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A CAPACITOR
LEAKAGE
ADAPTOR
FOR DMMS
By JIM ROWE
Here’s a cut-down version of the Digital Capacitor Leakage Meter we described
in December 2009. Instead of using a PIC microcontroller and an LCD panel to
display the leakage current, this version connects to your DMM to provide the
readout. It provides the same range of seven different standard test voltages (from
10V to 100V) and can measure leakage currents down to 100 nanoamps!
28 Silicon Chip
siliconchip.com.au
W
hy would you need to measure capacitor leakage
current? In case you missed the December 2009
article, here’s a summary of the introduction we
provided there.
In theory, capacitors are not supposed to conduct direct
current apart from a small amount when a DC voltage is
first applied to them and they have to ‘charge up’.
With most practical capacitors using materials like
ceramic, glass, polyester or polystyrene - even waxed paper - as their insulating dielectric, the only time they do
conduct any DC is during charging. That’s assuming they
haven’t been damaged, either physically or electrically. In
that case they may well conduct DC as a steady ‘leakage
current’, showing that they are faulty.
But as many SILICON CHIP readers will be aware, things
are not this clear cut with electrolytic capacitors, whether
they be aluminium or tantalum. All brand new electrolytic
capacitors conduct a small but measurable DC current, even
after they have been connected to a DC source for sufficient
time to allow their dielectric oxide layer to ‘form’. In other
words all electrolytic capacitors have a significant leakage
current, even when they are ‘good’.
The range of acceptable leakage current tends to be
proportional to both the capacitance and the capacitor’s
rated voltage. Have a look at the figures given in the Leakage Current Guide opposite. The current levels listed
there are the maximum allowable before the capacitor is
regarded as faulty.
So an instrument capable of measuring the leakage
current of capacitors can be very handy in many areas of
electronics.
Commercially available capacitor leakage current meters
are expensive (ie, over $1000) and even the Capacitor Leakage Meter we described in the December 2009 issue will
probably cost you over $100 to build. That’s why we’ve
developed a cut-down version described in this article,
which lets you make all of the same measurements with
your existing digital multimeter (DMM).
The Adaptor is easy to build and will have a much lower
cost than the December 2009 meter while still providing
the same choice of seven different standard test voltages:
10V, 16V, 25V, 35V, 50V, 63V or 100V. It is also able to make
current measurements from 10mA down to a fraction of a
microamp. So it’s capable of making leakage current tests
on the vast majority of capacitors in current use.
It’s built into a compact UB1 size jiffy box and is battery
powered (6 x AA alkaline cells). This makes it suitable for
the workbench or the service technician’s tool kit.
CAPACITOR LEAKAGE CURRENT GUIDE
TYPE OF
CAPACITOR
Maximum leakage current in microamps A) at rated working voltage
10V
16V
Ceramic,
Polystyrene,
Metallised
Film (MKT,
Greencap
etc.), Paper,
Mica
25V
35V
50V
63V
100V
LEAKAGE SHOULD BE ZERO FOR
ALL OF THESE TYPES
Solid
Tantalum*
< 4.7 F
1.0
1.5
2.5
3.0
3.5
5.0
7.5
6.8 F
1.5
2.0
3.0
4.0
6.5
7.0
9.0
47 F
10
10
15
16
17
19
24
Standard
Aluminium
Electrolytic#
<3.3 F
5.0
5.0
5.0
6.0
8.0
10
17
5.0
6.0
8.0
12
15
23
8.0
13
18
25
35
50
4.7 F
5.0
10 F
15 F
8.0
11
19
25
38
100
230
100 F
50
230
300
330
420
500
600
150 F
230
280
370
430
520
600
730
680 F
500
600
780
950
1100
1300
1560
1000 F
600
730
950
1130
1340
1500
1900
4700 F
1300
1590
2060
2450
2900
3300
4110
* Figures for Solid Tantalum capacitors are after a charging period of one minute.
# Figures for Aluminium Electrolytics are after a charging/reforming period of three minutes.
source (on the left) which generates one of seven different preset voltages when the TEST button is pressed and
held down.
