ohm meter battery

I found this circuit somewhere, (I can't remember) to replace the 1.50 volt battery in a VTVM. I have an RCA WV98C Senior Voltohmyst VTVM that is in p erfect condition but doesn't get used very often. This battery retrofit cir cuit takes power from the 6.30 volt filament transformer and is built aroun d an LM317. With a trim pot you set it for 1.55 volts and you never have to change a battery again. It also needs to occupy an area no larger than the battery presently does.

As much as I like keeping things original, I'm worried that the battery is going to leak and rot the inside of the unit. I was going to build the circ uit but it occurred to me that I should probably be concerned about the cur rent that the LM317 will be dissipating when the meter is used on the low o hms range. Does anyone have a feel for the size that an adequate sized heat sink should be in order to handle this? Thanks, Lenny

Reply to
captainvideo462009
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What's the worst case current out of the 1.5V cell?
Reply to
John Fields

How about installing a fresh battery and measuring the current drain on it while making a few resistance measurements?

John S

Reply to
John S

Was it originally a mercury battery?

--sp

Reply to
Spehro Pefhany

Please don't do that. Sell it to me instead.

John S

Reply to
John S

On Thursday, March 20, 2014 12:13:46 AM UTC-4, snipped-for-privacy@gmail.com wrote :

lt battery in a VTVM. I have an RCA WV98C Senior Voltohmyst VTVM that is in perfect condition but doesn't get used very often. This battery retrofit c ircuit takes power from the 6.30 volt filament transformer and is built aro und an LM317. With a trim pot you set it for 1.55 volts and you never have to change a battery again. It also needs to occupy an area no larger than t he battery presently does.

s going to leak and rot the inside of the unit. I was going to build the ci rcuit but it occurred to me that I should probably be concerned about the c urrent that the LM317 will be dissipating when the meter is used on the low ohms range. Does anyone have a feel for the size that an adequate sized he at sink should be in order to handle this? Thanks, Lenny

The meter is 200uA full scale, that will be your maximum current draw. The LM317 does not regulate at such low currents, you need to hang a resistor o n its output of about 1.5/0.01= 150 ohms. The LM317 current load then wil l be approximately constant at 10mA for thermal consideration purposes.

Reply to
bloggs.fredbloggs.fred

I don't think you are asking me, but my research indicates a carbon-zinc D cell was the norm.

Reply to
John S

On Thursday, March 20, 2014 12:13:46 AM UTC-4, snipped-for-privacy@gmail.com wrote :

lt battery in a VTVM. I have an RCA WV98C Senior Voltohmyst VTVM that is in perfect condition but doesn't get used very often. This battery retrofit c ircuit takes power from the 6.30 volt filament transformer and is built aro und an LM317. With a trim pot you set it for 1.55 volts and you never have to change a battery again. It also needs to occupy an area no larger than t he battery presently does.

s going to leak and rot the inside of the unit. I was going to build the ci rcuit but it occurred to me that I should probably be concerned about the c urrent that the LM317 will be dissipating when the meter is used on the low ohms range. Does anyone have a feel for the size that an adequate sized he at sink should be in order to handle this? Thanks, Lenny

And it was soldered in place to avoid problems with contact resistance. Len ny

Reply to
captainvideo462009

This is a typical conversion: I did this to several VTVMs in the '70s.

View in a fixed-width font such as Courier.

+---+ 1N4001 TIP29 | | +-----++ C E 1.5VDC | +---+ |+---+-----+-----\ /---------+-------o | | +-----++ | | \ / | 6.3VAC| |+ +++ --+-- | | | -+- | | B | | | | 50uF -+- | | +-----+ | | +---+ | | |1k | | |+ | | | -+- +++ | | -+- +---+ | Gnd | | | -+- -+- +--------+ | | 10uF Gnd | |+ | | -+- | +++ -+- | | | | 10uF | 1N4001 | | | | +-+ | -+- +++ | Gnd | | +-+ | | | -+- 1N4001 | | Gnd +-+ +++ | -+- Gnd
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Anyone wanting to run for any political office in the US should have to 
have a DD214, and a honorable discharge.
Reply to
Michael A. Terrell

After pondering the schematic, I now believe that there is a 9.75 ohm resistor in series with the battery and probe. That can cause about

150mA drain on your battery with the probe shorted.

Cheers, John S

Reply to
John S

Needs another diode in the string--the base will be near 1.5V, but the emitter will be lower by a diode drop. (That was back before LM317s, of course.)

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

On Thursday, March 20, 2014 12:13:46 AM UTC-4, snipped-for-privacy@gmail.com wrote :

lt battery in a VTVM. I have an RCA WV98C Senior Voltohmyst VTVM that is in perfect condition but doesn't get used very often. This battery retrofit c ircuit takes power from the 6.30 volt filament transformer and is built aro und an LM317. With a trim pot you set it for 1.55 volts and you never have to change a battery again. It also needs to occupy an area no larger than t he battery presently does.

s going to leak and rot the inside of the unit. I was going to build the ci rcuit but it occurred to me that I should probably be concerned about the c urrent that the LM317 will be dissipating when the meter is used on the low ohms range. Does anyone have a feel for the size that an adequate sized he at sink should be in order to handle this? Thanks, Lenny

I finally found the Website where I originally found the circuit. I liked t he second schematic. It's isolated from chassis ground. Here's the link:

formatting link

Reply to
captainvideo462009

As John S replied, the schematic shows a 9.75 ohm resistor in series with the battery, which makes the maximum current draw about 154 mA at 1.5 volts. The LM317 will drop about 6.2 volts, so it will need to dissipate close to a watt.

I'd recommend an LM317T with a small heat sink. You could likely get away with an LM317T with no heat sink, but you have room for one, so use it. Worst case, temperature rise without a sink will be about 50C, or, in other words, the LM317T temperature will rise to 122 degress Fahrenheit above ambient. With a heat sink like Digikey # HS115-ND, you'll cut that worst case temperature rise roughly in half.

Ed

Reply to
ehsjr

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This one uses a reactive dropper, regulates from low mains to high 
mains - 108 to 132VRMS - and dissipates less than 1/2 watt from no 
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Reply to
John Fields

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