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Constructional Project
TIM BLYTHMAN’S
ESR
TEST T
EEZERS
We have produced a few variants of our Test Tweezers since the original
version in the October 2022 issue. Still, none has yet had the handy
feature of measuring capacitor ESR (equivalent series resistance). Our
new ESR Test Tweezers can measure ESR and capacitance while being
significantly more compact than all our previous ESR meters!
E
SR (equivalent series resistance)
is an inherent but undesirable
property of capacitors that acts
like a resistance in series with the
capacitive element. Fig.1 shows this
and the other factors that can be used
to model a real capacitor, as opposed
to an ideal, purely capacitive one.
For good performance, especially
at high currents (as in a switch-mode
supply), a capacitor’s ESR and ESL
(equivalent series inductance) should
be low and the leakage resistance
should be high. That combination
best approximates an ideal capacitor.
Generally, the ESL is relatively
small and is often lumped together with ESR by specifying it at a
known frequency, often 100-120Hz
or 100kHz (the former being relevant
when rectifying mains AC). The total
series impedance can then be specified in ohms.
If the ESR is high, the capacitor
will dissipate a significant propor-
Fig.1: the behaviour of real
capacitors, especially electrolytic
types, deviates from the ideal
model of capacitors found in
textbooks. ESR (equivalent series
resistance) is one of the more
prominent unwanted phenomena;
capacitors often fail due to the
ESR rising to unacceptable levels.
38
tion of the energy that passes through
it, unlike purely reactive elements
such as ideal capacitors and inductors, which have no losses. It is well
known that high-ESR electrolytic
capacitors can cause problems, but
they are not the only type of capacitor that can suffer from high ESR.
Other types, such as plastic film, can
be affected too.
In a power supply, a high ESR manifests as a voltage drop due to the current flowing in and out of the capacitor. That will decrease the voltage
available to the circuit and heat up
the capacitor, sometimes to the point
that its contents boil and spill out!
Electrolytic capacitors depend on
an electrolyte as the current path between the oxide dielectric layer and
the cathode. If this electrolyte dries
out, its resistance and thus the ESR
will increase. Increasing ESR will
also cause an increase in dissipation
inside the capacitor, further heating
and drying out the electrolyte.
A high ESR capacitor will often
cause mysterious or intermittent
faults, as documented extensively in
our Serviceman’s Log pages, where
replacing the electrolytic capacitors
usually fixes a power supply. The
conductivity of the electrolyte can
also change with temperature, leading to problems that appear or disappear as the capacitor heats up after
the equipment is turned on.
In audio circuits, the higher-than-
expected ESR can change the fre-
quency response of a circuit and may
increase distortion. These are just
some of the scenarios where a high
ESR can cause problems.
If you have a device that has failed
or isn’t working correctly, after checking for obvious visual faults like
burned components or failed solder
joints, the next step is usually to test
the electrolytic capacitors. If any are
found to have a low capacitance, high
leakage or high ESR, they may well
be the culprits. Often, several are
found to be on the way out.
So, an ESR meter is a very valuable piece of equipment for making
repairs and even checking new components to verify that they will perform as expected.
One frequently-seen piece of advice
is a warning not to connect the ESR
Meter to charged capacitors. We have
included some protection circuitry,
but large capacitors can pack enough
of a punch to render that protection
moot! The same advice applies to our
ESR Test Tweezers.
Like Bob Parker’s classic and now
20-year-old ESR Meter Mk2 design
(https://zilogbob.com/esr_meter/
esrmeter.htm), the ESR Test Tweezers
are also well suited to measuring low
resistances, such as current shunts.
So they are sure to come in handy for
other sorts of troubleshooting.
The Arduino-based LC and ESR
Meter from August 2024 uses the same
‘front-end’ design as the ESR Meter
Mk2 to measure ESR, but piggy-backs
Practical Electronics | April | 2025
ESR Test Tweezers
ing roughly acceptable values for a
range of capacitors.
Table 1 shows these values. Some
data sheets might specify a dissipation factor or loss angle instead of an
ESR value; page 47 has information
about what those parameters mean and
how to convert them to an ESR value.
The ESR Test Tweezers are much
smaller than the earlier devices, so
we have not been able to include the
table on the equipment, but you can
download it, print it out and keep a
copy handy.
Features & Specifications
❎ Measures ESR/resistance from 0.01Ω to 1kΩ
❎ Measures capacitance from 100nF to 50μF
❎ Can perform in-circuit testing as long as capacitors are discharged
❎ Compact Tweezers format makes probing parts easy
❎ Runs from a single 3V lithium coin cell
❎ Will operate down to a cell voltage of 2.4V
❎ Displays results on a clearly visible OLED screen
❎ Typical accuracy better than 10%
❎ Adjustable sleep timeout and brightness
❎ Display can be rotated to suit left- and right-handed use
❎ Simple calibration of most parameters
❎ The standby cell life is close to the cell shelf life
Design compromises
Silicon Chip ESR Test Tweezers Kit (SC6952, ~£30 + P&P)
This kit includes everything in the parts list except the coin cell & optional header
CON1. The three resistors & one capacitor needed for calibration are included.
onto the Wide-range digital LC Meter
from the June 2019 issue, using its
processor to drive the measurement
circuitry and display the results.
