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Assembly Manual
ESR & Low Ohms Cat No. K 7204
Meter
by Bob Parker
ACN 000 908 716
K I T
Reproduced in part by arrangement Please read Disclaimer carefully as we
with Electronics Australia, from their can only guarantee parts and not the
January 1996 edition. labour content you provide.
If you repair switch-mode power supplies, TV receivers, computer monitors, vintage radios,
or similar equipment, and/or if you need to measure very low values of resistance, this proj-
ect can save you lots of time and aggravation - as it has for me. It measures an aspect of
electrolytic capacitor performance which is very important, but normally very difficult to
check: the equivalent series resistance, or `ESR'.
ome wise person once said, "The can do is give a low reading if the elec- currents, the electrolyte will start to
S reliability of any piece of electronic
equipment is inversely proportional to
tro is nearly open circuit. decompose and the dielectric may dete-
riorate - and the ESR will increase far
the number of electrolytic capacitors in About ESR... more rapidly.
it", and I doubt that many service techni- To make things worse, as the ESR
cians would disagree! So what exactly is an electrolytic's increases, so does the internal heating
Especially now that switch-mode equivalent series resistance? caused by ripple current. This can lead to
power supplies (SMPSs) have been Electrolytics depend on a water-based
commonly used in domestic VCRs and electrolyte, soaked into a strip of porous
TVs, etc for a decade or so, one of the material between the aluminium foil
most likely components to fail is the plates, to complete the `outer' electrical
humble electrolytic. The symptoms can connection to the aluminium oxide
be as diverse as a VCR's playback pic- dielectric coating on the anode foil.
ture swimming in tiny dots, up to SMPSs The electrolyte has electrical resist-
mysteriously self-destructing. ance which, along with the (negligible)
As a service technician myself (though resistance of the connecting leads and
I'd prefer to be a full-time designer!), I aluminium foil plates, forms the capaci-
was just about tearing my hair out tor's equivalent series resistance.
because of the difficulty in determining Normally the ESR has a very low
which electros were faulty and which value, which stays that way for many
ones were still OK, in SMPSs and other years unless the rubber seal is defective.
equipment. I wanted to be able to check Then the electrolyte's water component
electros in circuit, with the power safely gradually dries out and the ESR creeps
disconnected. up with time. The electro gradually acts
Why not use a readily-available capac- more and more like a capacitor with its
itance meter? Because when electros go own internal series resistor...
faulty, they normally don't lose their
capacitance significantly (as many tech- Heat makes it worse
nicians assume they do). Rather their
equivalent series resistance (ESR) `goes If an electro is subjected to high tem-
through the roof'. Capacitance meters peratures, especially from heat generat-
don't tell you this; about the best they ed internally as a result of large ripple
Text and illustrations courtesy of Electronics Australia
only a few thousand hours at their max- needed because the display decimal
imum rated temperature and ripple cur- points are on the wrong side for this
rent. (A year is only 8766 hours!) application.
Switch-mode power supplies place If you forget to turn the power off, the
quite severe stresses on filter capacitors. micro will do it for you when the dis-
Because of their compact construction, played reading has remained the same
temperatures are high (that's why your for two minutes. This feature can be dis-
PC's power supply is equipped with a abled for uninterrupted operation from a
fan), and the capacitors have to endure 9V optional plugpack.
large ripple currents. When the battery voltage is nearly too
low for the circuit's 5V regulator to
Micro-based ESR meter function correctly, the Z86 reduces the
power to the LED displays and flashes a
Necessity is supposed to be `the moth- `b' on the right-hand one in the `off-
er of invention', but desperation works scale' condition, to warn you to look for
even better and I designed this ESR a new battery.
meter from scratch. It's based on a ver-
satile Zilog Z86E0408 or Z86E0412 Meter operation
microcontroller - which already has two
voltage comparators and two flexible There's a single pushbutton, which has
counter/timers built in, greatly simplify- three functions: one press turns the
ing the rest of the circuit. A micro also power on, and another press will switch
allows the easy incorporation of some it off again if the measured resistance is
A close up view of the ESR meter `user-friendly' features... 1 or more.
