| Some wise person once said, "The
reliability of any piece of electronics is inversely
proportional to the number of electrolytic capacitors
in it", and I doubt that many service technicians
would disagree.
Especially now that switch-mode power supplies (SMPS's)
have been commonly used in domestic VCR's and TV's etc.
for a decade or so, one of the most likely components to
fail is the humble electrolytic. The symptoms can be as
diverse as a VCR's playback picture swimming in tiny
dots, up to SMPS's mysteriously self-destructing.
As a service technician you have probably been close
to tearing your hair out because of the difficulty in
determining which electrolytics were faulty and which
ones were still OK in SMPS's and other equipment. You
want to be able to check electrolytics in circuit, with
the power safely disconnected.
Why not use a readily available capacitor meter? Because
when electrolytics go faulty, they normally don't lose
their capacitance significantly ( as is normally assumed
). Rather their equivalent series resistance (ESR) goes
through the roof. Capacitance meters won't tell you
this: about the best they can do is give a low reading
if the electrolytic is nearly open.
About ESR...
So what is an electrolytic's ESR?
Electrolytics depend on a water-based electrolytic,
soaked into a strip of porous material between the aluminum
foil plates, to complete the outer electrical connection
to the aluminum oxide dielectric coating on the anode
foil.
The electrolytic has electrical resistance which, along
with the (negligible) resistance of the connecting leads
and aluminum foil plates, forms the capacitors equivalent
series resistance.
Normally the ESR has a very low value, which stays that
way for many years unless the rubber seal is defective.
Then the electrolytics water component gradually dries
out and the ESR creeps up with time. The electrolytic
gradually acts more and more like a capacitor with it's
own internal series resistor.
Heat makes it worse...
If an electrolytic is subjected to high temperatures,
especially from heat generated internally as a result
of large ripple currents, the electrolytic will start
to decompose and the dielectric may deteriorate and
the ESR will increase far more rapidly.
To make things worse, as the ESR increases, so does
the internal heating caused by ripple current.This can
lead to an upward spiral in the capacitors core temperature,
followed by complete failure - sometimes even explosive!
The service life of electrolytics is approximately halved
for every 10 degrees C increase in temperature and,
surprisingly, many are only designed for a few thousand
hours at their maximum rated temperature and ripple
current. (A year is 8766 hours!)
Switch mode power supplies place quite severe stresses
on filter capacitors. Because of their compact construction,
temperatures are high and the capacitors have to endure
large ripple currents.
Necessity is
supposed to be the mother of invention, but desperation
works even better. |
