Author Topic: Electro Instruments (Non Linear Systems related?) DVM  (Read 4475 times)

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Offline SeanB

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Re: Electro Instruments (Non Linear Systems related?) DVM
« Reply #25 on: July 20, 2016, 05:16:54 am »
I came across an old Heathkit bench meter a whle ago that used a mercury button cell as a reference, but the cell had long before corroded away, and some previous owner had replaced it with the next best thing, a single LR44 cell, which of course was also dead and corroded.
 

Online Cubdriver

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Re: Electro Instruments (Non Linear Systems related?) DVM
« Reply #26 on: July 20, 2016, 05:31:07 am »
I came across an old Heathkit bench meter a whle ago that used a mercury button cell as a reference, but the cell had long before corroded away, and some previous owner had replaced it with the next best thing, a single LR44 cell, which of course was also dead and corroded.

I recently got an HP 740B DC Standard/Differential Voltmeter and it has a dead mercury battery in it as a reference.  I need to figure out what to do to replace it at some point.

-Pat
If it jams, force it.  If it breaks, you needed a new one anyway...
 

Offline SeanB

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Re: Electro Instruments (Non Linear Systems related?) DVM
« Reply #27 on: July 20, 2016, 06:58:41 am »
LR44 and a shottky diode is a quick and dirty substitute, but if there is room put in a TLV431 and adjust it to the right voltage. 1.35V is within the adjustment range, though if you want a better one just take a LM10 and use it, which will give a buffered reference with slightly better performance, or at least improved PSSR.

http://www.ti.com/lit/ds/symlink/tlv431.pdf

Just remember not to use the TL431, very easy to confuse the 2.
 
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Online Cubdriver

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Re: Electro Instruments (Non Linear Systems related?) DVM
« Reply #28 on: July 20, 2016, 07:42:36 am »
The mention of the reference got me searching on mercury battery substitutes, and I saw SR44 silver oxide button cells suggested as a possible alternative (with a series diode to drop the voltage) on several camera sites - apparently this chemistry has a relatively flat discharge curve similar to that of the old mercury cells.  However, seeing as the 740B is an old HP boat anchor, the installation of a TLV431 or LM10 and their associated support circuitry should not be a huge issue space-wise.  Thanks for the suggestions - more things to dig into.  (in my 'spare' time!!)

-Pat
If it jams, force it.  If it breaks, you needed a new one anyway...
 


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