That was a popular project, but
we reckoned we could simplify the
all-important ESR sensing circuitry
and fit it into a much more compact
instrument that costs less to build.
Measuring ESR
Measuring ESR is not difficult in
theory, although we must be able to
separate the effects of the main capacitance and leakage resistance from
the ESR (see Fig.1). As we noted, the
ESR is often taken to include ESL
at a specific frequency, so we don’t
need to concern ourselves with ESL
too much.
The ESR Test Tweezers use the
same philosophy as the other ESR
Meters. Relatively low currents are
briefly pulsed into the capacitor, and
the voltage across the capacitor is
measured. It is allowed to discharge
between tests.
The brief pulses do not have time
to significantly charge the capacitor (assuming it is above 1μF); the
capacitance acts like a short-circuit
in this testing, so it does not affect
the reading.
Since the capacitor is practically
always discharged, the leakage resistance has no effect; the capacitance effectively short-circuits it. The pulses
can also be considered analogous to
an AC signal, so the capacitor’s imPractical Electronics | April | 2025
pedance is low enough that the ESR
dominates.
Knowing the ESR is not enough to
tell whether a capacitor is faulty. It’s
a good idea to verify that its capacitance hasn’t dropped, and this Meter
can do that, too, up to about 50μF.
Beyond that, most DMMs will have
a capacitance measurement mode
that works up to a few thousand microfarads.
Any decent capacitor will specify
its expected ESR value (or equivalent) in the data sheet, and you can
compare that value to the Meter’s
reading. However, when servicing
equipment, the exact part number
may not be known, so the earlier
ESR Meters provided a table show-
This device is patterned on the
very popular Advanced Test Tweezers
from February & March 2024. They
are a compact and elegant device
with many useful functions. So we
have kept the ESR Test Tweezers to
much the same form factor, using
differently-coloured PCBs to make
the two tools easier to tell apart. We
know that many readers will end up
with both!
The Advanced Test Tweezers performed most of the tests in software
running on a microcontroller, so they
needed relatively few external components. For testing ESR, we need
more complicated circuitry, so we
have had to use more components.
They are the same M2012 (0805 imperial) SMD parts that measure 2.0 ×
1.2mm along with a few other parts
in small packages. Apart from there
being more components, construction
should not be any harder than for the
Advanced SMD Tweezers.
The ESR Test Tweezers use simplified circuitry compared to the earlier
Table 1: typical ESR readings for good capacitors
25V
35V
63V
160V 250V
1μF
10V
5
4
6
10
20
2.2μF
2.5
3
4
9
14
4.7μF
6
3
2
6
5
10μF
16V
1.6
1.5
1.7
2
3
6
22μF
3
0.8
2
1
0.8
1.6
3
47μF
1
2
1
1
0.6
1
2
100μF
0.6
0.9
0.5
0.5
0.3
0.5
1
220μF
0.3
0.4
0.4
0.2
0.15
0.25
0.5
0.3
470μF
0.15
0.2
0.25
0.1
0.1
0.2
1000μF
0.1
0.1
0.1
0.04
0.04
0.15
4700μF
0.06
0.05
0.05
0.05
0.05
10mF
0.04
0.03
0.03
0.03
If your
capacitor’s
data sheet does
not mention
a typical or
maximum ESR
value, this table
can be used
as a guide.
If your data
sheet mentions
a dissipation
factor or loss
angle, refer to
our panel on
page 47. This
table can be
downloaded
from
siliconchip.com.
au/Shop/11/238
39
Constructional Project
ESR Meter designs. That’s partly to
help us fit the parts on the board but
also because we were able to reduce
the parts count without compromising performance, saving on parts cost
and assembly time.
For example, the older designs feature a pulse injector with 11 parts
and a pulse amplifier made from 17
parts. The corresponding sections of
our circuit have only five and nine
parts, respectively (50% less overall!).
We are not using a voltage regulator
either; instead, our software compensates for any variations in the supply
voltage from the cell.
The earlier designs used a comparator (built into the processor) alongside a voltage ramp to measure the
pulse amplitude, requiring eight more
parts. Our circuit uses the 12-bit ADC
(analog-to-digital converter) peripheral built into the microcontroller
and no external parts.
Instead of a multiplexed LED display
driven by a shift register IC, requiring
several more parts, we are using the
same graphical OLED display module
as in the Advanced Test Tweezers (although it’s white this time rather than
blue/cyan). It sits over the main PCB,
occupying only the size of a four-pin
header on the main PCB.
The earlier ESR Meters could apply
test pulses up to 50mA. Given that
the ESR Test Tweezers are designed
to run from a coin cell, we aimed to
use lower amplitude pulses to avoid
excessive drain from the cell.
Despite all this, the ESR Test Tweezers can measure fairly accurately
down to 10mW (just like our previous ESR meters) and will draw less
than 1μA of current when in lowpower mode; that’s low enough that
the standby life of the cell will be
close to its shelf life.
We tested our prototype using our
Coin Cell Emulator that was described
in the November 2024 issue. It reported a current of 0.0μA while the
ESR Test Tweezers were sleeping,
less than the 100nA minimum that
the Coin Cell Emulator can display.
The typical operating current is
around 3.5mA with no components
connected to the test leads, rising
Fig.2: the ESR Test Tweezers use a 16-bit, 28-pin PIC24 microcontroller to drive the measurement circuitry and a
small OLED display. Different test currents are applied to the DUT via the 300W, 3kW and 30kW resistors, while Q2
amplifies the voltage across it for the micro to sense using its internal ADC. The diodes protect the micro in case the
probed capacitor has some charge left.
40
Practical Electronics | April | 2025
ESR Test Tweezers
to 5mA when a component is being
tested or settings are being modified.
About half of that current is due to the
OLED screen, which is set to near its
lowest brightness setting by default.
The current draw increases if you
need to operate the OLED at a higher
brightness, but we found that was not
necessary for indoor use.
Circuit details
Fig.2 shows the full circuit diagram
of the ESR Test Tweezers. Many components are common to the Advanced
Test Tweezers: IC1, MOD1 and CON1
are much the same, with IC1 being
the PIC24FJ256GA702 16-bit microcontroller.
IC1 is powered by coin cell BAT1.
The two 100nF capacitors bypass its
two positive supply pins, while the
10μF capacitor provides bypassing
for a 1.8V regulator internal to IC1.
Practically nothing else is connected
directly to the cell, meaning that IC1
has total control over what can draw
current from it.
The 22μF capacitor provides a reserve of power to assist the coin cell in
delivering the test pulse current. This
is about the highest value of capacitor
commonly available in the M2012 size
we are using for this project; it is sufficient for our needs.
The highest pulse current is 10mA,
applied for no more than 50μs. With
a 22μF capacitor, the nominally 3V
rail dips by about 0.02V, rather than
the 0.2V expected without the capacitor. This also means that the coin cell
is subjected to a lower average load;
it does not see the heavy peaks that
would otherwise shorten its useful
life considerably.
CON1 is the ICSP (in-circuit serial
programming) header and the 10kW resistor on IC1’s pin 1 sets the micro to
run normally unless a programmer is
connected. We mainly included CON1
to simplify software development; you
shouldn’t need it in regular operation,
although it may be useful if we ever
release a firmware update.
MOD1 is an I2C OLED module powered at its Vcc pin by one of IC1’s I/O
(input/output) pins. Pulling that pin
low shuts off the display module completely. The other two connected I/O
pins provide the I2C serial control interface.
Tactile pushbuttons S1-S3 connect to
three more I/O pins. Each is furnished
with an internal pullup current from
Practical Electronics | April | 2025
The ESR Test Tweezers PCB (shown enlarged) looks similar to the Advanced
Test Tweezers, but it has different capabilities. We used white PCBs to set them
apart and will provide white arm PCBs to match.
IC1, so their state can be easily detected without external parts. Debouncing
is done by the software.
The parts below MOD1 form the
pulse injection circuitry. The 300W,
3kW and 30kW resistors allow nominal currents of 10mA, 1mA and 100μA
to be generated from a 3V supply rail.
IC1’s I/O pins can source 1mA with
only a small (less than 0.1V) voltage
drop. At 10mA, the drop would be
around 0.6V, so the 300W resistor is
provided with PNP transistor Q1 for
switching; the second 3kW resistor
provides the base current when Q1
is driven.
The 22μF and 100nF capacitors in
parallel are present to limit the amount
of charge that can be injected if a large,
charged capacitor is connected to the
TP+ and TP− terminals. They act together as a low impedance when the
pulses are applied. Silicon diodes D2
and D3 clamp any voltage from the capacitor being tested that exceeds their
forward thresholds.
The presence of D2 and D3 also
means that the maximum pulse that
can be applied is less than 1V. So even
if you test a capacitor in reverse, the
voltage should be low enough to avoid
damaging it.
IC1’s pins 21 and pin 22 are normally kept low, and pin 18 is kept high,
turning Q1 off. The PULSE OUT line
sits at 0V and the 22μF and 100nF capacitors are discharged via the 10kW
resistor at bottom left. Any connected
device is also discharged.
Just before a pulse is applied, pins
21 and 22 are put in a high-impedance
state by the processor. The appropriate
pin is driven high (or low in the case
of pin 18) to start the pulse. A measurement is then taken, and the pins
revert to their idle state, ready for the
next measurement.
