Hi,

I've seen this done before on low current designs. Sometimes when you
replace the batteries in this type of design the circuit voltage does not
have time to drop completely away due to the charge saved on bulk
capacitors. When the new batteries are added the circuit comes up in a
peculiar state. This is particularly true of uP power on reset circuits.
There are more elegant ways to take care of this problem but I suppose a
single resistor would be the cheapest (if one ignored battery life).

Regards,

Kevin Harris
Manager, Approval Services
Digital Security Controls



-----Original Message-----
From: Maxwell, Chris [mailto:chr...@gnlp.com]
Sent: Monday, September 25, 2000 11:31 AM
To: 'EMC-PSTC Internet Forum'
Subject: Battery Safety



All,

We have inherited a design from a company which we purchased.  The product
is a handheld and can be operated from a pair of Alkaline batteries.  Inside
the unit, there is a 91 KOhm resistor across the + and - terminals of the
batteries.  Since the people who designed the instrument are long gone, some
of my collegues have asked me if this resistor could be a safety  feature.

I can't think of any way this resistor would help the safety of the
instrument.  I did read through the safety test report; and I found no
reference to this resistor being required.   All it does is provide a
constant drain on the battery (reducing battery life).  It has been
suggested to me that some designers put resistors across batteries to reduce
the electrical noise in a product.  To me a capacitor would be better for
this because it wouldn't drain the battery while it was filtering.  Even so,
isn't a battery the ultimate capacitor?  I'm just drawing a blank why anyone
would do this.  I'd love to recommend that we pull this resistor out because
it's a pain to solder and it affects battery life.  However, I don't want to
sacrifice the safety of the product.

Anybody want to take a guess at this one?

Thanks.

Chris Maxwell, Design Engineer
GN Nettest Optical Division
6 Rhoads Drive, Building 4  
Utica, NY 13502
PH:  315-797-4449
FAX:  315-797-8024
EMAIL:  chr...@gnlp.com



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