The second section is a simple current to voltage converter (on the right) which is used to generate a voltage
proportional to the direct current passed by the capacitor
under test, so that it can be measured easily using your
DMM.
Any direct current passed by the capacitor being tested
flows down to ground via resistor R2, which therefore
acts as a current shunt. The voltage drop across R2 is then
passed through an output buffer which feeds your DMM.
The DMM is set to its 0-2.0V DC voltage range, which allows its readings to be easily converted into equivalent
current levels.
So that’s the basic arrangement. The reason for resistor
R1, in series with the output of the test voltage source, is
How it works
to limit the maximum current that can be drawn from the
source, in any circumstances. This prevents damage to
The Adaptor’s operation is straightforward, as you can
either the voltage source or the current-to-voltage converter
see from the block diagram of Fig.1. There are two funcsections, in the event of the capacitor under test having
tional circuit sections, one being a selectable DC voltage
an internal short circuit. It also protects
CAP UNDER TEST
R2 and the output buffer from overload
+
when a capacitor (especially one of high
R1
+
+Vt
+
TEST
SELECTABLE
value) is initially charging up to one of
+
–
OUT TO DMM
OUTPUT
DC VOLTAGE
R2
the higher test voltages.
(1V = 10mA
BUFFER
SOURCE
OR 100 A)
(S2)
TEST
–
–
R1 has a value of 10k, which was cho(7 VOLTAGES)
TERMINALS
sen to limit the maximum charging and/
(IC1)
(IC2)
or short circuit current to 9.9mA even
Fig.1: block diagram of the adaptor shows it has two elements: a selectable
on the highest test voltage range (100V).
DC voltage source and a simple current-to-voltage converter.
At this stage you may be wondering
siliconchip.com.au
April 2010 29
how the Adaptor can allow your DMM to read leakage
currents down to less than a microamp, when it also has to
cope with charging currents of up to 9.9mA. The answer is
that the current-to-voltage converter section of the Adaptor actually has two current ranges, which are selected by
switching the value of shunt resistor R2.
The default value of R2 is 100, which provides a
0-10mA range for the capacitor’s charging phase (ie, when
TEST button S2 is first pressed). But when (and if) the
measured current level falls below 100A, pushbutton
S4 can be pressed to switch the value of R2 to 10k, providing a 0-100A range for more accurate leakage current
measurement.
The 270k resistor forms the top arm of the feedback divider, while the 36k and 2.4k resistors from
pin 5 to ground form the fixed component of the lower
arm. These give the divider an initial division ratio of
308.4k/38.4kor 8.031:1, to produce a regulated output
voltage of 10.04V. This is the converter’s output voltage
when selector switch S1 is in the ‘10V’ position.
When S1 is switched to any of the other positions, additional resistors are connected in parallel with the lower
arm of the feedback divider, to increase its division ratio
and hence increase the converter’s output voltage.
For example, when S1 is in the ‘25V’ position, this
connects the 270, 8.2k, 5.1k, 2.0k, 200, 2.4k,
150 and 3.6k resistors (all in series) in parallel with
the divider’s lower arm, changing the division ratio to
283.954k/13.954k or 20.35:1. This produces a regulated
output voltage of 25.44V.
The same kind of change occurs in the other positions
of S1, producing the various preset output voltages shown.
(Although the test voltages shown are nominal, with the
specified 1% tolerance resistors used for the divider resistors, they should all be well within ±4% of the nominal
values because the 1.25V reference inside the MC34063
is accurate to within 2%.)
Note that IC1 only generates the selected test voltage
Circuit description
Now have a look at the full circuit schematic of Fig.2.
The selectable DC voltage source is based around IC1,
an MC34063 DC/DC controller IC, used here in a ‘boost’
configuration in conjunction with autotransformer T1 and
fast switching diode D2.
We vary the circuit’s DC output voltage by varying the
ratio of the voltage divider in the converter’s feedback
loop, connecting from the cathode of D2 back to IC1’s pin
5 (where the voltage is compared with an internal 1.25V
reference).
