[mdlug] [Fwd: Re: bios battery]

[Raymond McLaughlin]

Jeff Hanson wrote:
> On 1/16/07, Raymond McLaughlin <driveray at ameritech.net> wrote:
> 
>> Also, one question for anyone who might know: The caps I'm looking at
>> are marked 1500 mfd, 6.3V, 105 deg C. How much lee way do I have on any
>> of these?
>>
>> OK, the temp is obviously a maximum not a minimum, I just included it
>> for completeness.
>>
>> I'm pretty sure the voltage is more of a maximum rating than a rigid
>> spec, so a little higher should be OK, I think.
>>
>> What about the capacitance? My understanding is that these capacitors
>> serve as "power smoothers" i.e. by holding a charge, at an applied
>> voltage (not likely to be exactly 6.3V), on a DC power circuit, they
>> compensate for microsecond dips in the supplied voltage as power usage
>> fluctuates among the components. If this is the case then you should be
>> able to get away with a little over capacitance, but not under.
>>
>> No doubt an exact match if probably best, and these are probably not
>> that hard to find if you really look, so I'm just wondering.
> 
> Actually it is a little more complicated.  In switching power supplies
> the Equivalent Series Resistance is critical.  If you give me all the
> numbers and letters on it, the diameter, length, and in spacing in mm,
> and a picture of the manufacturer's logo I may be able to find an
> exact match.

Really? I'm surprised that an exact match for the specs above would'nt
be good enough.

These caps 15 mm high, 9mm diameter cylinders, with leads 5mm apart and
black jacket with copper colored printing. The only markings are
(parallel to the axis):
  a brand name(?) of  'I.Q'   then below that
  '6.3V 1500uF',              then below that
  '+105[deg]C',               then below that
a arrow-stripe, containing what looks like a '-' inside an ellipsoid,
and ending in a downward pointing triangle. I take this '-' to indicate
the negative terminal. At first I had been looking at the + sign above
this, but now I think it is part of the temperature spec.

Examining the board I can see that the six capacitors share a common
pair of fairly wide traces. The capacitor leads are spaced 11mm apart.

I'm kind of skeptical about the importance of the particular brand of
capacitor. You mention "switching power supplies", so I must make sure
you are clear on the fact that we are talking about capacitors on the
computer's motherboard, not in the power supply unit it self. The fact
that the motherboard is populated by over two dozen other electrolytic
caps (the other caps are blue and branded Sanyo or Taepo rather than
I.Q), and countless other components leads me to question how much
effect these caps have on the over all impedance of the motherboard as a
whole. If I get 6.3V 1500uF electrolytics of more or less the same
physical dimensions, shouldn't I be good in this case?

If you know different I'm willing to listen, but I have the impression
that these electrolytic capacitors are pretty much a commodity items.
It's good to know the limits of such a generality too. :)

Raymond McLaughlin