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RE: Variac

Subject:  RE: Variac
  Date:   Sun, 20 Apr 97 16:24:58 UT
  From:   "William Noble" <William_B_Noble-at-msn-dot-com>
    To:   "Tesla List" <tesla-at-pupman-dot-com>


Those prices for variacs are quite high - I see them at local swap meets
for 
much less than half that (at the high priced vendors).  If I were you,
I'd 
keep looking a bit - perhaps also you could  get someone on the list to 
volunteer to mail you one from one of their more reasonably priced
sources in 
exchange for some suitable consideration.  I saw but did not buy what I
recall 
as a 20?? amp variac in a dented case (it was about 12 inches in
diameter) for 
$85 last month.  There were some larger units there too, as well as some
huge 
rheostats from navy??surplus that would do - they were about $15 if I
remember 
right.  And there is always the cheapo way - get about 6 light bulbs -
say 60 
watt, 150 watt, 300 watt, 1000 watt, 1000 watt, 1000 watt (If you can
get 
bigger than 1000 watt, then use that at high end) - put the bulbs in
series 
with your xformer and provide shorting switches to bypass the bulbs one
at a 
time to raise power level.  There are some advantanges to this approach
since 
the bulbs will protect the load by limiting power (as opposed to
voltage) and 
will also give you a visual indicaiton of true power level.   Edample 
schematic:



   110 AC  
-------------------------------------------------------------------|
                                                                Xformer
                      NO switches                               |
   110 AC
--60W-------\----|-----------------------------------------------|
           |----150w-----\---|
           |----300w-----\---|
           |----1000w---\---|
           |----1000w---\---|
           |----1000w---\---|
           |---------------\---| 
as you can see, to go from 0 power to full power, the sequence of switch 
closures is (numbering switches from top to bottom) 

Switches closed:        1 2 3 4 5 6 7
60w                     x 0 0 0 0 0 0
150                     0 x
300                     0 0 x
1000                    0 0 0 x
1500                    x x x x
2000                    0 0 0 x x
2500                    x x x x x
3000                    0 0 0 x x x
3500                    x x x x x x
full                    - - - - - - x

where X=on, 0=off, and -=don't care(recommend on)

by the way, be sure to read the table in a non-proportional font (like 
courrier) or it gets all shifted around.  Anyway, this is a really cheap
way 
of making a high power rheostat with calibrated steps.  The switches can
be 
toggle switches, or a high power rotary switch, or even relays
controlled from 
a nice rotary switch (or maybe even fiberoptically from a computer if
you want 
to be ultra sexy)
         

Hi,

I`m on my way of finishing a nice 900 watt Tesla coil, I`ve finished the
cap, the RQ spark gap and cleaning up the neons, Im now looking for a
good
variac, there`s a store in Ohio wich sells surpluss electrical eq., I
asked if they had any good 10 amp variacs, they returned me a list of
the
variacs they had:

        For a new 18 amp 0-140v with knob and dial $185
        For a new 10 amp with dial and plate $130
        For a new 10 amp without dial and plate $105
        For a used 10 amp wihtout dial and plate $ 85

        
Are these prices fair, should I go for the 18 amp one, or does anyone
know
a cheaper mail-order place where to get this beasties, I know I can get
them for less at ham festivals but there are none here in MX.