D3 1N4004
POWER
+8.4V
K
A
S3
470 F
16V
9V BATTERY
(6xAA ALKALINE)
Q1
BC327
+8.4V
T1
1
15T
DrC
GND
SwE
4
Cin-
A
K
200
100V
SET
TEST
VOLTS
63V
5.1k
25V
10V
TP1
TPG
36k
TEST
TERMINALS
2.4k
100nF
100
–
IC2: LM358
3
1k
100 F
16V
LOW
LEAKAGE
+
OUT TO
DMM
(0–1V)
RLY1
K
6
K
10k
6
ZD1
10V
5
A
D1
IC2b
470
7
–
4
A
7,8
8.2k
1k
270
2
D1: 1N4148
A
K
CAPACITOR LEAKAGE ADAPTOR FOR DMMS
ZD1
A
1N4004, UF4003
K
A
K
BC327
LEDS
K
A
Fig.2: here’s the complete circuit diagram for the adaptor. At the beginning of each test it
measures on its 10mA range but if the current drops below 100A it can switch to a 100A range.
30 Silicon Chip
470
1
IC2a
100
1,14
1M
2
8
+
–
16V
33k
10mA
RANGE
LED2
K
2.0k
35V
A
+
50V
S1
SC
10k
2.4k
LED1
2010
2.2 F
250V
MET.
POLY
+1.25V
150
TEST
VOLTS
K
270k
3.6k
2
68k
45T
8
1
IC1
SwC
MC34063
5
Ct
820pF
B
D2 UF4003
A
7
Ips
6
Vcc
3
S4
PRESS
FOR
100 A
RANGE
C
TEST
S2
2.2k
E
B
E
C
siliconchip.com.au
when test pushbutton S2 is pressed and
held down. This is because IC1 only
receives power from the battery when
S2 is closed, allowing the converter
circuit to operate and thereby charging
the 2.2F/250V metallised polyester
reservoir capacitor. The test voltage is
then made available at the positive test
terminal via the 10k current limiting
resistor, R1.
Now let us look at the current-tovoltage converter section, which is
virtually all of the circuitry below and
to the right of the negative test terminal.
The 100, 1M and 10k resistors connected between the negative
–
470 F
9V BATTERY
(UNDER)
POWER
IC2
LM358
1
S3
TEST
S2
0102 ©
4004
10140240
D3
470
470
+
–
Test voltages: ........... 10V, 16V, 25V, 35V, 50V, 63V or 100V.
Leakage current: ...... from 10mA down to less than 100nA (0.1A), via two ranges:
0-10mA (default) and 0-100A (manually selected).
Both ranges convert these current values into an output
voltage range of 0-1000mV DC, allowing all measurements
to be made on the DMM’s 0-1V or 0-2V range.
The Adaptor’s default 10mA range is current limited to
provide protection from damage due to shorted capacitors
or the charging current pulse of high-value capacitors.
Power:......................... Internal 9V battery (6 x AA alkaline cells).
Current drain:............ Varies between 1mA and 125mA, depending on the test voltage
and the current range in use.
E GAKAEL R OTI CAPA C
S M MD R OF R OTPADA
+
OUTPUT
BANANA
JACKS
(TO DMM)
Specifications
100nF
10V
ZD1
TEST
VOLTS
S1
LED1
4
7
1.0
3
6
5
8.2k
270
1
IC1
34063
2
SET VOLTS
1
2.2k
33k
1k
820pF
T
S
10mA
RANGE
5.1k
2.0k
100 A
RANGE
S4
68k
Q1
BC327
LED2
36k
2.4k
1k
3.6k
T1
150
2.4k
200
15T + 40T
F
D1
4148
RLY1
2.2 F 250V
METAL POLYESTER
270k
TPG
1M
10k
100 F
LL
T+
10k
TEST
TERMINALS
100
T–
D2
4003
100
TP1
Fig.3: with the exception of the test terminals, DMM output
jacks and three of the switches, all components mount on
one PC board.
siliconchip.com.au
Here’s a photograph which matches the diagram at left. In
this case, the terminals and the two push-button switches
are not shown on the board because they mount on the
front panel and connect to the PC board via short lengths of
tinned copper wire (one of the last steps in assembly).
