control of the bridge rectifier (good move) ;
Low current SCRs (anything less than a few hundred amps is 'low current' for an SCR) will probably have an 'on' junction drop of 1.2 .. 1.8 volts.
Presume 1.8v here (because this is only a 'thought experiment').

So your voltage drop chain looks like:
1.8v + 2.2v + 1.8v == 5.8v
Your power loss (at 5 amps) looks like:
8 watts + 11 watts + 8 watts == 37 watts, 1 watt of light, 36 watts of heat.
Those are: Rect SCR + Laser + Rect. SCR voltage and power vectors.
Call it 40 watts consumed.

The transformer for an 800 watt microwave has enough iron in its core to support that flux density.
So if you could (you can't) just scale the above, then as a 'first guess' think of supporting a 20 watt output power laser diode.
Yeah, the other 780 watts is wasted as heat from something (including the transformer - one reason for the (you can't) above).

How many turns?
The bridge rectifier needs a center tapped winding, at least 6 volts per side - since this is an actively controlled bridge, 8 volts per side (no load) would be nice to have.
Transformers for microwave ovens are run at a flux density that amounts to about 0.8 .. 1.0 volts per turn.
Naturally you would make a small gauge wire test coil of a known turn count to check what your transformer is working at.

Err on the side of two many turns, you can always just 'dial back' the rectifier bridge controller -
0.8 volts per turn, 10 turns per side. I.E: a 20 turn, center tapped, secondary , OR
1.0 volts per turn, 8 turns per side. I.E: a 16 turn, center tapped, secondary.

Now you have a lot of the example ballpark figures -
All you need now is to get out the reference chart to figure out what the cross sectional area of the copper strip needs to be for a less than 40c rise, tightly packed, coil operating at the currents you need for your project (a 20watt output power laser diode might require 100 .. 140 amps drive - just about the limits for a microwave oven transformer core.

Way off topic - but more fun than making serial port connectors.
Attached Files
File Type: pdf Polyimide-Tape.pdf (35.8 KB, 25 views)

Last edited by knc1; 10-30-2012 at 06:46 AM.
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Old 10-30-2012, 01:13 PM   #57
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I want a microwave serial cable!!! made out of cnc'd glass.

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Old 10-30-2012, 03:19 PM   #58
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I want a microwave serial cable!!! made out of cnc'd glass.

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I recommend copper vapor deposition inside the glass, to help guide the microwaves.
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Old 10-30-2012, 06:55 PM   #59
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@twobob : your microwave runs at the low end of the radar S-Band. That microwave serial port is physically larger than you might think.

@geekmaster : too much surface loss at S-Band for copper, your choices are silver or gold, but not too much of it, due to the "skin effect" at S-Band (and above / below S-Band).

PS: Do not use "Brasso" to polish the silver (or gold) - it leaves a film that destroys the surface conduction of the metal. Been there, made that mistake; spent a long weekend taking the radar cavities back apart and cleaning off the "Brasso" film.
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Old 10-30-2012, 07:18 PM   #60
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@geekmaster : too much surface loss at S-Band for copper, your choices are silver or gold, but not too much of it, due to the "skin effect" at S-Band (and above / below S-Band).
I only said "copper vapor" in lieu of my little "variac" story above, to fit in with twobob's off-topic topic concatenation.

I have a bunch of old coax with silver core and silver braid. But it is obsolete. I use LMR-400 these days. Very limited waveguide collection at this time...

And besides, a glass waveguide would look prettier with gold than with copper.

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