mounted on 1/4” standoffs on one wall of the enclosure,
with appropriate holes bored in the front panel to allow
access to these three components. Refer to Figures 4, 5,
The ExpressPCB schematic and PCB files can be
found at the article link.
Each of the test points for ground — +5V, -5V, TP1,
TP2, and TP3 — is made of a short length of solid hook-up
wire. One end is soldered into a hole in the PCB, and the
free end is formed into a loop for easy grabbing by clip
leads or test probes.
Figure 6 is an inside view of the enclosure, showing
the internal wiring. Here you can see that connections to
the front panel meter and binding posts are brought out
from the PCB by four-pin male connector J2, and power
from the rear panel via two-pin male connector J3.
Raw DC power ( 9 to 16
VDC) is supplied through a
2.1 mm coax jack and SPST
rocker switch on the rear
panel as shown in Figure 7.
The current requirement is
fairly modest. The whole
circuit runs on less than 40
mA. A good quality wall wart
type of power source works
very well, as does a 9V
The front panel label
sheet and a new face for the
panel meter were drawn using
Microsoft Visio, printed on
heavy paper stock, and glued
There are two adjustment
trimmer potentiometers on
the circuit board. One (R8) is
used to adjust the output of
the phase-shift oscillator to
about 1.8V peak-to-peak, and
the other (R19) sets the meter
sensitivity. Full details of this
procedure can be found in the
downloads at the article link.
Figure 4 shows the result
of this setup with a one ohm
resistor connected across the
CUT binding posts. In Figure
5, a 100 µF tantalum capacitor
is being measured for ESR.
28 January 2016
■ FIGURE 6. Internal wiring, showing the mounting of
the circuit board and the cabling to the front and rear
What a Capacitor Really Looks Like
Nothing is perfect in this world, and that includes electronic components. Resistors have a little
bit of capacitance and inductance; inductors have a smidgeon of resistance; and capacitors have all
of the above. Fortunately, most of the time these "parasitic" quantities can be ignored and we can
treat the components we use as ideal resistors, inductors, and capacitors.
Notice I said "most of the time." Capacitors — especially large value electrolytics — can suffer
from an illusory low value resistor that appears to be in series with an ideal capacitor. This is referred
to as the Equivalent Series Resistance (ESR) of the capacitor. It's "illusory" because ESR is not a true
resistance; rather, it's the result of a combination of many factors — all of which contribute in some
way to power loss in the capacitor. Figure A is the equivalent circuit model of a typical real world
capacitor and gives a better picture of what I'm talking about. For high value capacitors and at low
frequencies, the stray inductance shown in the model can usually be ignored and the two resistances
combined into one.
Since you're reading
this magazine, you
probably already know
that every capacitor is
basically just a pair of
conductors separated by
a dielectric. The
conductors in a large
capacitor are usually
strips of foil. The
dielectric is an insulating
oxide layer formed on
one of the strips (the
"anode," or positive
electrode), plus a liquid
or paste electrolyte
which acts as the second
electrode of the
capacitor (the "cathode").
This stuff can be
corrosive, so if you have a capacitor which is physically damaged and oozing electrolyte, be careful
of getting it on your skin.
Losses in the dielectric plus leakage across the capacitor and resistance in the welds and
mechanical crimp contacts to the terminals all contribute to the ESR.
Here's the problem: Over time — especially at elevated temperatures — the liquid electrolyte
component of the dielectric dries (or leaks) out. The capacitance may not change very much, but
there will be an increase in resistivity; therefore, the ESR rises. To make matters worse, depending on
the dielectric substance, the ESR can vary with frequency. This can be a problem if the capacitor
must handle substantial alternating current, as in a switching power supply, for example. High ESR
combined with high current means extra power dissipated in the capacitor. The resulting temperature
rise can cause further degradation and premature failure.
Aluminum electrolytic capacitors are particularly prone to this problem — especially if they've
been around for a long time. Solid tantalum capacitors also have ESR problems but to a lesser
degree. Small ceramic capacitors are essentially free of this plague.
■ FIGURE A.
(top) and how
it simplifies to