PCB, showing the position and orien- This instrument has three ESR ranges, A push of the button with the leads
tation of all the parts - except the test
leads jacks, push button and battery.
with full-scale readings to 0.99, 9.9 shorted together will cause their resist-
Note that the LED displays and deci- and 99 respectively. The range is auto- ance value to be subtracted from all sub-
mal point LEDs are mounted via a 28- matically selected by the micro, so your sequent readings, as long as it's less than
pin IC socket. hands are free to hold the test leads. The 1.
an upward spiral in the capacitor's core accuracy of the prototypes was better Before making a measurement on an
temperature, followed by complete fail- than +/-5% of displaying reading, +/-1 electrolytic, first discharge it. I find a
ure - sometimes even explosive! digit. A single `-' on the left-hand dis- 100 5W does this well. Then connect
The service life of electros is approxi- play indicates a reading above 99. the test leads - either way around. If your
mately halved for every 100C increase in The readout is on two 0.5'' (13mm) capacitor is still in circuit (with the
temperature and, surprisingly, many are seven-segment LED displays, plus two power OFF!), you're likely to get quite
designed for a reliable operating life of 3mm decimal point LEDs which are an accurate reading, because it should be
The schematic may look a little complicated, but the circuit is actually quite elegant. Everything is under the control of
IC2, a low cost microcontroller. As well as auto ranging, it also compensates for test lead resistance.
Page 2 Text and illustrations courtesy of Electronics Australia
the component with by far the lowest Parts List
high-frequency impedance.
The circuit's test signal has a peak Resistors
(All 0.25W 5% unless noted) 4-band 5% 5-band 1%
open-circuit voltage of 600mV (maxi-
R1,19,20,21,23,24 10k brn-blk-org-gld brn-blk-blk-red-brn
mum 100mV peak at full-scale reading), R2,27,28 4.7k yel-vio-red-gld yel-vio-blk-brn-brn
so it won't make diodes or transistors R3,26 15k brn-grn-org-gld brn-grn-blk-red-brn
conduct and cause measurement errors. R4,25 47k yel-vio-org-gld yel-vio-blk-red-brn
Compare the reading you get with the R5,7,9,15 2.2k red-red-red-gld red-red-blk-brn-brn
front-panel table, to get an idea of R6 10k 1% M/F brn-blk-org-brn brn-blk-blk-red-brn
R8 1k 1% M/F brn-blk-red-brn brn-blk-blk-brn-brn
whether the electro's ESR is about nor- R10 100 1% M/F brn-blk-brn-brn brn-blk-blk-blk-brn
mal or significantly above it. R11 220 red-red-brn-brn red-red-blk-blk-brn
The table's approximate `worst ESR' R12 1k brn-blk-red-gld brn-blk-blk-brn-brn
figures were taken from the Nippon R13 100k brn-blk-yel-gld brn-blk-blk-org-brn
Chemi-Con Aluminium Electrolytic R14 220k red-red-yel-gld red-red-blk-org-brn
Capacitors Catalog Number 4, as R16 180 brn-gry-brn-gld brn-gry-blk-blk-brn
R17 6.8k blu-gry-red-gld blu-gry-blk-brn-brn
100kHz impedance figures for their R18 680 blu-gry-brn-gld blu-gry-blk-blk-brn
`SXE', `SXG' and `LXA' capacitor R22 470k 1% M/F yel-vio-yel-brn yel-vio-blk-org-brn
series. They agree pretty closely with R29 2.7k red-vio-red-gld red-vio-blk-brn-brn
my own measurements of many new VR1 10k hor. trimpot
electros of assorted brands, styles, sizes VR2 200 hor. trimpot
and ages. Capacitors
C1 100uF 16/25 VW RB IC1 78L05 voltage regulator
From my actual faultfinding experi-
electrolytic IC2 Z86E0408/Z86E0412
ence with the prototypes, an electro's C2,4,5,13 0.1uF/100n/104 50VW microcontroller
ESR needs to be many times the table disc/monolithic (with custom firmware)
value before it's likely to cause trouble. C3,9 220uF 16/25VW RB IC3 4094 CMOS shift register
electrolytic DIS1,2 Seven-segment LED