Sense amplifier
The DUT (device under test), usually a capacitor or low-value resistor,
connects between the TP+ and TP−
pins. The test current applied to the
PULSE OUT line induces a voltage at
TP+ relative to circuit ground. The
circuitry below IC1 amplifies the resulting voltage. When IC1’s pin 25 is
low, this circuitry is powered off via
the AMP POWER line, but it is brought
high during testing.
The 1MW/470kW divider ensures that
Q2 is biased on slightly, as long as the
supply is above about 2V. The 100nF
capacitor at Q2’s base will have the bias
voltage across it. Before a pulse is applied, the voltages at LOW ANALOG
(pin 24, AN7) and AMP OUT (pin 23,
AN8) can be sampled by IC1’s ADC to
record a baseline voltage.
The LOW ANALOG line will be
close to 0V, and the AMP OUT pin
will be close to the voltage provided
by the AMP POWER line, which will
be reduced slightly due to Q2 being
biased on slightly.
When a pulse is applied, the voltage
rises at the TP+ pin, and the voltage at
Q2’s base rises by a similar but slightly
smaller amount. The reduction is due
to the signal being attenuated by the
surrounding components, such as the
10kW resistor and 1MW/470kW divider.
Q2 behaves as an emitter follower,
so its emitter will rise by much the
same voltage, and the current through
the 100W resistor will be proportional
to the emitter voltage.
Since the collector current will
match the emitter current (give or
take the much smaller base current),
41
Constructional Project
the current through the 2.2kW resistor will be the same as that through
the 100W resistor, meaning that the
voltage across the 2.2kW resistor is
22 times that across the 100W resistor.
The microcontroller then takes another sample to compare with the baseline values. In practice, the change
at the AMP OUT pin is 10-15 times
the change at the LOW ANALOG
line. Of course, the AMP OUT line
will fall during a pulse, while the
LOW ANALOG line will rise, but it
is simple enough to take the difference either way.
The 1kW resistor and dual diode
D1 provide another level of protection against external voltage sources
(such as charged capacitors).
While it appears that we effectively
have six ranges to read (two analog
inputs multiplied by three current
sources), they overlap. We use four
ranges: the 100μA source sensed at the
LOW ANALOG input and all three test
currents sensed at the AMP OUT input.
Note that neither the LOW ANALOG
or AMP OUT signals can swing railto-rail. Diode D1 clamps the LOW
ANALOG level between AMP POWER
and ground. Due to the 100W resistor,
the AMP OUT signal cannot reach 0V,
even if Q2 is saturated.
Several calibration factors are programmed into the ESR Test Tweezers, including the levels at which
the LOW ANALOG and AMP OUT
signals are valid.
Firmware
The firmware driving the ESR Test
Tweezers has much in common with
the Advanced Test Tweezers since
they use the same microcontroller.
However, the ESR Test Tweezers do
not have as many features.
We have implemented three measurement modes, labelled ESR, RES
and CAP. The ESR mode provides a
function similar to our previous ESR
meters.
The main ESR testing mode uses the
Parts List – ESR Test Tweezers
1 double-sided main PCB coded 04105241, white solder mask, 36 × 28mm
2 double-sided arm PCBs coded 04106212, white solder mask, 100 × 8mm
1 double-sided back panel PCB coded 04105242, white solder mask, 36 ×
28mm
1 0.96in 128×64 I2C OLED module, white (MOD1)
1 surface-mounting 32mm coin cell holder (BAT1)
3 SMD two-pin tactile switches (S1-S3)
1 3-pin gold-plated header, 2.54mm pitch (for tips and mounting MOD1)
1 4-pin header, 2.54mm pitch (to mount MOD1; usually comes with MOD1)
1 5-way header, 2.54mm pitch (CON1; optional, for ICSP)
1 M2 × 6mm Nylon panhead machine screw
2 M2 Nylon hex nuts
1 CR2032 or CR2025 lithium coin cell
1 small piece (eg, 2 × 2cm) of double-sided foam-core tape
2 100mm lengths of 10mm diameter clear heatshrink tubing
Semiconductors
1 PIC24FJ256GA702-I/SS microcontroller programmed with 0410524A.HEX,
SSOP-28 (IC1)
1 BC859 PNP transistor, SOT-23 (Q1; marking 4C)
1 BC817 NPN transistor, SOT-23 (Q2; marking 6C)
1 BAT54S dual schottky diode, SOT-23 (D1; marking KL4)
2 1N4007WS silicon diodes, SOD-323 (D2, D3)
Capacitors (all SMD M2012/0805 size 6.3V+, X5R or X7R)
2 22μF
1 10μF
4 100nF 50V X7R
extra 10μF (could be any type) for capacitance calibration
Resistors (all SMD M2012/0805 size, 1/8W, 1% – codes in brackets)
1 1MW (105 or 1004)
2 10kW (103 or 1002) 1 1kW (102 or 1001)
1 470kW (474 or 4703) 2 3kW (302 or 3001)
1 300W (301 or 300R)
1 30kW (303 or 3002)
1 2.2kW (222 or 2201) 1 100W (101 or 100R)
extra 10W, 100W and 1kW resistors for calibration
42
100μA source and the LOW ANALOG
input to detect if a component is present across TP+ and TP−. If so, it runs
pulses from each of the 100μA, 1mA
and 10mA sources, taking measurements using the AMP OUT signal from
the pulse amplifier.
