[R-390] Isolation xfmrs available

[Jim M.]

I wonder if using a low-cost UPS (battery backup computer supply - as a 
substitute for an isolation transformer) would work for the 390 to prevent 
GFCI tripping?


----- Original Message ----- 
From: "Gary Gitzen" <r390a at uwave.com>
To: <r-390 at mailman.qth.net>
Sent: Saturday, September 30, 2006 11:43 PM
Subject: [R-390] Isolation xfmrs available


> Hi R-390,
>
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> Please send all replies "off list" to "user3 at uwave dot com".
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>
>
> 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
>
>
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