4
open circuit at DC, and will show less and less resistance as the frequency across it
increases. Some inexpensive cap meters utilize this fact by measuring a cap's impedance at a
fixed frequency such as 1KHz and translating the reading to capacity. In reality, checking a
cap at 1KHz only works if the cap is being used in a circuit that also utilizes 1KHz.
Television and VGA monitor video circuits use frequencies into the megahertz, and in
PWM power supplies, frequencies of 100KHz and higher are commonly used. It makes
more sense to forget about capacitance altogether and use our knowledge that high
frequency ESR increases with age and as a cap dries up.
Several ESR cap checkers have appeared throughout the years. The originator was a simple
meter that used 50KHz and a simple mechanical meter. A more recent meter is a slightly
improved version, still using an old-fashioned mechanical meter, but running at a more
accurate 100KHz. However, these meters have their limitations; they cannot check for leaky
or shorted caps and the technician must individually discharge each cap before testing. Also,
the cheap test probes add their own capacitance and readings varied depending on the
position that the technician held the probes (and whether or not he or she was sweating!)
The most widely used in-circuit ESR/DCR tester is the
CapAnalyzer 88 series
by EDS. This
unique meter uses a test frequency higher than most others, automatically discharges the cap
under test, checks for low
DCR
, then checks and displays
ESR
on a 20 segment LED bar
scale. It includes a low-capacitance one-handed tweezer test probe, and beeps from one to
five beeps depending on the ESR reading of the cap. Because it is microprocessor
controlled, it has more features and is much more accurate than the older designs. Possibly
its best attribute is a three-colored chart on the front panel that shows typical ESR readings
of good, fair, and bad caps depending on their capacitance. There have been cheap Oriental
copies made for years, so always make sure your
CapAnalyzer
is purchased from EDS or an
authorized EDS distributor. All
CapAnalyzers
made after 2001 are series II units, even
though the front labels don't show "series II".
The
CapAnalyzer 88A
claims 100% accuracy in circuit because of its testing parameters.
The frequency is variable and high enough to make the cap's actual capacity insignificant,
so it measures only the ESR. The high test frequency also helps isolate the cap under test
from the rest of the circuit via the high inductance of the pc board copper foil. This
frequency is also high enough to ignore any coils over 5 uH. ESR testing is done with a
calibrated low resistance at the test point which allows it to compensate for normal circuit
resistance. Both DCR and ESR measurements are under 50 millivolts so that no active
devices are turned on. Therefore only the component at the test point will rerspond.
However, because it checks DCR first, it will alert the technician immediately if the cap or
anything else in that circuit is shorted or leaky,
before
it checks ESR. This test parameter
(DCR OHMS SET ALERT) is user adjustable from zero to 500 ohms.