April 2010 31
NEGATIVE TEST TERMINAL
(POSITIVE TERMINAL
BEHIND IT)
LED2
BEHIND S4
T1
LED1
BEHIND S1
PC BOARD MOUNTED
BEHIND LID USING
4 x 25mm M3
TAPPED SPACERS
S1
RLY1
S2
IC2
ZD1
100n
TRANSFORMER T1 POTCORE
HELD TO PC BOARD USING
25mm x M3 NYLON SCREW
WITH NUT & FLAT WASHERS
470F
S4
NEGATIVE OUTPUT JACK
(POSITIVE JACK
BEHIND IT)
S3 BEHIND S2
6xAA CELL HOLDER
(CUT DOWN FROM
10xAA HOLDER)
MOUNTED IN
BOTTOM OF BOX
USING DOUBLESIDED TAPE
Fig.4: a side-on view “through” the wall of the jiffy box, showing how everything goes together. The 6xAA cell holder must
be mounted at one end, as shown here, to avoid fouling the screw holding the transformer to the PC board.
test terminal and ground correspond to the current shunt
labelled R2 in Fig.1, with the contacts of reed relay RLY1
used to change the effective shunt resistance for the Adaptor’s two ranges. For the default 0-10mA ‘charging phase’
range RLY1 is energised and connects a short circuit across
the parallel 1M/10k combination, making the effective
shunt resistance 100. For the more sensitive 100uA range
RLY1 is turned off, opening its contacts and connecting
the parallel 1M/10k resistors in series with the 100
resistor to produce an effective shunt resistance of 10k.
Relay RLY1 is turned on or off by transistor Q1. When
power is first switched on via switch S3, Q1 is switched
on by forward bias current applied to its base via the 68k
resistor to ground. It therefore conducts about 10mA of collector current, which energises RLY1 and also causes LED2 to
light – indicating that the Adaptor is operating in the 10mA
current range. But if the capacitor’s current reading (on the
DMM) drops down to below 100A, pressing pushbutton
switch S4 and holding it down causes Q1 to switch off. As
a result LED2 and RLY1 both turn off as well, switching the
Adaptor to its 0-100A range.
The 100F low leakage capacitor in parallel with the
shunt routes any AC signal from the capacitor being tested
around the shunt. This prevents ripple from the switchmode supply from corrupting the reading.
Regardless of which current range is in use, the voltage
drop developed across the shunt resistance (as a result of
any current passed by the capacitor under test) is passed
to the non-inverting input of IC2a, one half of an LM358
dual op amp. IC2a is configured as a voltage follower with
a voltage gain of unity, feeding the positive output terminal
of the Adaptor via a 470 isolating resistor.
So what is the purpose of IC2b? It is connected as a voltage follower in much the same way as IC2a, except that its
non-inverting input is connected directly to ground and
its output is used to drive the negative output terminal. Its
purpose is to balance out most of the input offset of IC2a,
so that the Adaptor’s effective output voltage, when there
is no current flowing through the test terminals, is much
less than 1mV.
All of the Adaptor’s circuitry operates directly from the
9V battery, via polarity protection diode D4 and of course
32 Silicon Chip
S3. The total current drain when in ‘standby’ (ie, with TEST
button S2 not pressed) is about 11mA in the default 10mA
current range or 1mA if S4 is pressed to switch it into the
100A range. The current level increases to between 25mA
and 125mA when S2 is pressed and held down to generate
the test voltage and perform the actual leakage current test.
Construction
Virtually all of the circuitry and components used in the
Capacitor Leakage Adaptor are mounted on a single PC
board measuring 145 x 84mm and coded 04104101. This is
mounted under the lid (which becomes the Adaptor’s front
panel) of a UB1 jiffy box (157 x 95 x 53mm) via four 25mm
long M3 tapped spacers. Six AA alkaline cells provide
power, mounted in a cut-down 10-cell holder secured to
the bottom of the box.
Both the voltage selector switch (S1) and the DC/DC converter’s step-up transformer (T1), wound on a 26mm ferrite
pot core, mount on the board, the latter using a 25mm long
M3 Nylon screw and nut.
