RE: [AMRadio] VTVM Battery Eliminator
> 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. There was some discussion regarding this topic on the Tempe Heathkit reflector in the 10/04 timeframe. At lest on the Heath VTVM, there is apparently an issue wrt current draw from the power transformer, regardless of how simple or complex one chosses to go with the idea of feeding a 1.5v DC supply to replace the battery. I've kept a battery going in my v-7A since 1957. I had a bit of corrosion once, many years ago, but cleaned it up -- and have checked it once a year or so since, with no more problems. Grant/NQ5T
RE: [AMRadio] VTVM Battery Eliminator
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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] 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 <[EMAIL PROTECTED]> To: Discussion of AM Radio ; 'Glowbugs (E-mail) <[EMAIL PROTECTED]> 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 The information contained in this e-mail may be confidential and is intended solely for the use of the named addressee. Access, copying or re-use of the e-mail or any information contained therein by any other person is not authorized. If you are not the intended recipient please notify us immediately by returning the e-mail to the originator.(17b) __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net AMfone Website: http://www.amfone.net AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net AMfone Website: http://www.amfone.net AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb
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 <[EMAIL PROTECTED]> To: Discussion of AM Radio ; 'Glowbugs (E-mail) <[EMAIL PROTECTED]> 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 The information contained in this e-mail may be confidential and is intended solely for the use of the named addressee. Access, copying or re-use of the e-mail or any information contained therein by any other person is not authorized. If you are not the intended recipient please notify us immediately by returning the e-mail to the originator.(17b) __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net AMfone Website: http://www.amfone.net AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb
