ohm meter battery

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What's the worst case current out of the 1.5V cell?

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

John S

Was it originally a mercury battery?

--sp

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

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.

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

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

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

--
Anyone wanting to run for any political office in the US should have to 
have a DD214, and a honorable discharge.

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

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

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

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:

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

On Fri, 21 Mar 2014 14:43:13 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

--- 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 load to full load from 108 to 132VRMS in:

Version 4 SHEET 1 1868 680 WIRE 320 -128 224 -128 WIRE 368 -128 320 -128 WIRE 480 -128 432 -128 WIRE 576 -128 480 -128 WIRE 480 -80 480 -128 WIRE 576 -80 576 -128 WIRE 1200 16 1152 16 WIRE 1264 16 1200 16 WIRE 1344 16 1264 16 WIRE 1424 16 1344 16 WIRE 576 32 576 -16 WIRE 704 32 576 32 WIRE 816 32 704 32 WIRE 864 32 816 32 WIRE 1264 48 1264 16 WIRE 320 80 320 -128 WIRE 704 80 704 32 WIRE 1200 80 1200 16 WIRE 1200 80 1152 80 WIRE 816 128 816 32 WIRE 864 128 816 128 WIRE 1264 144 1264 112 WIRE 1264 144 1152 144 WIRE 1344 144 1344 16 WIRE 480 208 480 -16 WIRE 704 208 704 144 WIRE 704 208 480 208 WIRE 480 256 480 208 WIRE 576 256 576 32 WIRE 320 368 320 160 WIRE 320 368 224 368 WIRE 480 368 480 320 WIRE 480 368 320 368 WIRE 576 368 576 320 WIRE 576 368 480 368 WIRE 704 368 704 208 WIRE 1008 368 1008 192 WIRE 1008 368 704 368 WIRE 1344 368 1344 224 WIRE 1344 368 1008 368 WIRE 1424 368 1344 368 WIRE 704 448 704 368 FLAG 704 448 0 FLAG 1424 16 BAT+ FLAG 1424 368 BAT- FLAG 224 -128 T1-BLU FLAG 224 368 T1-BWN SYMBOL voltage 320 64 R0 WINDOW 0 -81 36 Left 2 WINDOW 3 -77 71 Invisible 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V4 SYMATTR Value SINE(0 7.7 60) SYMBOL diode 592 320 R180 WINDOW 0 -38 32 Left 2 WINDOW 3 -98 2 Left 2 SYMATTR InstName D3 SYMATTR Value MURS120 SYMBOL cap 432 -144 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1

SYMBOL PowerProducts\\LT1962-1.5 1008 80 R0 SYMATTR InstName U1 SYMBOL cap 1248 48 R0 SYMATTR InstName C3 SYMATTR Value 10n SYMBOL diode 464 256 R0 WINDOW 0 -40 32 Left 2 WINDOW 3 -100 63 Left 2 SYMATTR InstName D2 SYMATTR Value MURS120 SYMBOL diode 560 -80 R0 WINDOW 0 47 33 Left 2 WINDOW 3 27 64 Left 2 SYMATTR InstName D1 SYMATTR Value MURS120 SYMBOL diode 496 -16 R180 WINDOW 0 41 32 Left 2 WINDOW 3 26 -2 Left 2 SYMATTR InstName D4 SYMATTR Value MURS120 SYMBOL polcap 688 80 R0 SYMATTR InstName C4

SYMBOL res 1328 128 R0 SYMATTR InstName R1 SYMATTR Value 10K TEXT 720 392 Left 2 !.tran 1 TEXT 856 392 Left 2 ;RCA WV98C BATTERY ELIMINATOR TEXT 856 424 Left 2 ;JOHN FIELDS 21 MARCH 2014 TEXT 432 408 Left 2 ;FOR SIM ONLY,DO NOT TEXT 360 432 Left 2 ;CONNECT TO UNIT COMMON