The only components not mounted directly on the main
board are power switch S3, pushbutton switches S2 and
S4, the two test terminals and the two output banana jacks.
These are all mounted on the box front panel, with their
rear connection lugs extended down via short lengths of
tinned copper wire to make their connections to the board.
All of these assembly details should be fairly clear from the
diagrams and photos.
To begin fitting the components on the PC board I suggest
you fit the wire link, located just to the right of the position for rotary switch S1. Next fit the four 1mm terminal
pins to the board – two for the test point at upper left and
two at upper right for the battery clip lead connections.
Follow these with the sockets for IC1 and IC2, which are
both 8-pin devices.
Now fit the fixed resistors. These are 1% tolerance metal
film components, apart from the 1.0 resistor just to the
right of T1 and to the left of IC1. This resistor should be a
0.5W carbon composition type. Check each resistor’s value
with a DMM as you insert and solder them to ensure they
all go in the right places.
Next, you can fit the two lower-value capacitors and the
siliconchip.com.au
large 2.2F metallised polyester capacitor, followed by the
(polarised) 470F electrolytic.
Then fit the mini DIL relay, making sure its locating groove
is at the top end. Then you can fit voltage selector switch
S1, which mounts with its indexing spigot at 3-o’clock. Just
before you fit it you should cut its spindle to a length of
about 13mm and file off any burrs, so it’s ready to accept
the knob during final assembly.
After it has been fitted to the board, remove its main nut/
lock washer combination and turn the spindle by hand to
make sure it’s at the fully anticlockwise limit. Then refit the
lock washer, making sure that its stop pin goes down into
the hole between the moulded ‘7’ and ‘8’ digits. Check that
the switch is now ‘programmed’ for the correct seven positions, simply by clicking it around through them by hand.
With S1 fitted, you can add the four diodes. Don’t mix
them up! D1 is a low power 1N4148 ‘signal’ diode, D2 is a
UF4003 ‘fast’ rectifier, D3 is a 1N4004 1A power diode and
A zener. Use the overlay diagram as a guide
ZD1 is a 10V/1W
to their orientation when you’re fitting each one to the board.
Next fit transistor Q1, followed by the two 5mm LEDs.
The red one is used as LED1 and
60.5the green one as LED2.
They are both mounted vertically with their leads left at
almost full length, so that the lower surface of their bodies
38.5
5
is about 23mm
above the surface of the board.
E
This allows them to just protrude
through the matching
B
B
holes in the lid/front panel when the board assembly is
attached
behind it.
9.5
At this stage your board assembly is very close to complete, with the main task remaining being to wind transE
9.5
former
T1 and fit it to the board. You’ll find the full details
on how to do this in the separate panel.
C
Once the transformer has beenF fitted to the board, you
can attach the four 25mm M3 tapped spacers to it as well.
38.5
These each attach very close to each corner of the board,
using 6mm long M3 screws passing up from
the underside.
37
Now all that remains to complete the board assembly is
to plug IC1 and IC2 into their sockets. Place it aside while
you prepare the case to receive it.
Preparing the case
There are no holes to be drilled in the lower part of the
case (the battery holder can be held securely in place using strips of ‘industrial’ double-sided adhesive foam tape)
but the lid does need to have holes drilled for the various
switches, LEDs and input/output connectors.
The location and dimensions of all these holes are shown
in the diagram of Fig.5, which is actual size so it (or a photocopy) can also be used as a drilling template. The larger
holes are easily made by drilling them all first with a 7mm
twist drill and then carefully enlarging them to size using
a tapered reamer.
We have prepared an artwork for the front panel if you
would like to make it look neat and professional. This can
be either photocopied (Fig.6) or downloaded as a PDF file
A
from our website and then printed
out.
Either way the resulting copy can either be covered
with self-adhesive clear film or, better still, laminated, for
HOLE
protection60.5
against finger grease, etc before it isDIAMETERS:
glued to the
lid/panel.
A: 3.0mm
B: 5.0mm
Mount switches S2, S3 and S4 on the panel,
followed
7.0mm
by the binding posts used as the meter’s testC:
terminals
D: 8.0mm
D
and the banana sockets
used for Dthe output connections
E: 9.0mm
to your DMM.
