mica pf caps

[Edward R Morris]

Hello. When I need to replace the mica caps and can't find an exact
replacement value should I choose the next highest value or the next lowest
value or should I choose the closest value?
Thanks for your time,
Eddie

[Todd H.]

"Edward R Morris" > writes:
> Hello. When I need to replace the mica caps and can't find an exact
> replacement value should I choose the next highest value or the next lowest
> value or should I choose the closest value?

This question is impossible to answer well.

Provide more details of what equipment you're working on, perhaps a
schematic, the voltage rating of the problem cap, and what the relvant
existing value, and the closest ones available are, and perhaps people
can assist. Personally, I'd be very hesitant to change values on
replacement. Going to a different type of capacitor is probably the
best way to go in general provided the voltage rating of the
replacements meets or exceeds that of the mica cap you're replacing.

sci.electronics.* would be a better hierarchy for this question.

Good luck!

Best Regards,
--
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[Michael Black]

"Edward R Morris" > wrote in message >...
> Hello. When I need to replace the mica caps and can't find an exact
> replacement value should I choose the next highest value or the next lowest
> value or should I choose the closest value?
> Thanks for your time,
> Eddie

You need to specify where these are. Small value capacitors tend to
be small for very good reasons. In virtually all cases, they are
there as frequency determining components (ie in tuned circuits
in radio circuits, or as the controlling elements in active filters
at audio), and messing with them may affect things considerably.
Since any change in value will tend to be a good percentage of
the original value, you may find any change is bad. Likewise,
when small value capacitors are chosen, the designer may choose
higher tolerance capacitors specifically to address this issue.

One place where small value capacitors may appear in audio equipment
where their values may not be so important is for RF suppression.
You'd find these at inputs to a piece of audio equipment, and
they are there to "short" radio frequencies to ground instead
of getting into the active stages. Small values are used so
they don't affect audio response, but small values will still
have sufficiently low impedance at the needed frequencies. In
this use, the exact value does not tend to be important.

Contrast all of this with larger value capacitors. Take a bypass
capacitor, .01uF. A larger value, say ten times that, will simply
lower the frequency where it has low impedance at. A few pF will
not be noticed one bit. Lower it, and it will still be useful
as a bypass capacitor at those frequencies, though the value
chosen may tend to be the minimum in such cases. These capacitors
tend to be low tolerance (am I getting that right? I mean there
can be lots of variation from the marked value), because the variation
will not affect things

Michael

[unitron]

(Michael Black) wrote in message >...
> "Edward R Morris" > wrote in message >...
> > Hello. When I need to replace the mica caps and can't find an exact
> > replacement value should I choose the next highest value or the next lowest
> > value or should I choose the closest value?
> > Thanks for your time,
> > Eddie
>
> You need to specify where these are. Small value capacitors tend to
> be small for very good reasons. In virtually all cases, they are
> there as frequency determining components (ie in tuned circuits
> in radio circuits, or as the controlling elements in active filters
> at audio), and messing with them may affect things considerably.
> Since any change in value will tend to be a good percentage of
> the original value, you may find any change is bad. Likewise,
> when small value capacitors are chosen, the designer may choose
> higher tolerance capacitors specifically to address this issue.
>
> One place where small value capacitors may appear in audio equipment
> where their values may not be so important is for RF suppression.
> You'd find these at inputs to a piece of audio equipment, and
> they are there to "short" radio frequencies to ground instead
> of getting into the active stages. Small values are used so
> they don't affect audio response, but small values will still
> have sufficiently low impedance at the needed frequencies. In
> this use, the exact value does not tend to be important.
>
> Contrast all of this with larger value capacitors. Take a bypass
> capacitor, .01uF. A larger value, say ten times that, will simply
> lower the frequency where it has low impedance at. A few pF will
> not be noticed one bit. Lower it, and it will still be useful
> as a bypass capacitor at those frequencies, though the value
> chosen may tend to be the minimum in such cases. These capacitors
> tend to be low tolerance (am I getting that right? I mean there
> can be lots of variation from the marked value), because the variation
> will not affect things
>
> Michael


The low tolerance is probably because making them to higher
tolerences would result in much higher prices and, since they are used
in applications where having a capacitor of "about" a particular value
in that part of the circuit is a lot more important than hitting a
specific target value, the demand is more often for "inexpensive and
around the right value" than "within a millionth of a percentage point
and the expense be damned".