cuit. It measures a
characteristic of electrolytic capacitors
which is very important: the equivalent
series resistance or ESR.
Back then, no-one (including myself)
expected that a meter designed to meas-
ure a capacitor characteristic hardly any-
one had ever heard of would become
popular in Australia, let alone overseas.
However, we didnt consider the explo-
sive growth of the Internet. Thanks to
people discussing it on various news-
groups and by email, about 12,000 ESR
meter kits have now been sold and sales
(mainly outside Australia) continue to be
strong.
Over those eight years, both Dick
Smith Electronics (which sells the kit)
and the author have received many sug-
gestions from constructors on improving
the ESR meter kit particularly on mak-
ing the construction easier. This upgrad-
ed version is the result and incorporates
many of those ideas. As before, it will be
available as a complete kit from DSE.
Whats ESR?
Before taking a look at whats changed
in this Mk2 version of the ESR meter,
lets take a look at what an ESR meter
does. First, we need to get into a bit of
boring theory to understand how elec-
trolytic capacitors (which Ill refer to
simply as electrolytics from here on)
are constructed and work. This is neces-
sary to understand why they cause so
many electronic faults.
Fig.1 is a simplified cross-section
drawing which shows the basics. As with
many other kinds of capacitors, the
plates of an electrolytic consist of two
long aluminium foil strips wound into a
cylinder. The big difference is that the
Assembly Manual
K
K II T
T
ESR Meter Mk.2
K 7214
Please read Disclaimer carefully as we
can only guarantee parts and not the
labour content you provide.
Cat No.
Forget about capacitance meters - an ESR meter is the way to go when it comes to identifying
faulty electrolytics. This well-proven design is autoranging, low in cost and simple to build.
WEBSITE:
www.siliconchip.com.au
E-MAIL:
silchip@siliconchip.com.au
by Bob Parker
SILICON CHIP - March/April 2004 Issue
A.B.N. 34 000 908 716
In-circuit testing, made possible by using <100mV test voltage which wont
forward bias diodes or transistors.
Auto-ranging to cover 0.01-99 .
Non-polarized test leads due to no DC component in the test signal.
Single pushbutton to easily control all functions.
Test lead resistance zeroing.
Automatic switch-off after three minutes when the meter is idle.
Low battery voltage warning b blinks on the display.
13mm LED displays for easy viewing from a distance.
Chart of typical electrolytic capacitor ESR figures on the front panel.
ESR Meter: Main Features dielectric isnt a strip of plastic or other
insulating material separating these
plates, but an extremely thin layer of alu-
minium oxide which is formed directly
onto the anode foil itself during the man-
ufacturing process.
As part of an electrolytics electro-
chemical operation and to achieve the
closest possible electrical contact with
the cathode side of the oxide layer, a sep-
arating strip of porous material (general-
ly paper) is sandwiched between the
plates. This separator is soaked with a
highly conductive liquid called the elec-
trolyte, which effectively connects the
negative plate to the oxide layer and
gives the capacitor its name. In very old
electrolytics, the electrolyte was water-
based but they now use water-free for-
mulas.
Because electrolytics make use of a
conductive liquid to complete the electri-
cal circuit between the cathode plate and
one side of the dielectric, the elec-
trolytes electrical resistance is critical. It
is the major component of the capacitors
equivalent series resistance or ESR.
Other components of ESR are the induc-
tance of the wound capacitor element,
the resistances of the internal connec-
tions and the impedance of the capaci-
tance itself.
In operation, electrolytic capacitors
can function perfectly for decades.
However, there are some conditions
which will cause the electrolytes resist-
ance (ESR) to increase. This can eventu-
ally reach a point where it causes prob-
lems for the circuit.
Normally, a flexible rubber seal keeps
the electrolyte contained inside the alu-
minium case of the capacitor. If the seal
fails (as it regularly does in surface-
mount electrolytics), the electrolyte will
leak and/or dry out.
The two other big killers are: (1) high
temperatures where the electrolytic is
located; and (2) high levels of ripple cur-
rent through the capacitor, which cause
elevated temperatures inside it. These
conditions cause chemical changes to the
electrolyte, increasing its resistance.