Low ohms uses C6 47uF 50VW bipolar RB displays LTS 5503AE
electrolytic (see text)
C7 33nF/0.033uF/333 63/100VW LED1,2 3mm LEDs (see text)
The instrument is very handy for C8 22uF 16/25 VW RB
measuring low values of resistance, too. electrolytic Miscellaneous
But note that because it makes an AC C10 0.47uF/470n/474 Resistor 82R 1% & resistor 5.6R 5% (used
measurement, it can't give a sensible 63/100VW MKT for calibration); XTAL-1 3.58MHz crystal,
indication of the DC resistance of induc- C11,12 27pF 50V NPO disc ceramic HC-49/U case; One each 16, 18 28-pin IC
sockets; `216' type battery snap lead; plas-
tive components, such as transformer Semiconductors tic utility box, type UB3; PC board; 95 x
windings or chokes. D1,2,5,6 1N4148/1N914 etc signal 57mm, code ZA1245 (96ESR1); push-on
I've used it to `roll my own' low value diode momentary pushbutton switch; two x 4mm
resistors, by measuring off the required D3,4 1N4002/4 etc power diode banana sockets; pre-punched silk-
number of (milli) ohms of DSE catalog Q1,3,4,5 BC328 PNP transistor screened front panel; 4 x 15mm spacers;
Q2,6,11, 2 x 4mm banana plugs; 2 x mini alligator
number W-3200 resistance wire, which
12,13 BC338 NPN transistor clips; PCB pins; piece of foam; red per-
I then formed into a spiral by wrapping Q7 BC548 NPN transistor spex display filter; hookup wire; solder; 8 x
it around a drill shank. Q8,9,10 BC558 PNP transistor washers; test leads.
It can also locate short circuits on PC
boards, by measuring actual track resist- radiated by the horizontal output stage so that only one display and its decimal
ances. If the reading increases as you can be picked up by the test leads and point LED can illuminate at a time,
probe further along the track, you know cause unstable readings. under control to the Z86's pin 18.
you're going in the wrong direction! Every 5ms the Z86 serially updates the
You could also use it to confirm solid The circuit segment and decimal point data in IC3,
continuity of mains lead earth conduc- and swaps displays; at this speed it
tors, etc, etc... Now let's have a brief look at how it appears both displays are continuously
all works, by reference to the schematic. illuminated.
What NOT to do A push of the button switches on Q1, The Z86 starts a measurement by
supplying +9V to IC1, the 5V regulator. grounding its pin 4, switching on current
There are a few minor points to keep Once the Z86E0408 or Z86E0412 mirror transistors Q9 and Q10 and caus-
in mind, when using the meter. First, it microcontroller (`Z86' for short) starts ing C10 to begin charging at a constant
can't identify leaky or short-circuited up, it forward biases Q2. This holds Q1 rate. Under control of its firmware, the
capacitors - that's what the resistance on, maintaining the 5V supply after you Z86 drives Q3, 4 or 5 to apply short cur-
ranges of normal multimeters are for! release the button. rent pulses of 0.5, 5 or 50mA, via C5/6,
Also, avoid using self-retracting `curly' The displays are driven by IC3, an to the capacitor being tested. The result-
test leads, because their inductance can eight bit serial-in parallel-out shift regis- ing voltage pulses, proportional to the
cause small measurement errors. ter. IC3's outputs are connected to both electro's ESR, are amplified by Q7 and
Lastly, don't use the meter right next seven segment displays and the decimal Q8 (no relation to the one in the Persian
to an operating TV set or computer point LEDs in parallel. Cathode driver Gulf!), and compared with C10's volt-
monitor - the high amplitude pulses transistors Q12 and Q13 are configured age. Q6 functions as a ground reference
Text and illustrations courtesy of Electronics Australia Page 3
plays the value.