F: 12.0mm
Tighten the binding post and banana
socket
mounting
9.5
nuts firmly, to make sure that they cannot come loose with
C
use. Then use the second nut of each post andLsocket to
9.5
D
attach
16.5 a 4mm solder lug plus a 4mm lockwasher to make
sure they don’t work loose either.
Now you can turnF the lid assembly over and solder ‘extension wires’ to the connection lugs of the three switches,
and also the solder lugs fitted to the rear of the binding posts
and
sockets. These wires should all be about 30mm long
37
and cut from tinned copper wire (about 0.7mm diameter).
A
A
A
ALL
DIMENSIONS
IN
MILLIMETRES
A
CL
60.5
38.5
5
E
D
B
B
D
9.5
9.5
9.5
HOLE
DIAMETERS:
A: 3.0mm
B: 5.0mm
C: 7.0mm
D: 8.0mm
E: 9.0mm
F: 12.0mm
60.5
CL
E
D
16.5
F
9.5
F
C
38.5
37
37
A
A
siliconchip.com.au
CL
Fig.5: a 1:1
drilling
template for
the front
panel
ALLof the
DIMENSIONS
specified
jiffy
IN
box.
MILLIMETRES
April 2010 33
Parts List – Capacitor
Leakage DMM Adaptor
1 PC board, code 04204101,
145 x 84mm
1 UB1 jiffy box, 158 x 95 x 53mm
1 Single pole rotary switch,
PC mounting (S1)
2 SPST mini pushbutton switch
(S2, S4)
1 SPDT mini toggle switch,
panel mounting (S3)
1 Mini DIL reed relay, SPST
with 5V coil (RLY1)
2 Premium binding posts,
1 x red and 1 x black
2 4mm banana jack sockets,
1 x red and 1 x black
1 16mm diameter fluted instrument
knob
1 Ferrite pot core pair, 26mm OD
1 Bobbin to suit pot core
1 3m length of 0.5mm diameter
enamelled copper wire
1 25mm M3 Nylon screw and nut
and two flat washers
2 8-pin DIL IC sockets
4 1mm dia. PC board terminal pins
4 25mm long M3 tapped spacers
8 6mm long M3 machine screws,
pan head
1 10x AA battery holder (flat, side
by side)
Semiconductors
1 MC34063 DC/DC converter
controller (IC1)
1 LM358 dual op amp (IC2)
1 BC327 PNP switching transistor
(Q1)
1 10V 1W zener diode (ZD1)
1 5mm red LED (LED1)
1 5mm green LED (LED2)
1 1N4148 100mA diode (D1)
1 UF4003 fast 1A diode (D2)
1 1N4004 1A diode (D3)
Capacitors
1 470F 16V PC electrolytic
1 100F 16V low leakage electro
1 2.2F 250V (or 100V)
metallised polyester
1 100nF multilayer monolithic
ceramic
1 820pF disc ceramic
Resistors (0.25W 1% unless specified)
1 1M
1 270k
1 68k
1 36k
1 33k
1 22k
2 10k
1 8.2k
1 5.1k
1 3.6k
2 2.4k
1 2.2k
1 2.0k
2 1k
2 470
1 270
1 200
1 150
2 100
1 1.0 0.5W carbon (5%)
34 Silicon Chip
“Opened out” view showing the PC board “hanging” from the front panel.
The next step is to prepare the battery holder. Because you can’t buy a
six-way flat AA holder (at least we
couldn’t find one!) we cut down a tenway AA holder.
The last three cell positions are removed altogether (at the ‘negative lead’
end) and then the eyelets are drilled out
and used to attach the contact spring
for the sixth cell position and also the
contact spring and negative lead connection lug at the end of the removed
section.
This will allow you to re-attach the
negative lead’s connection lug to the
contact spring for the sixth cell using
a 6mm long M2 machine screw and
nut. The seventh cell position is still
retained to support the sixth cell connection spring and the negative lead
connection lug.
The converted battery holder can
now be fitted inside the main section
of the box at lower right, with the connection lead side uppermost. Mount it
using double-sided adhesive foam as
mentioned earlier, or simply a strip of
‘gaffer’ tape.
