[sdiy] [OT] electrolytics specs

[harrybissell]

Dear Groovy...

I'll try to comment. For regular linear PS apps, maybe no special
thigs are needed... inline

groovyshaman at snet.net wrote:

> Ok, I need some electrolytics for +-15V & +5V power filtering on one of my
> synth projects.  Typically I would just order general purpose types, but I
> think it's time I gain some understanding of the various specs.  Some specs
> are simple enough, others not, and some are mutually exclusive.  So I humbly
> ask... which of the following specs are the most important to look for when
> purchasing electrolytics?
>
> 1) low ESR :

needed especially for switching power supplies.  High ESR + Ripple = heat =
early death.  Bypass a DC application with some monoltyhic ceramic cap (.1uF)
and you're OK.

> 2) high temperature:

You live in the Sahara desert, play in an oven, or have an industrial
application...
or you have a switching supply see #1 above.

> 3) low leakage current

An important factor if you are doing a timing circuit... for which most sane
designers
eschew electrolytics anyway... Might be important in interstage coupling
though...

> 4) low dissipation factor

Important where high frequency ripple currents are a factor... switching power
supplies.

> 5) high permissible ripple current

Important where high ripple currents are found... switching power supplies AND
speaker (passive) crossovers.  Usually for a crossover you want a bipolar
electrolytic (which was not on your list ???) and even better, use a film
dielectric
cap, which has ripple current capacity to shame all 'lytics...

BTW... most cap manufacturers can spec the same cap to get whatever parameter
you want... if you specify higher ripple current they specify lower ambient
temperature, or shorter life...

> 6) low impedance

Combination of low ESR and low series inductance... these are important in HF
apps like a switching power supply. Series impedance will make the caps that
much
less effective.

> 7) long life/high reliability

Bigger caps can disspate more heat and are more reliable. Long life is
everyone's
desire... and it comes from keeping temperatures down. Electrolytic life goes
down
(help me here list I might remember incorrectly) by half every +10C

> 8) safety vented

Well SH!T they all ahve a safety vent... and when it opens you are hosed. Really

small ones just explode... Big ones with high voltage or very high energy
storage
will sometimes NOT go through the vent anyway... they will blow right through
the
can.  E-mail me tomorrow (offlist) and I'll send you some HORROR photos for
Halloween ;^)

Not on your list:

Bipolar electrolytics... the capacitor of choice when you cannot be assured of
the
polarity of the voltage applied. This comes up a LOT in synth applications...
where
signals and offsets might be positive or negative. Some folks like to use two
electrolytics with a bias resistor to the negative rail, I don't.  If I can use
a film cap
I do... but they become really impractical above 1uF and bipolar electrolytics
become a better choice. They come in High ripple and ordinary ripple... for most

synth use ordinary is the correct (and less expensive) choice...

Hope this helps

H^) harry

> Any info to help my education along would be greatly appreciated!
> George