Now for the `frills'...
Battery voltage check: The Z86 regu-
larly allows C10 to charge to 2V. If the
battery voltage sample on VR1's wiper
(fed in via pin 9) drops below this 2V
level, the `battery low' warning condi-
tion is triggered.
Protection: Sooner or later this meter
will find itself connected to an unexpect-
edly charged capacitor (or worse), so
some protection has been built in. D3,
D4, D5 and D6 are strategically placed
to prevent destructive voltages from
finding their way back to the microcon-
troller; D4 also limits the test voltage to
0.6V peak.
The rest: When the Z86 detects the
button is pushed while measuring a
resistance of one ohm or more, it switch-
es off Q2 and displays questions marks
until your finger leaves the button. Then
Q1 switches off the battery supply.
Crystal XTAL1, C11 and C12 com-
plete the Z86's clock oscillator, which
provides the timing for all the meter's
operations.
Here is the PCB diagram showing where everything goes. Use it in conjunction Construction
with the photo when you are assembling your own board, to ensure that you get
everything in the right place. As the photos show, every component
switch. ESR. At the end of each measurement, except the pushbutton is mounted on a
A simple time measurement allows Q11 is used to discharge C10. 57 x 95mm PC board, coded ZA1245
the Z86 to determine the amplitude of Then the Z86 subtracts the test lead (96ESR1).
the voltage pulses and so the capacitor's resistance (if it's been zeroed), and dis- Before fitting any components to the
The display window (perspex) is inserted into the panel cut-out from inside the flush with the front panel.
Fig.1: Shows how the
front panel is mounted to
the PCB using 15mm
insulated tapped spacers.
Note, if more clearance is
needed between the LED
display and perspex win-
dow we suggest to insert
one or two flat washers
beneath each spacer as
shown in the diagram.
Page 4 Text and illustrations courtesy of Electronics Australia
supplied) 9V plugpack, leave R25 range.
(47k) off the PCB. This will dis- Using alligator clip leads or similar,
able the two minute automatic connect a variable DC power supply set
switchoff function, and also the for 9V (and preferably with about 50mA
low battery warning... current limiting) to the battery snap ter-
When you're soldering the com- minals. Make sure the positive lead goes
ponents to the PCB, using the to the clip, and the negative to the stud.
overlay diagram as a guide, bear in You can use another battery snap to
mind that the PCB is tightly make a connection, but remember that its
packed and the solder pads are black lead will be the positive one!
small. The last thing this circuit Hold the button down and check that
needs is solder bridges and bad pin 5 of IC2's socket and pin 16 of IC3's
joints. socket have +5V on them, relative to the
Begin installing the compo- power supply negative. If they have, dis-
nents, starting with the lowest- charge any static electricity by touching
height ones (resistors and diodes), something earthed, then install IC2 and
and working your way up to the IC3 in their sockets and double-check
tall ones. Note that the displays their orientation.
and LEDs are mounted on a 28-pin Push the button for a second or so, and
IC socket, not soldered directly to you should see `EA' flash on the displays
the PCB! for a moment, followed by a single `-' on
It's a good idea to double-check, the left-hand one. If not, carefully
especially at the beginning, that recheck the PCB and its soldering and
you're putting each component's component placement, until you find the
leads into its correct holes. problem.