You should now be ready for the
only slightly fiddly part of the assembly
operation: attaching the PC board as-
sembly to the rear of the lid/front panel.
This is only fiddly because you have
to line up all of the extension wires
from switches S2, S3 and S4, the two
test terminals and the output banana
sockets with their matching holes in
the PC board, as you bring the lid and
board together. At the same time you
have to line up the spindle of switch S1
and the two LEDs with their matching
holes in the front panel.
This is actually easier to do than it
sounds, so just take your time and the
lid will soon be resting on the tops of
the board mounting spacers. Then you
can secure the two together using four
6mm long machine screws.
Now it’s simply a matter of turning
the complete assembly over and soldering each of the switch and terminal
extension wires to their board pads.
Once they are all soldered you can clip
off the excess wires with sidecutters.
If you find this description a bit confusing, refer to the assembly diagram
in Fig.4. This will hopefully make
everything clear.
Next solder the bared end of the red
(positive) battery holder lead to the
positive battery terminal pin at the
upper right on the PC board, and the
siliconchip.com.au
Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
No.
1
1
1
1
1
1
2
1
1
1
2
1
1
2
2
1
1
1
2
1
Value
1M
270k
68k
36k
33k
22k
10k
8.2k
5.1k
3.6k
2.4k
2.2k
2.0k
1k
470
270
200
150
100
1.0 (0.5W)
black (negative) battery holder lead to
the negative pin alongside.
You can now fit six AA-size alkaline
cells into the battery holder (make sure
you fit them with the correct polarity) and your new Capacitor Leakage
Adaptor should be ready for its initial
checkout.
Initial checkout
You’ll need to use a twin test lead
to connect the Adaptor’s output to the
input jacks of your DMM. The DMM
should also be set to measure DC voltage, and to its 0-1V or 0-2V range if it’s
not auto ranging.
Switch on the Adaptor’s power using S3 and green LED2 should light –
showing that the Adaptor is operating,
in standby mode and in the default
10mA current range. Then if you press
S4, the range change button, LED2
should go dark. This shows that the
range switching circuitry is operating.
But your DMM should still be giving
a zero reading. At this point you can
stop pressing S4.
Next try pressing test button S2.
This should cause red LED1 to glow,
indicating that power is now being
applied to the test voltage generation
circuitry. If there is no capacitor or
other component connected across the
test terminals, your DMM should still
be giving a reading of zero.
Assuming all has gone well at this
point, your Adaptor is probably worksiliconchip.com.au
4-Band Code (1%)
brown black green brown
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blue grey orange brown
orange blue orange brown
orange orange orange brown
red red orange brown
brown black orange brown
grey red red brown
green brown red brown
orange blue red brown
red yellow red brown
red red red brown
red black red brown
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yellow violet brown brown
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brown green brown brown
brown black brown brown
brown black gold gold (5%)
ing correctly.
However if you want to make sure,
try shorting the two test terminals.
Then set S1 to the ‘100V’ position, and
press Test button S2. The DMM reading
should change to a value corresponding
to 9.9mA (i.e., 990mV), representing
the current drawn from the nominal
100V source by the 10k current limiting resistor and the 100 current shunt
resistor inside the Adaptor.
Don’t worry if the current reading is a
bit above or below the 9.9mA figure, by
the way. As long as it’s between about
9.2mA (920mV) and 10.6mA (1.06V),
things are OK.
With the terminals still shorted together, you can try repeating the same
test for each of the other six test voltage
positions of switch S1.
You should get a reading on the
DMM corresponding to approximately
6.25mA (625mV) on the 63V range,
4.95mA (495mV) on the 50V range,
3.46mA (346mV) on the 35V range,
2.48mA (248mV) on the 25V range,
1.58mA (158mV) on the 16V range and
990uA (99mV) on the 10V range.
If the readings you get are close to
these, your Capacitor Leakage Adaptor
is working correctly.
This being the case, switch off the
power again via S3 and then complete
the final assembly by lowering the lid/
PC board assembly into the case and
securing the two together using the four
small self-tapping screws supplied.
