[AMRadio] VTVM Battery Eliminator


John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Thu Jul 20 19:45:23 EDT 2006


The thing about batteries is they give great isolation.  A VTVM uses a
differential amplifier for the metering.  It may be a problem getting
another supply in place with the isolation that is needed.  I would need to
study it a little.  It could be difficult or it might be easy.  If either
side of the battery goes to the common ground then easy it is.

John

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jim Candela
Sent: Thursday, July 20, 2006 3:21 PM
To: Discussion of AM Radio
Subject: Re: [AMRadio] VTVM Battery Eliminator

Don,

   I never tried this idea but here is what I would do. I'd try to simulate
a 1.5v battery where it was a floating supply with low noise, and low source
impedance (Rs). I would take an isolated DC-DC converter, say 12 volts input
to 5 volts output at 100ma. Then I'd use a 3 terminal adjustable regulator
like a LM-317 (or any of the many similar parts). This regulator works down
to 1.25 volts, and it is simple to adjust to 1.5v with two resistors. So now
you have a floating 1.5 v source that is clean, and has low source
impedance. To get the 12 volts I'd use a 1/2 wave voltage doubler off the
6.3v filament circuit and another LM-317 to provide 12v to the DC to DC
converter.

  There might be easier ways for sure, but emulating a floating dry cell, if
done properly will work. Some ohm-meters may pull 100 ma or more on the R X
1 range so you need to maintain 1.5 volts at whatever the shorted test lead
current is when on the R X 1 range.

Using a fresh battery once a year might be easier! :-)

Regards,
Jim

----- Original Message ----
From: Merz Donald S <merz.ds at mellon.com>
To: Discussion of AM Radio <amradio at mailman.qth.net>; 'Glowbugs (E-mail)
<glowbugs at piobaire.mines.uidaho.edu>
Sent: Thursday, July 20, 2006 11:26:52 AM
Subject: [AMRadio] VTVM Battery Eliminator

Going through these old magazines, one idea that pops up a few times is
replacing the OHMS battery in old VTVMs with some sort of regulated 1.5V
derived from the VTVM's AC supply. The circuit designers seem to be a bit at
odds over the best way to accomplish this.

Has anyone does this with a circuit that they are happy with? I'd like to do
this to one or more of my VTVMs but I don't want to experiment with it--I
want an optimal circuit that will be reliable. Anybody got one?

73, Don Merz, N3RHT
 
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