If the 10mA pulse gives a valid AMP
OUT reading, an ESR value is calculated using this data and a calibration
factor. The 1mA pulse is checked next;
if this is not valid, the ESR reading is
taken from the 100μA pulse.
You can tell which range has been
used from the number of decimal
places displayed. The 10mA pulse
gives a result to two decimal places
(0.01W), while the 1mA pulse gives a
result to the nearest tenth of an ohm
and so on.
The RES mode (for resistance) is
intended to measure the values of
resistors, and it does so using only
the 100μA source. That makes it a bit
easier on the cell since there are no
high-current pulses. The resolution
of the RES mode is only around 10W;
we expect it to be useful if you have
many parts to sort through.
The CAP mode gives a reading for
both capacitance and ESR for the
device under test. It also uses the
100μA source but applies it for long
enough to charge up the capacitor, although this is somewhat limited by
the 22μF capacitance in series with
the DUT.
It takes readings at 40μs, 400μs
and 4ms from the start of the pulse.
Our prototype gave us fairly accurate
readings up to 50μF, so we’ve specified that as the maximum. The display
will show dashes if the measured capacitance is higher than 50μF.
The lower limit of 100nF is due to
the resolution being about 10nF; the
readings will tend to be inaccurate
below 100nF. Since we have collected
much the same data as the RES mode,
an ESR reading is given too, with the
same limitations as that mode.
The firmware is also responsible
for monitoring button presses and
putting the processor to sleep when
the device is not being used. There
is a SETTINGS mode where preferences and calibration parameters can
be changed, including the option to
save the calibration and settings to
flash memory.
We’ll delve into the calibration,
setup & operation of the ESR Test Tweezers once construction is complete.
Practical Electronics | April | 2025
ESR Test Tweezers
Construction
The SSOP-package microcontroller
and M2012 parts mean assembly is not
overly difficult, but it best suits constructors with some experience working with SMDs. If you have built the
Advanced Test Tweezers, you should
have little trouble with the ESR Test
Tweezers.
You will need a fine-tipped soldering iron, solder, flux paste and solder-
wicking braid. You should also have a
magnifier, SMD tweezers and a means
of holding the PCB in place, such as
Blu-Tack. Good lighting is highly recommended, along with fume extraction (or work outdoors or near a large
open window).
Start by placing a little flux paste
on the PCB pads for IC1 and rest it
in place, checking that the pin 1 dot
is in the correct position. Looking at
the PCB with CON1 at the bottom,
the text on the chip should be rightway-up. Check your build against the
Fig.3 overlay diagram and accompanying photos.
Note that our photos show CON1
fitted (which isn’t necessary unless
you need to program the chip onboard).
We also fitted a socket for MOD1 so
we could remove the OLED if necessary; you can hard solder it using a
standard pin header.
Tack solder a couple of IC1’s leads
and check that the other pins on both
sides are correctly aligned. Adjust it if
needed before carefully soldering the
remaining pins. When finished, clean
away any flux residue (eg, using alcohol) and closely inspect the soldering
before proceeding, as it will be much
easier to correct problems you find
before more components are fitted.
If you have bridged any of the pins
of the IC, add a dab of flux paste on top
and then use solder-wicking braid to
clear it. Verify that all pins have had
solder flow onto both the pin and the
pad; if it’s just on the pin, it will not
make a good connection to the PCB.
Fit the three SOT-23 devices next,
being careful not to mix them up.
Dual diode D1 is near the top of the
PCB, with PNP transistor Q1 near
the bottom. Q2, the NPN transistor,
is near IC1. If you aren’t sure which
is which, they should have codes
printed on the top. The parts list has
likely codes (although they can vary
by manufacturer).
In each case, apply a little flux paste
to the pads, tack one lead, then check
Practical Electronics | April | 2025
Fig.3: fit the components to both sides of the main PCB as shown here. Most
of them are moderately easy to solder apart from IC1, which has closely
spaced pins. Don’t mix up the different SOT-23 devices and note that D2 and
D3 are connected in opposite directions. You don’t need to fit the headers
for CON1 and MOD1; we did so to simplify the development process.
These photos show a number of the important construction details. The
arms attach to the main PCB with chunky solder fillets and are protected by
heatshrink tubing. The white screw and nuts prevent the coin cell from being
easily removed. A header pin soldered between the main PCB and the OLED
PCB helps to reinforce the OLED mounting. A solder fillet mechanically secures
the tips to the arms. Ensure that the solder surrounds one end of the header pin
and flows into the holes in the arm PCB.
that the other two leads are within their
pads before soldering them.
The two single diodes, D2 and D3,
face in opposite directions, so check
that the PCB’s cathode markings match
the devices’ cathode stripes.
