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<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=883513815-28032007>Chuck,</SPAN></FONT></DIV>
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class=883513815-28032007></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=883513815-28032007>Right you are. Making something work is relatively
easy. Making it work within the constraints of environment, regulations, and
cost is something else entirely.</SPAN></FONT></DIV>
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class=883513815-28032007></SPAN></FONT> </DIV>
<DIV dir=ltr align=left><FONT face=Arial color=#0000ff size=2><SPAN
class=883513815-28032007>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.)</SPAN></FONT></DIV>
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<P><SPAN lang=en-us><FONT face="Arial Black" color=#800000
size=2>Rick</FONT></SPAN></P>
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<FONT face=Tahoma size=2><B>From:</B> dprglist-bounces@dprg.org
[mailto:dprglist-bounces@dprg.org] <B>On Behalf Of </B>Chuck
McManis<BR><B>Sent:</B> Tuesday, March 27, 2007 4:58 PM<BR><B>To:</B>
dprglist@dprg.org<BR><B>Subject:</B> RE: [DPRG] Re: 9 volt batteries in
parallel<BR></FONT><BR></DIV>
<DIV></DIV>Hi Rick,<BR><BR>This is a good example of where the requirements can
be harder to get right than the design! So you wrote:<BR><BR>> You are
correct - 9V batteries are short on current capacity, which is <BR>>
precisely why I need to use them. There's more to the story. The circuit
<BR>> I'm building has to be mounted in an intrinsically safe area, so I have
to<BR>> limit the total power capacity of the batteries.<BR><BR>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?" <BR><BR>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. <BR><BR>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?" <BR><BR>--Chuck<BR><BR></BODY></HTML>