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TIMESTAMPS
Capacitor ESR Tester
Project Description
This project was conceived as a way to enhance the collection of test equipment on my test bench.
I buy a lot of older HP test gear off ebay as well as older radios. Most of this gear is 25-60 years old
and needless to say, the condition of the electrolytic capacitors is somewhat suspect. I needed a way to
quickly weed out bad caps with an in circuit tester.
At the present time, there are no plans to market circuit boards, kits, parts or complete units
for sale.
What is ESR?
"ESR" stands for equivalent series resistance. ESR is one of the
characteristics that defines the performance
of an electrolytic capacitor. Low ESR is highly desirable in a capacitor
as any ripple current through the
capacitor causes the capacitor to heat up due to the resistive loses.
This heating accelerates
the demise of the capacitor by drying out the electrolyte at an ever
increasing rate.
Over the lifetime of a capacitor, it is not uncommon for the ESR to
increase by a factor of 10 to 30 times
or even go open circuit. Typical lifetime ratings for electrolytics are
2000-15000 hours and are very dependant on ambient operating
temperature. As the ESR increases, the filtering operation of the
capacitor becomes impaired
and eventually the circuit fails to operate correctly.
Why are ESR Meters so Useful?
A typical capacitor checker measures the capacity (usually in micro
farads) of the test capacitor.
Some advanced units also test for leakage current. Most of these testers
require that the capacitor be
removed from the circuit. Unless the capacitor has totally failed, they
will not detect a high ESR value.
In a typical circuit, there may be 10's or 100's of capacitors. Having
to remove each one for testing is very
tedious and there is a great risk of damaging circuit boards. This
tester uses a low voltage ( 250mv )
high frequency (150khz) A/C current to read the ESR of a capacitor in
the circuit. The in circuit testing
is possible because of the low voltage used for obtaining the
measurement. The voltage is low enough that
solid state devices in the surrounding circuitry are not activated and
do not affect the low resistance reading we are attempting to obtain. A
lot of capacitor checkers will be damaged if you happen to test a
charged capacitor.
This circuit is A/C coupled and will withstand up to 400vdc of charge on
a capacitor (but watch your fingers!).
The ESR checker will not detect shorted capacitors as they will read
with a very low ESR value. If you are trouble shooting a circuit, you
will have to use several instruments including your nose, voltmeter and
oscilloscope to locate all the possible failure modes. My experience has
found that the ESR meter catches about 95% of capacitor problems and
potential problems.
Features
Tests electrolytic capacitors > 1uf in circuit.
Caps may be tested in circuit or by bridging them across the terminal posts or using test leads.
Polarity insensitive testing.
Tolerates charged capacitors up to 400vdc.
Low battery draw (approx 25ma) resulting in about 20 hours of battery time when using 4 cheap AA nicads.
Easy to read analog meter.
Measures ESR range from 0-75ohms with an expanded scale A/C ohmmeter.
Circuit Description
See the schematic for component designations.
The circuit starts with a 150khz oscillator using one gate of a 74hc14.
The rest of the gates are used as
buffers to increase the current drive to the low pass filter. The single
pole low pass filter is necessary because the square wave signal
contains a lot of energy in high frequency odd order harmonics. The
output of the filter is applied across a 10 ohm load resistor that
provides the low impedance drive signal to the test capacitor. Diodes D5
and D6 protect the circuit from discharge spikes if you happen to test a
charged capacitor. R18 is used to discharge C5 so that you don't blow
up anything if you alternately test charged high and low voltage caps.
C5 isolates the test circuit from DC voltages up to 400 volts.
Be careful if you are testing high voltage capacitors... the safest
way to work is to make sure the test capacitors are discharged before
testing them. Be aware that high voltage capacitors can hold a lethal
charge for several days depending on the circuit design. I learned this
first hand in high school electronics class. Students (not to be named!)
used to charge the caps and put them back on the shelf for the next
unsuspecting victim to pick up.
The rest of the circuit is a very straight forward transistor
amplifier with a gain of about 10.5. This amplifies the
A/C signal passed through the test capacitor up to several volts in
amplitude. The signal is then coupled to a full
wave bridge rectifier that has the meter as its load. The threshold
voltage of the bridge rectifier is used to an advantage and provides the
expanded scale function of the ohm meter. The amplified voltage from
the test capacitor must be great enough to overcome 2 diode drops before
the meter will start to respond. This sets the high resistance
threshold for the meter at somewhere between 75-100 ohms. The meter is
zeroed by shorting the test leads together and adjusting the pot in the
meter circuit for a full scale ( 0 ohms )reading on the meter. Proper
operation of the circuit can be verified by checking several values of
resistors with the meter. Shorted leads should indicate full scale, a 1
ohm resistor should read about 90% of full scale, a 10 ohm resistor
should read about 40% of full scale and a 47 ohm resistor should barely
move the needle to about 10% of FSD. The absolute readings will vary
with temperature and battery voltage, but the idea is that most ESR
values should be much less than 10 ohms which means good caps test at
very close to full scale and bad caps test at little or no deflection.
The board below shows my second prototype. The top 2 wires run to the
front panel banana test jacks, the middle 2 wires are the incoming
switched 5 volts from the 4 AA nicads, and the bottom 2 wires run to the
series combination of the zero adjust pot and the meter on the front
panel.
This circuit was first prototyped using a homebuilt prototyping panel.
As you can see by the picture above,
it was ugly but it worked. My final version was hand built on a "pad per
hole" prototyping board with 0.1" spacing.
I used the PCB layout as a guide for placing and wiring the components
(printed and glued to proto board). I did the PCB design as part of the
project
so that I could optimize the board size and parts placement. The pcb is
roughly 2x3" and is single sided with no
special or critical parts required.
Updates:
September 2009
The zero of the meter tends to drift with changes in the battery
voltage. Several builders have emailed me with modifications to
use a low dropout regulator to keep the voltage more constant. Since my
unit runs on 4 nicad cells, there is not enough headroom for a 5volt low
dropout
regulator. The charged voltage of my nicad pack is 5.6volts, but most of
the packs useful life is down around 4.8volts. I added a
15 ohm 1/2w resistor in series from the battery pack and a 4.7volt 1
watt zener to ground. This modification really helps keep the readings
more stable as the
battery discharges. With the lower supply voltage, I had an issue with
driving the meter to full scale. I ended up reducing R17 from 10k down
to 4k7.
The latest success story using the ESR meter was finding an open
330uF/25v cap on the output of a wall wart supply used with a wireless
router. Replacing
the $0.25 part saved me buying a $100.00 router.
Capacitor ESR Tester
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