[R-390] Bucking Transformer / Autotransformer Question

[Bob kb8tq]

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
>>> ______________________________________________________________
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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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