Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!ub!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: What is the noise voltage of a 1.5V cell
Summary: Internal resistance of cells & standard cells
Message-ID: <4291@kitty.UUCP>
Date: 4 Jan 91 15:32:18 GMT
References: <1991Jan3.001303.483@ae.chalmers.se> <4285@kitty.UUCP> <13749@milton.u.washington.edu>
Organization: Recognition Research Corp., Clarence, NY
Lines: 62

In article <13749@milton.u.washington.edu>, whit@milton.u.washington.edu (John Whitmore) writes:
> >	The essential noise component of a simple electrochemical primary
> >cell is thermal noise (Johnson Noise, for Johnson fans). 
> >
> >	If you were drawing some serious power from the alkaline cell,
> >I would consider some other factors in evaluating noise (like temperature
> >shifts, gas generation, etc.)
>
> 	The folks found that their best bet (Zeners and IC references
> were too noisy) was a stack of standard cells (which have no gaseous
> byproduct).  They bought a few dozen for their 90V bias battery.

	The standard cell (Weston cell) is certainly a stable voltage
source, but from an electrical noise standpoint is undesirable to bias a
device subject to great amplification due to the high internal resistance
of the cell.  Typical unsaturated (the most common secondary standard
variety) standard cells have internal resistances between 500 and 1,200
ohms.  When used as a voltage standard, the noise component is not
significant when compared to the accuracy and precision of the cell, and when
one considers that standard cells are usually compared potentiometrically
in a system having virtually zero frequency response bandwith.

> They bought a few dozen for their 90V bias battery.
> It was expensive.

	I bet it was expensive, considering that one would require at
least 88 unsaturated Weston cells to get 90 volts!  Even the miniature
unmounted Weston cells made by Eppley sell for at least $ 50.00 apiece
in quantities.

> Then they put the cells in a temperature-regulated
> box (chilled, I believe).

	The most common standard cell temperature is 28 deg C.

	Current drain from standard cells must be kept to a minimum.
Eppley recommends an absolute maximum of 100 uA for 5 minutes.  A
definite breach of etiquette is to measure the voltage of a standard
cell using a conventional VOM.  It's really a silly thing to do, when
one considers that the precision of a standard cell is many orders of
magnitude greater than a VOM meter movement!

	However, put a standard cell and a VOM in a lab full of students,
and once can rest assured that at least one student will use the VOM to
measure the voltage! :-)  It seems to be an irresistable temptation, and
I freely admit that I have committed this fau paux in my younger days.

> 	Different battery systems have different stabilities; get
> a few different types and test 'em.  The only ones I KNOW won't work
> are carbon/zinc.

	I certainly agree with that!  Bear in mind that lower internal
resistance will result in lower thermal noise.  Mercury cells make
great stable voltage sources, but they have fallen into disfavor for
obvious reasons.  A favorite "casual" voltage standard during the
1960's and 1970's was a multi-tapped mercury cell made by Mallory.
It had taps every 1.34 volts, had 8 cell sections, and was sealed in
a robust metal case with binding posts on the top.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
VOICE: 716/688-1231   {boulder, rutgers, watmath}!ub!kitty!larry
FAX:   716/741-9635                  {utzoo, uunet}!/      \aerion!larry