Fit the capacitors next, being careful not to mix them up, as they are
not marked. There are four 100nF capacitors on the front of the PCB plus
one 10μF capacitor. One of the 22μF
capacitors is on the front, while the
43
Constructional Project
Screen 1: the default display at
power-on. Touching the tips together
will show a low readings in ohms.
The cell voltage is displayed next to
a countdown timer; when the timer
expires, the Tweezers enter a lowpower sleep mode.
other mounts on the back of the PCB.
Now carefully work through the
11 resistors, matching the markings
to the PCB silkscreen. The parts list
shows the typical markings for the
values we are using. Note that one
of the 3kW parts is also on the back
of the PCB.
Next, solder the cell holder to the
back of the PCB. Make sure that the
opening faces towards the screw hole;
you can compare it to our photos.
Now thoroughly clean the flux residue off the PCB using a suitable solvent. Your flux might recommend one
on its data sheet, but isopropyl alcohol is a good all-round alternative.
Methylated spirits can be used, although it might leave residue. Allow
the PCB to dry and inspect it again
before proceeding.
Next, solder the three tactile switches, S1-S3. We do this now to avoid
getting solvent in their mechanisms.
They are fitted in much the same way
as the other surface mounting parts but
are a bit larger and easier to manage.
Screen 2: the second operating mode
uses the low-current range to measure
resistance without unnecessarily
loading the cell. If S1 is pressed in
any operating mode, the timer is
paused and dashes are displayed, as
seen here.
You can carefully clean up any flux
residue from this step using a cotton
tip or similar moistened by a small
amount of solvent.
Programming the
microcontroller
You won’t need to perform this step
if you have a pre-programmed microcontroller from the Silicon Chip Online
Shop (including the one in our kit).
If you have a blank micro, it’s best to
program it now before the arms and
display are fitted, as they might get
in the way.
You’ll need a Snap, PICkit 3, PICkit
4 or PICkit 5 programmer to program
the PIC24FJ256GA702 microcontroller.
The Snap cannot provide power, so
you can temporarily fit the coin cell
while programming occurs.
We suggest using Microchip’s free
MPLAB X IPE for programming. It’s
available as part of the MPLAB X IDE
download and can be installed on
Windows, Mac and Linux computers. Choose the PIC24FJ256GA702
Screen 3: the third mode gives
readings for capacitance (between
100nF and 50μF) and ESR using
low-current pulses. A typical 10μF
capacitor is connected here. Pressing
S2 will resume the timer, as will
changing modes with S3.
and open the 0410524A.HEX file in
the IPE. Enable power from the programmer if you need it.
To avoid permanently soldering the
header to the PCB, you can push the
5-way header into the socket on your
programmer while holding the other
ends of the pins in place through the
pads of CON1. It’s a bit of a juggle,
but it will make the Tweezers easier
to use later.
Click the button to program the
chip and check that the IPE verifies
the program correctly.
Fitting the arms
The arms are each formed from a
long, thin PCB, with the tips using
gold-plated header pins to offer a lowresistance contact surface that will not
corrode. Tin each arm tip generously and remove the header pins from
their shroud.
Using a pair of tweezers, solder a pin
in position to the end of each arm, as
shown in the photos. Try to line them
up so they are centred. Note that the
The rear of the ESR Test Tweezers before the
protective panel is attached.
Coin Cell Precautions
The ESR Test Tweezers make use of a coin cell. Even though
we have added protections such as the locking screw, there is no
reason for this device to be left anywhere that children could get hold of it. Also,
the tips are pretty sharp and might cause injury if not used with care.
44
Practical Electronics | April | 2025
ESR Test Tweezers
Screen 4: the Calibrate step takes
readings with open and shorted
tips and automatically sets the ADC
saturation settings and probe (contact)
resistance. Leave the tips open, press
S1, then hold the tips together and
press S1 again. Then release the tips.
You can try again if you get an error.
pins face inwards once the Tweezers
are assembled.
The arm PCBs slot over the larger
pads in the corners of the main PCB.
We recommend not fully pushing
the main PCB into the slot; leave
some room. Take care that the arms
do not contact any other pads on the
PCB.
Fitting the arms is a bit like fitting
the SMD components. Tack them
roughly in place and check that they
are aligned well, then add more solder
to secure them firmly. Check the action
and see that the tips meet correctly.
Finally, add solid fillets of solder allround to make them mechanically
secure.
Slide the heatshrink tubing over
the arms, leaving the tips clear, then
shrink it in place. Doing this now
avoids damage to the OLED screen
from excessive heat.
We’ve taken some photos of the ends
of the arms so you can see how the
tips are attached and how the arms
mount to the main PCB.
Screen 5: the bandgap voltage is the
nominally 1.2V reference used by
IC1 for voltage measurements. At
the bottom is the calculated supply
(cell) voltage; use S1 & S2 to trim the
bandgap until the displayed voltage
matches the cell voltage, measured
using a multimeter or similar.