Above is a component overlay showing They're rather closely-spaced... Now short the `capacitor' leads, and
component designations that can be used Take care with the orientation of the display should indicate a resistance
in a servicing or assembly situation to the polarised components, and of around .03 ohms. Push the button
cross-reference between the circuit dia- make sure the different transistors again and this should change to `.00',
gram and board overlay.
types and IC1 all go in their correct possibly with the least significant digit
PCB, first illuminate its component side places. Don't install the socketed parts hesitating between 0 and 1.
with a bright light, and examine the cop- just yet, though.
per side carefully for fine track breaks When everything's on the PCB, once Calibration
and especially whiskers or bridges - par- again illuminate it from the top, and
ticularly where tracks pass between IC check for and correct any solder bridges Connect a resistor of known accuracy
socket pads. or other problems. of around 68 to 82 ohms to the `capaci-
If you're planning to run the ESR & tor' leads, and adjust VR2 for a correct
Low Ohm Meter from an optional (not Making a display reading. Now try a resistor in the range
of 1 to 9 ohms, and check that the meter
Fitting Perspex Window Next study the display area in the reads close to the correct value.
photo of the PCB. Then, keeping the Battery Warning: Skip this bit if you
cathode (short) leads of the decimal disabled the automatic switchoff func-
point LEDs to the right, cut the leads tion by leaving R25 off the PCB.
down to about 9mm long. Use long-nose Leaving the `capacitor' leads separat-
pliers to `dog-leg' the leads about 3mm ed, turn the power supply voltage down
from the ends, so the LEDs will be about to 7.0V, preferably confirmed by a digi-
level with the bottom of the seven-seg- tal voltmeter. Slowly turn VR1 anti-
ment displays when installed. clockwise until the display brightness
Push the LEDs into their places in the suddenly drops noticeably, and a `b'
The milled perspex window should 28-pin socket, followed by the seven- begins flashing on the right-hand dis-
be of snug fit inside the front panel segment displays themselves, ensuring play.
cut-out. At times, paint over-spray their decimal points are at the bottom, Push the button again and displays
on the inner edge of the front panel and they are properly seated. should switch off. Wind the power sup-
cut-out could make mounting diffi- ply back up to 9V, switch the ESR meter
cult whereby a little filing may be First test back on by pushing the button, and
needed. recheck that the battery warning triggers
We suggest to file smooth the verti- Solder the battery snap leads to their when you drop the supply to 7.0V.
cal milled edge of the perspex win- pads, making certain their polarity is
dow as shown above until perfect correct, and solder long component lead General assembly
snug fit is formed. Now a small drop offcuts to the `capacitor' and `button'
of glue at each corner can be used to pads. Temporarily solder the pushbutton Now you only need to install the com-
secure the window in place. to the ends of its leads, then turn VR1 pleted electronics in the box. The front
fully clockwise, and set VR2 to mid- panel is supplied pre-punched and silk-
Text and illustrations courtesy of Electronics Australia Page 5
screened. So mount the button and 15mm spacers (or a similar scheme). the battery.
banana sockets as shown in the photos, Finally solder the `capacitor' and Alternatively, you can bring the lead
then position a milled edge red filter per- `pushbutton' leads in place and connect a from an optional 9V plugpack into the
spex under the display window - holding 9V alkaline battery (not supplied), which box via a suitable grommeted hole.
it in place with a small drop of contact can be held snugly against the bottom of Now screw the lid down, plug in some
adhesive or similar on each corner. Be the box by the PCB via a piece of foam probe (or clip) leads, and prepare for a
careful not to get any glue onto the view- strip provided, trim foam if necessary. future of easy identification of faulty
ing section of the window. The cardboard piece is to protect the bot- electrolytics, and low resistance meas-
Now bolt the PCB to the lid, using four tom of the PCB from shorting out against urements!
Assembly Notes
ACN 000 908 716
CNR Lane Cove & Waterloo Roads
North Ryde NSW 2113
PH: (02) (lnt 612) 9937 3200 Fax: (02) 9888 3631
Text and illustrations courtesy of Electronics Australia Dick Smith Electronics