Re: JU570-2 caps and fix

Julian Perry <jp@digitaltapestries.com> writes:

> I can't see any reason why using a ceramic capacitor WOULDN'T work
> just fine (if not better - but one wonders why that wasn't done in the
> first place)

Ceramic capacitors have very different characteristics compared to an
electrolytic cap. The two main ones that can cause problems if they are
used as substiture are the drastically lower ESR and capacity variation
depending on the DC bias.

While low ESR is desireable for some circuits, it can also cause issues
if the circuit relies on a capacitor's ESR being in a certain range. For
example you can often find both lower and upper ESR limits in the data
sheets of linear voltage regulators, sometimes with a note that an
additional series resistor must be used if a ceramic capacitor is to be
used.

The other issue is that the capacity of ceramic capacitors varies with
the DC bias applied to them, which doesn't happen with
electrolytics. The amount of loss depends on the amount of bias and the
capacitor itself, it is usually specified in a diagram in the data
sheet. The nominal capacity assumes a zero volt DC bias, but if you're
unlucky the circuit you substituted the capacitors in could happen to
use an operating point where the ceramic cap only has 20% of its rated
capacity. If this reduction is problematic for the circuit in question,
it can be compensated by choosing ceramic capacitors with a higher
voltage rating and/or capacity.

Fun fact: It appears to be a common sport to replace the SMD caps in
Amiga 600/1200 computers with ceramic capacitors, but some people report
a loss of color on the composite video output after this
modification. The characteristics of the ceramic caps are different
enough to cause problems in one part of the circuit.

-ik

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