The OLED screen
The OLED is mounted next. You
should be able to simply slot the
four-way pin header into the pads
of the MOD1 footprint on the PCB.
We recommend temporarily placing
a piece of card behind the OLED to
prevent it from shorting the main PCB
or arms. This will also help to add a
small space between them. Tack one
pin and check that the display is neat
and square.
Solder the remaining pins and
remove the piece of card. You can
fit the battery at this stage and check
that everything works. You should see
something like Screen 1 when it is
first powered on. The reading should
show a low value (under 0.1W) when
the tips are shorted together.
Remove the battery and solder a pin
header or piece of solid wire to the top
right corner of MOD1 and through to
the main PCB underneath. This provides extra support for the OLED to
prevent it from flexing and touching
the main PCB.
Screen 6: the display can be rotated
by 180° to suit left- or right-handed
use. Press S1 to toggle it and the
display will rotate immediately to the
new setting. Like all the other settings
here, these new values are used
immediately but are not automatically
saved to non-volatile flash memory.
The back panel PCB can be soldered to the ground pins of MOD1
and CON1 or simply stuck to the back
of the cell holder using double-sided
tape. Ensure that the ESR TWEEZERS
legend faces outwards (it’s a dual-use
panel; the other side has the legend for
the Advanced Test Tweezers).
Finally, fit and secure the cell using
the M2 Nylon screw and nuts. The
nuts go on the same side as the cell,
giving the depth needed to prevent
the cell from being easily pulled out.
The photos show how we have done
that on our prototype. This is to prevent a child who might get hold of
the Tweezers from removing the cell,
which could be dangerous (it is hard to
pull out regardless, but this is worthwhile extra security).
Calibration and operation
In regular operation, pushbutton
S3 cycles between the modes, while
S1 pauses the countdown timer. S2
(or any S3 mode change) will enable
it again. The timer is shown at upper
The ESR Test Tweezers shown at actual size. It’s easy to read the screen while
probing components. Most constructors do not need to solder the programming
pin header.
Practical Electronics | April | 2025
45
Constructional Project
Screen 7: as with our other Tweezers,
the OLED current draw is the single
most significant drain on the cell.
Setting the display brightness as
low as possible (using S1 & S2) will
prolong the cell life. The default level
of 30 is the lowest usable setting; it can
be changed in steps of five up to 255.
right and defaults to 10 seconds. When
it expires, the low-power sleep mode
is activated. Normal operation is resumed by pressing any button.
Screen 2 shows the RES mode, with
a 510W resistor connected. The three
dashes at upper right indicate that the
timer is paused. That means the ESR
Test Tweezers will not go to sleep; it
will probably drain the battery within
a day or two if left like this.
Screen 3 shows a 10μF capacitor
connected in CAP mode; similarly,
the timer has been paused to allow
continuous readings to be made. All
three operating modes also show the
cell voltage at the top of the screen.
Our prototype could function down
to around 2V. This is about the point
at which the PIC24 processor stops
working. We specify 2.4V as the minimum supply voltage, as the accuracy of readings declines significantly
below that.
A long press of S3 (about two seconds) switches between operating
and settings modes, with S3 then
cycling through the various parameters and S1 and S2 adjusting them.
The ESR Test Tweezers are usable
Screen 8: the timer is displayed in
the ESR, RES and CAP modes. The
Tweezers go into a low-power sleep
when it counts down to zero. The time
can be set in multiples of five seconds
up to 995 seconds (about 16 minutes).
Since the timer can be paused, you
might not need to change this setting.
without calibration, but the calibration steps are easy. There are also a
couple of customisation preferences
you can apply.
Many calibration steps involve measuring a known value or voltage with
the Tweezers and trimming the calibration factor until the displayed value is
accurate, which is quite simple and
intuitive.
The suggested parts to use are 10W,
100W and 1kW resistors for calibrating
ESR and a 10μF capacitor for calibrating capacitance. These values are near
the top of their ranges, so they will
provide the best resolution when performing the calibration.
The calibration factors are shown in
ohms because they are analogous to
providing an exact value for the second
resistor in a divider. However, because
of the circuit’s complexity, they don’t
correspond to any measurable resistance value.
If you don’t have these exact value
resistors, a lower value (preferably
within that decade) will be adequate.
Higher values might be outside the limit
of their respective range, in which case
the display will show “OPEN”.
We designed this PCB to protect the back
of the Test Tweezers. It can be attached
to the cell holder with double-sided tape.
It has markings on the opposite side so
that it can also be used for the Advanced
Test Tweezers. This blue version will
be available on our website for users of
the Advanced Test Tweezers, although a
white version will be included in ESR Test
Tweezers kits.
46
Screen 9: four screens like this
calibrate the current pulse values.
Connect the recommended resistor or
capacitor value (100W here) across the
probes and trim the value until the
smaller text (99.90W) is close to the
actual value connected. The default
values are based on our prototype.
