[R-390] Isolation xfmrs available

[Gary Gitzen]

Hi R-390,

Please do not reply to the sending address of this mail. It has
been compromised by an infected computer on the R-390 list.
All mail to that address is trashed.
Please send all replies "off list" to "user3 at uwave dot com".
Please do _not_ add it to your address book. Thanks!


I recently came across some nice isolation transformers, and
want to give the R-390 list first crack at them.

First, the basic specs:
	Input:	90-126, 180-264 VAC, 47-63Hz [Two windings]

	Output #1: 90-126VAC, 300VA
	Output #2: 90-126VAC, 300VA
	Output #3: 90-126VAC, 80VA

	Dual bobbin & dual "U" core construction
	Stranded, PVC insulated, color coded wire leads.
	Size: Approx 5" X 4 3/4" X 4 3/8"
	      Flat sheet steel mounting base approx 5" X 4 3/8",
	      mounting holes on approx 4 1/16 X 3 3/4" centers.
	Weight: approx 13 lbs.

Now the gory details:
The two input windings are identical, each about 1 ohm. The high
line voltages don't track because the specs appear to include the
effect of winding resistance voltage drop in the series (180-264)
configuration.

Two of the output windings are identical, each about 1.6 ohms.
The output voltages at no load are about 5% high to compensate
for voltage drop under load.
Output #3 has similar characteristics, just lower power.
Calculations indicate that under full load the output voltage
will equal the input voltage.

So these would make a nice isolation transformer from either
120VAC or 240VAC to 120VAC, with a rating of 680VA.
Lots of power for three R-390As even with heaters on.
Caveat: when lightly loaded, the output voltage will be 5% high.
At 122V in, the output will be 128-129VAC.
But: that 5% voltage difference also works the other way.
If input & output are swapped, a 122VAC input becomes 116VAC out
at no load, and will drop roughly 1.25V/A due to winding resistance.
The end result is that with a line voltage of 122-125VAC (not
uncommon today) the output voltage under moderate load will
be 113-117VAC; just about perfect for older vacuum tube gear. 

"But I want the same voltage in and out, without that 5% step up
or step down." Simple: series connect one "input" and one "output"
winding and use it as the primary. Series out the other windings
as the secondary for a 1:1 isolation xfmr. Use the 80VA output
winding in parallel with the 300VA winding for the "primary".
Feed with 120VAC in and get 120VAC out. Caveat: the transformer
will only be good for 300VA in this configuration, and will have
more resistive voltage loss. The last can be good, if you want
closer to 115VAC.

For those of you who want or need to use GFIs to power your
R-390X, one of these transformers can be ideal. Connect the
input and output windings in series (as in 240V), but apply
120VAC to the inputs as suggested above. Ground the center tap
of the two seriesed output windings, and use the two "hot" legs
as a 300VA grounded center tapped source for your R-390X.
Reverse input & output if desired to reduce output by 5%.
Connect the output legs to a GFI for added safety in a basement
or outbuilding. This gives 120VAC in two balanced 55-60VAC legs.

Now the pricing:
	Quantity 1: $15.00
	Quantity 2: $28.00	$14/ea
	Quantity 3: $39.00	$13/ea
	Quantity 4: $48.00	$12/ea
Thanks to the miracle of USPS Flat Rate shipping, and the fact
that _four_ of these will (just) fit in an FRB1 box, shipping
for one to four of them should be $8.10 to "domestic US".
It is unclear to me if "domestic US" includes Alaska & Hawaii.
You folks who live there probably know.

[ I can just imagine the postal guy/gal lifting a 52 lb
box of transformers. :-) ]

Terms:
Quantity available per person will depend on demand.
I don't have a lot of these. I'd rather each person get one
than some people get four and others get none. Quantities over
one will be available only after "one of" requests are filled.
If demand exceeds supply, a random drawing will take place
and/or quantities will be reduced.
Unless demand exceeds supply before then, orders will be
accepted until midnight Pacific Time, Sunday 10/8/06.

Payment must be made in advance. Personal checks are OK.
Details to be arranged after orders close.

If you are interested, please reply off-list to
"user3 at uwave dot com".

Gary Gitzen
San Jose, CA