On Fri, 2 Jan 2004, James Lerch wrote:
Greetings All,
Just thought I'd share the initial results on my latest project, a 24" mirror
coater :) Today I fabricated the Square O-Rings from stock 10mm O-Ring cord,
and ran the first leak test (which it failed, but not to terribly bad, I still
made it down to the 18 torr range on the rough pump..)
I note for ultrahigh vacuum copper seals are used. Just a thought. Also,
you cannot mean 18 Torr. I presume you mean something like 10e-3 Torr,
which is characteristic for a common roughing pump.
I would be interested in your solution for firing up the tungsten
filaments, presumably you used (green) TIG welding electrodes. I am
testing this now. I note on your web page you mention wanting to improve
your heating control. I have built a high power (light) dimmer
circuit that works pretty well that uses minimal components and is
dirt cheap (a couple $USD). The thinnest TIG electrodes around here are
1 mm diameter and about 150 mm long so there is not much resistance. On the
other hand they are so cheap that one can use 10 or 15 of them in series
at which point the circuit cold resistance actually increases to sanity
levels, comparable to a light bulb. My current plan is to get the whole
series working exactly right and then replace the linear dimmer potentiometer
(there is also a threshold pot so the dimmer pot changes TIG temperature
thoughout its entire range) with a switch between two resistance values that
give either low heat (melts aluminum and wets the electrode) or high
heat (vaporizes molten aluminum). In other words, the final control unit
will not have a range of temperatures. It will only have two; melt and
vaporize. Although, I will keep the dimmer pot as a third setting in case
I try other adventures (anyone that know me at all knows this is likely).
If you or some else want the circuit I can arrange this. Basically,
it uses a BT139 triac (600 V @ 16 Amp) and a diac tuned by a couple
of resistors and potentiometers that regulate the rate of charge/discharge
of a capacitor (600 V @ 33 nF) that triggers the triac. Circuit includes a
filter for radio interference, but this can be skipped because high power chokes
add signicantly to the price and might not even be needed. I think if you
get noise you can just as well try turning the potentiometer a tad. I have
now made a PCB layout and will post the final result on my website when I
get more time. It is built for 230 VAC mains, but can be adapted for 120
VAC. I of course welcome any criticism of such a circuit for this application.
Dominic-Luc Webb
