[AMRadio] VTVM Battery Eliminator

Jim Candela jcandela at prodigy.net
Thu Jul 20 16:20:31 EDT 2006


   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! :-)


----- 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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