[R-390] Bucking Transformer / Autotransformer Question

[Barry]

Actually, it is DC (output of the bridge rectifier but before the regulator).  Hmmm, that zener idea looks like a good one.

The other, similar, front panel lamp is fed from the regulated side so that one is okay.  I think it was inconvenient to power them both from the regulated side (and would add another 40ma to regulate as well).

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "Bob kb8tq" <kb8tq at n1k.org>
> To: "Glenn Little WB4UIV" <glennmaillist at bellsouth.net>
> Cc: r-390 at mailman.qth.net
> Sent: Thursday, July 16, 2020 5:01:10 PM
> Subject: Re: [R-390] Bucking Transformer / Autotransformer Question
> 
> Hi
> 
> Since it’s AC and not DC, you need a 24V zener and a bridge rectifier to make
> it “bi-directional”
> given the price of all the parts, that’s not a big deal in any way.
> 
> Bob
> 
> > On Jul 16, 2020, at 5:41 PM, Glenn Little WB4UIV
> > <glennmaillist at bellsouth.net> wrote:
> > 
> > View with fixed with font.
> > 
> >   Use a 24 Volt zener to feed the lamp.
> > 
> > -------------    Supply
> >              |
> >              |
> >             /
> >             \
> >              \  Resistor to limit current
> >              /
> >             /
> >             \
> >              |
> >              |-------- 24V reg
> >              |
> >              |
> >           /-----/
> >             /\
> >             --
> >              |
> > ----------------------- Return
> > 
> > 
> > 73
> > Glenn
> > WB4UIV
> > 
> > On 7/16/2020 3:55 PM, Barry wrote:
> >> I have a very similar box that I've been using to power the calibrator
> >> (the equipment I'm wanting to limit the input voltage at the moment).
> >> I'm wanting, however, to provide a separate supply of 115VAC that
> >> doesn't depend on my variable autotransformer.
> >> 
> >> Normally I don't worry too much about that but, in this particular
> >> instrument, one of the 28V indicator lamps is fed from one of the
> >> unregulated supplies (through dropping resistors) and when the input
> >> voltage is 10% high, then that poor lamp also gets over voltaged by that
> >> percentage and it worries me that this will greatly shorten the life of
> >> that bulb.  The unit came to me with that lamp burned out and I suspect
> >> higher than anticipated line voltages may have contributed to a shortened
> >> bulb life - particularly since this lamp can be on a significant
> >> percentage of the calibrator's "on" time.
> >> 
> >> Thanks,
> >> Barry - N4BUQ
> >> 
> >> ----- Original Message -----
> >>> From: "wli via R-390" <r-390 at mailman.qth.net>
> >>> To: r-390 at mailman.qth.net
> >>> Sent: Thursday, July 16, 2020 2:39:00 PM
> >>> Subject: Re: [R-390] Bucking Transformer / Autotransformer Question
> >>> 
> >>> I have chosen a different approach. I use a single AC source for all of
> >>> my
> >>> receivers... they can be powered up selectively since they are all
> >>> plugged
> >>> into a single switched multi-outlet AC plate. The plate is fed by a fused
> >>> switched home-brew box that sports an AC voltmeter, an AC ammeter, and a
> >>> 5A
> >>> VARIAC.  Obviously, this avoids messing with the stdby-off switch on the
> >>> R-390A's, (leaving it on at  MGC or AGC); so the receivers are "turned
> >>> on"
> >>> all the time. The AC plate controls which unit is being powered.  I just
> >>> bring up the AC slowly with the VARIAC, and can monitor the power drain
> >>> of
> >>> the unit being used... and preferentially run everything at 110VAC...
> >>> since
> >>> our local AC can vary up to 128VAC (!)
> >>> 
> >>> When I am experimenting on the bench, the VARIAC box is small, and can be
> >>> easily disconnected and moved to the bench for project work.
> >>> 
> >>> W. Li
> >>> ______________________________________________________________
> >>> R-390 mailing list
> >>> Home: http://mailman.qth.net/mailman/listinfo/r-390
> >>> Help: http://mailman.qth.net/mmfaq.htm
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> > --
> > -----------------------------------------------------------------------
> > Glenn Little                ARRL Technical Specialist   QCWA  LM 28417
> > Amateur Callsign:  WB4UIV            wb4uiv at arrl.net    AMSAT LM 2178
> > QTH:  Goose Creek, SC USA (EM92xx)  USSVI LM   NRA LM   SBE ARRL TAPR
> > "It is not the class of license that the Amateur holds but the class
> > of the Amateur that holds the license"
> > 
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