This is why time after time, repair
technicians find electrolytics failing in
switchmode power supplies, the deflec-
tion stages of CRT TVs and monitors,
and other power circuitry such as elec-
tronically-commutated motors where
both of those conditions are common.
Why high ESR causes trouble
The function of an electrolytic capaci-
tor is to block DC while acting as a low
impedance to any AC voltage across it.
As a power supply filter, an electrolytic
smooths rectified voltage and so has to
pass the AC ripple voltage on it. This
causes ripple current through the
capacitor.
In a perfect capacitor, such ripple cur-
rent causes no internal heating or other
problems but real world capacitors have
ESR. The ripple voltage across this
equivalent series resistance causes cir-
cuit losses as well as heating within the
capacitor, if it becomes excessive.
For example, in switchmode power
supplies, high ESR can cause starting
failure, loss of regulation and excessive
high-frequency noise on the outputs.
Similarly, deflection circuits can suffer
from distorted and reduced scanning
waveforms. In fact, high electrolytic
capacitor ESR often causes strange prob-
lems which are hard to make sense of.
Its worth noting that ESR increases
rapidly as the temperature drops. As a
result, defective electrolytics are often
indicated by faults which are worst in
winter and when the equipment is first
switched on, with the symptoms gradual-
ly diminishing as the temperature rises.
Capacitance vs ESR meters
In the past, technicians didnt have
much choice but to check suspect elec-
trolytics using a capacitance meter.
Unfortunately, capacitance meters are
Text and illustrations courtesy of Silicon Chip
Page 2
Fig.3: this block diagram shows the basic scheme for the ESR meter. S1 is an
electronic switch and it allows the test capacitor to be alternately charged for
8
�
�
s from a constant current source and then discharged for 492
�
�
s. The result-
ing voltage waveform is then amplified and fed to a comparator, where it is com-
pared with a reference voltage ramp.
Fig.1: simplified cross-section of an
electrolytic capacitor. The dielectric
consists of a thin layer of aluminium
oxide on the anode plate and this is
connected to the cathode plate via an
electrolyte-soaked separator.
Fig.2: as shown in this diagram,
the electrical resistance of the
electrolyte is in series with the
capacitance of the oxide dielec-
tric. It is the major component of
the equivalent series resist-
ance or ESR of an electrolytic
capacitor. generally useless for weeding out elec-
trolytics which are causing trouble.
Theyre generally designed to ignore the
ESR and show only the actual capaci-
tance which usually stays close to its cor-
rect value, even when the ESR has gone
through the roof! In addition, the capaci-
tor must be disconnected from the circuit
before making capacitance measure-
ments.
Now you can see why ESR meters
have become so popular with techni-
cians. Theyre designed to directly meas-
ure the very characteristic which is caus-
ing the fault.
Whats more, this measurement can be
made with the capacitor still in circuit
(while the equipment is safely discon-
nected from power). This avoids the
inconvenience of having to unsolder it,
which incidentally also heats it up and
makes the ESR drop, thereby masking
the problem.
Microcontroller-based meter
Unlike most other ESR meters, this
design is based on a microcontroller IC.
This custom-programmed chip makes
possible the extensive range of features
offered (see panel). It also greatly con-
tributes to the small size, low cost and
simplicity of the ESR meter.
The microcontroller drives two 7-seg-
ment LED displays to give a direct read-
out of ESR measurement.
How it works
An ESR meters job is to measure the
resistance of an electrolytic capacitors
electrolyte while (as far as possible)
ignoring the capacitive reactance. Fig.3
shows a simplified diagram of how this
is done in the ESR meter described here.
As shown, switch S1 (in reality, an
electronic switch driven by the micro-
controller) alternately connects and dis-
connects the capacitor being tested to a
constant current source of either 0.5mA,
5mA or 50mA (depending on the range).
In practice, the capacitor is alternately
charged for 8ms (S1 in the Charge
position) and discharged for 492
�
s (S1 in
the Discharge position).
Because the test current pulses are so
short, the voltage pulses developed
across the capacitor are essentially pro-
portional to its ESR. Thats because
capacitors with values above about 1
�
F
dont have time to charge enough to sig-
nificantly affect the reading.
The voltage pulses across the capacitor
are fed to a non-inverting wideband
amplifier with a gain of 20. The resulting
signal is then applied to the non-invert-
ing input of an op amp comparator
(inside the