Remember that while resistors are
readily available with 1% tolerance
or better, capacitors could vary up to
20%. If possible, measure your capacitor with an accurate capacitance
meter and use that instead of the
nominal value.
The panels above with Screens 4-12
detail the available calibration and
setup options. Be sure to do the steps
in the order listed, as some factors
depend on others being set accurately
beforehand.
To return to normal operation from
settings, press and hold S3 for about
two seconds. Be aware that the sleep
timer does not count down while in
Settings mode, so you should return
to operating mode immediately after
changing the settings to avoid draining the battery.
Using the ESR Tweezers
Connect the component to be tested
between the tips of the probes and
apply pressure to make sure they are
making good contact. Polarised components should have their positive
lead connected to the top (TP+) tip.
However, the test voltage is low and
should not cause damage if the component is reversed.
Diagnosing capacitor problems due
to high ESR is helpful for those in the
power and audio fields. Now you can
check that with a handy, compact tool
that doesn’t cost much to build.
The ESR Test Tweezers can measure
ESR, resistance & capacitance (albeit
over somewhat limited ranges), making
them more valuable than the 2004
design and in a smaller package. SC
Practical Electronics | April | 2025
ESR Test Tweezers
Screen 10: this is the last screen you
should need to use for setup and
calibration. Press S1 to save any
altered settings to flash memory;
S2 will load the defaults in case the
saved data becomes corrupted. The
defaults can also be loaded by holding
S3 while powering up the Tweezers.
Screen 11: after saving to or restoring
from flash, you should get a message
indicating it completed successfully.
This is the last necessary step for
setup and calibration; a long press of
S3 will return to operating mode. As
well as on the first use, you should
recalibrate when a new cell is fitted.
Screen 12: there are some screens
after Save/Restore that should not
need to be changed; they adjust the
factors set by the Calibrate step shown
in Screen 4. They include the probe
contact resistance (shown here) and
two pages with ADC limit values, used
to check that readings are valid.
Dissipation factor, loss angle and ESR
Dissipation factor (DF) and loss angle (δ) measure the
energy lost in an oscillating system. Many capacitor data
sheets specify these instead of providing an ESR value.
In our case, the dissipation factor and loss angle specifically refer to the losses in a capacitor due to ESR.
These terms are also used in other contexts in electrical
engineering, but we are looking specifically at capacitor
ESR. We want to relate the capacitive reactance to the
pure resistance due to ESR. Both can be plotted on the
complex number plane, hence the references to angles.
The loss angle is simply the inverse tangent function
of the dissipation factor; thus, you might also see ‘tangent of loss angle’, which means the same as ‘dissipation factor’.
Since the reactance changes with frequency, we need
to focus on a specific frequency.
For example, in a transformer-based mains power
supply, the capacitors will be subjected to predominantly 100Hz (50Hz mains) or 120Hz (60Hz mains) ripple. Capacitors in audio circuits will be subjected to a
broader range of frequencies, perhaps 20Hz to 20kHz.
Capacitors in switch-mode supplies will generally have
ripple at 20kHz to 2MHz.
Let’s take a concrete example of a capacitor, such as
the 4700μF 50V electrolytics we have previously in our
projects, in the form of Nichicon UVZ1H472MRD capacitors. We used them to filter the rectified output of a mains
transformer. Their data sheet lists a (maximum) tangent
of loss angle of 0.2.
That corresponds to a loss angle of 11.3° or 0.197
radians, ie, tan(11.3°) ≈ 0.2. Note that the loss angle (in
radians) is very close to the dissipation factor for typical
values. This is a well-known approximation for the tangent function at low values.
Using the impedance equation for capacitors of Z = 1 ÷
(2πfC), we get an impedance value of 0.34W for a 4700μF
capacitor at 100Hz. Multiplying this by the dissipation
factor of 0.2 gives an ESR of 0.068W, close to the 0.05W
noted in Table 1 for similar capacitors. If you measured an
Practical Electronics | April | 2025
ESR of 0.05W for such a capacitor, that would be acceptable, as it is below the specified maximum.
The loss angle (δ) can be visualised with a diagram of
the complex impedance (Fig.a), which shows the reactance due to capacitance in the imaginary plane (vertical)
and the resistance due to ESR in the real plane (horizontal).
The cosine of the loss angle relates to the proportion
of energy transmitted by the capacitor (compared to that
dissipated by the ESR). At low loss angles, the cosine of
δ is close to unity, and there are no losses, although they
rise sharply as the angle (and ESR) increases.
These ideas are similar to concepts like power factor
(and power angle), although, in AC power systems, the
capacitive element is undesirable and a purely resistive
load is preferred.
You can also see from this how a high ESR would create a phase shift for audio signals, increasing distortion.
Fig.a: this complex plot
shows how a capacitor’s
impedance (Z), ESR and
loss angle (δ) are related.
The dissipation factor
(DF) is the ratio of the
horizontal distance (ESR)
to the vertical distance (Z),
ie, DF = ESR ÷ Z = tan(δ).
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