Tungsten Emitter

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Tungsten Emitter

Postby David Kunkle » Fri Sep 11, 2015 6:53 pm

Looking for a decent passive electron emitter. Didn't find much on a search. Been using 60W incandescent bulb filaments and house current. They actually work OK, but I'm getting tired of them frying. The supporting wires seem to get hot, bend, and then short out on the chamber walls.

New plan:
I already have some of the W filaments in the photo below.

W filament.jpg


From what I can gather for these filaments, all I need is a Variac that can handle the amperage, and then run it at just 2 or 3 V?
What kind of amperage would you expect these to require?

Thanks for any input.
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Re: Tungsten Emitter

Postby Rich Feldman » Fri Sep 11, 2015 7:28 pm

On 120 volts, your household lamp guts will run hotter in vacuum chamber than in original argon-filled bulb. To be quantitative, you could use one as a Pirani gauge. At what reduced voltage and current does the filament's resistance in vacuum match its resistance on standard voltage in original bulb?


You should use a step-down transformer together with the variac, to have good control resolution at that kind of voltage.
How about using, uh, a filament transformer? A low voltage lighting transformer could serve.
Variac could go on either side of the fixed transformer, but its current would be much lower on the primary side.

You can estimate of the amperage requirement without actually heating one of the new filaments.
Hot:cold resistance ratio will be about the same as that of a light bulb filament at similar temperature.
Power will be similar to a light bulb filament of similar size and temperature or emission current.

When you go for first light, be careful. Once upon a time, I burned out a beautiful didactic Crookes tube using a variac and 2.5-volt filament transformer. Had forgotten that that tube's short, robust ribbon filament wanted much less voltage. :-(
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Re: Tungsten Emitter

Postby Jerry Biehler » Sat Sep 12, 2015 6:25 am

Like rich says, you need a filament transformer. A variac will have almost no control at that low end. And if you intend to put a bias on it to actually get it to emit more electrons you will need an isolated filament transformer that can handle the voltage you plan on running the filament at. BMI Surplus has a bunch of these Airco Temescal filament transformers. They are intended for use on electron beam guns for deposition. I got one for $75 from them and I think the darned thing is brand new. They are good for at least 10kv isolation from the primary and are rated at something like 6 volts at 40 amps.

http://www.bmisurplus.com/products/2174 ... ransformer
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Re: Tungsten Emitter

Postby Steven Sesselmann » Sat Sep 12, 2015 11:31 am

Search for electroplating transformers, that's what I was using, a great big monster, 6V 50 Amps. For the filament you can pull one from a commercial microwave magnetron. Somewhat tricky to extract, but after a few you become an expert.
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
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Re: Tungsten Emitter

Postby David Kunkle » Mon Sep 14, 2015 8:03 pm

No electroplating transformers came up on ebay. I decided on a used Airco filament transformer from BMISurplus for about $100. 6-12 volts in the secondary.

Also, a new 110V Variac, 20 A, 0 -130V output, on ebay for $120. I was using a cheapo Menards dimmer on the 110V bulb filaments. Worked well enough, but half the time a filament would short, it would take out the dimmer/rheostat too. Hopefully, between the 2 things, I'll have a much easier time of it with the emitters.

Thanks for the help guys.

Dave
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Re: Tungsten Emitter

Postby Jerry Biehler » Mon Sep 14, 2015 8:12 pm

You can also use a phase angle SCR controller like a Eurotherm 425A to control a resistive load on the transformer. Thats how I am controlling the filament on my e-beam gun.

Note that the Airco transformer is 240v in.
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Re: Tungsten Emitter

Postby Steven Sesselmann » Mon Sep 14, 2015 10:03 pm

David,

That's probably because its old school, they are called "plating rectifier" in the plating trade :)

http://www.ebay.com/sch/i.html?_from=R4 ... r&_sacat=0

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Re: Tungsten Emitter

Postby Rich Feldman » Tue Sep 15, 2015 12:56 am

Note that the Airco transformer is 240v in.


Jerry, is David's new transformer the same model as what you got? Would be handy if it comes with a dual primary (120 P / 240 S), but today's BMI catalog pictures look unpromising. If 240 only, David can still easily get up to half or 7/12 of nominal voltages, which ought to be plenty.

The pictures clearly show dual secondary windings, presently jumpered in series. Red potted shape suggests that there might be HV isolation between the two secondaries. Don't know about e-beam applications, but that could be handy for powering rectifier filaments. David: if you ever connect the secondaries in parallel, it would be bad to get the phasing wrong.
21745_1_.jpg
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Re: Tungsten Emitter

Postby Jerry Biehler » Tue Sep 15, 2015 1:19 am

No, they are intended for running on 208/240 only. They were never expected to run on 120v since the power supplies that these worked with required three phase 208 or 240 to run. You could run these at 120 and get 6v out probably up to about 70 amps. They are rated for 840VA at 240 in. The windings are certainly isolated from each other, I just dont know how well. they also have shielding between the primaries and secondaries too.

These are intended for electron beam deposition systems like the old Airco Temescal CV-8 and CV-14. I am putting together an e-beam system in my large chamber at home (26" dia, 30" tall, 2000l/s turbo) to do dielectric coatings and stuff. I ended up getting a Thermionics electron beam power supply that will output up to 1.5 amps at 10kv. To power it I bought a 12.5kw 3 phase generator, the e-beam gun I am using can only handle up to 12kw so I should be fine. More info here: https://hackaday.io/project/190-vacuum- ... deposition
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Re: Tungsten Emitter

Postby David Kunkle » Tue Sep 15, 2015 5:39 pm

S_Sesselmann wrote:David,

That's probably because its old school, they are called "plating rectifier" in the plating trade :)

http://www.ebay.com/sch/i.html?_from=R4 ... r&_sacat=0

Steven


Found 'em. Only thing is, even for a used one, it's still quite a bit cheaper to go with a filament transformer.
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