DPRG mailing list archives: [DPRG] Re: 9 volt batteries in parallel

Wed Mar 28 10:50:29 CDT 2007

Chuck,

Right you are. Making something work is relatively easy. Making it work
within the constraints of environment, regulations, and cost is
something else entirely.

Not wanting to bore everyone with the specifics of this thing, I did not
go into much detail; but when designing a circuit for intrinsically safe
applications, there are power limiting things you can do which will make
the circuit, or parts of the circuit "safe" as defined by UL standard
913 for intrinsically safe apparatus. What it boils down to is that by
placing some protection components on each battery and limiting the
voltage and current from it - and by separating circuits withing a
minimum creepage and clearance distance (and a few other requirements)
you can make the circuit "not subject to fault" as far as intrinsic
safety goes. 9 volt batteries are already accepted for these
applications, which means that UL and the electrical inspectors are
already familiar with them in this application and will accept them. The
9 volt batteries are also readily available to the customer who will be
replacing them. (Were usually lucky if he can insert them in the correct
direction.)

Rick

________________________________

From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On
Behalf Of Chuck McManis
Sent: Tuesday, March 27, 2007 4:58 PM
To: dprglist at dprg.org
Subject: RE: [DPRG] Re: 9 volt batteries in parallel

Hi Rick,

This is a good example of where the requirements can be harder to get
right than the design! So you wrote:

> You are correct - 9V batteries are short on current capacity, which is

> precisely why I need to use them. There's more to the story. The
circuit 
> I'm building has to be mounted in an intrinsically safe area, so I
have to
> limit the total power capacity of the batteries.

On the surface this appears to be a design requirement that defines the
design to be impossible. On the one hand you state "I use 9v batteries
because they have less power capacity for saftey reasons" on the other
hand you state (not in this message but to start this thread) "I need
more power capacity than a 9v battery can provide, can I use them in
parallel?" 

This is further complicated by the inference that "intrinsic safety" and
"power capacity" are somehow related. I expect you will be much more
successful if you treat each of these requirements separately, power
required and safety level required. It may be possible that what you are
trying to do is impossible given the requirements at which point you
have to go back to the folks who created the requirements in the first
place and explain their choice in terms of changing requirements or
picking a different project. 

The reason this is such a good example is that it crisply demonstrates a
key component of the engineer's life. Asking the question "Is there a
solution (design) within the volume constrained by the following
requirements?" 

--Chuck

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