5-Band Code (1%)
brown black black yellow brown
red violet black orange brown
blue grey black red brown
orange blue black red brown
orange orange black red brown
red red black red brown
brown black black red brown
grey red black brown brown
green brown black brown brown
orange blue black brown brown
red yellow black brown brown
red red black brown brown
red black black brown brown
brown black black brown brown
yellow violet black black brown
red violet black black brown
red black black black brown
brown green black black brown
brown black black black brown
Make sure you also remove the shorting
wire between the test terminals.
Using it
The Capacitor Leakage Adaptor is
very easy to use, because all you have
to do is connect the capacitor you want
to test across the test terminals (with
the correct polarity in the case of solid
tantalums and electrolytics), after connecting the Adaptor’s output sockets to
the input jacks of your DMM.
Then turn on the DMM and set it to
measure DC volts.
Now set the Adaptor’s selector
switch S1 for the correct test voltage
and turn on the power (S3), whereupon
LED2 should light. Then to begin the
actual test, press and hold down Test
button S2.
What you may see first on the DMM
is a reading of the capacitor’s charging current, which can be as much as
9.9mA (with high value caps) but will
then drop back as charging continues.
How quickly it drops back will depend
on the capacitor’s value.
With capacitors below about 4.7F,
the charging may be so fast that the first
reading will often be less than 100A
(10mV).
If the capacitor you’re testing is of
the type having a ‘no leakage’ dielectric
(such as metallised polyester, glass,
ceramic or polystyrene), the current
should quickly drop down to less than
10A (1mV).
April 2010 35
Winding autotransformer T1
The step-up autotransformer T1 has
around the hole in a circle, with a diam60 turns of wire in all, wound in four
eter of 10mm. Your ‘gap’ washer will
15-turn layers. And as you can see
then be ready to place inside the lower
UPPER SECTION
from the assembly diagram at right, all
half of the pot core, over the centre hole.
OF FERRITE
POT CORE
four layers are wound on a small Nylon
Once the gap washer is in position,
bobbin using easily handled 0.5mm
you can lower the wound bobbin into the
diameter enamelled copper wire. Use
pot core around it, and then fit the top
BOBBIN WITH
this diagram to help you wind the
half of the pot core. The transformer
WINDING
(4 x 15T OF 0.5mm DIA is now be ready for mounting on the
transformer correctly.
ENAMELLED COPPER
Here’s the procedure: first wind on
main PC board.
WIRE, WITH TAP AT END
15 turns, which you’ll find will neatly
First place a Nylon flat washer on the
OF FIRST LAYER &
INSULATING TAPE
take up the width of the bobbin provid25mm-long M3 Nylon screw that will be
BETWEEN LAYERS)
ing you wind them closely and evenly.
used to hold it down on the board. Then
Then to hold them down, cover this first
pass the screw down through the centre
FINISH
layer with a 9mm-wide strip of plastic
hole in the pot core halves, holding them
TAP
insulating tape or ‘gaffer’ tape.
(and the bobbin with gap washer inside)
START
Next take the wire at the end of this
together with your fingers.
first layer outside of the bobbin (via one
Then lower the complete assembly
'GAP' WASHER OF 0.06mm
of the ‘slots’) and bend it around by 180°
down
in the upper left of the board with
PLASTIC FILM
at a point about 50mm from the end of
the ‘leads’ towards the right, using the
the last turn. This doubled-up lead will
bottom end of the centre Nylon screw to
be the transformer’s ‘tap’ connection.
locate it in the correct position. When you
LOWER SECTION
The remaining wire can then be used
are aware that the end of the screw has
OF FERRITE
POT CORE
to wind the three further 15-turn layers,
passed through the hole in the PC board,
making sure that you wind them in
keep holding it all together but up-end
the same direction as you wound the
everything so you can apply the second
first layer.
M3 Nylon flat washer and M3 nut to the
(ASSEMBLY HELD TOGETHER & SECURED TO
Each of these three further layers
end of the screw, tightening the nut so
PC BOARD USING 25mm x M3 NYLON SCREW & NUT)
should be covered with another 9mmthat the pot core is not only held together
wide strip of plastic insulating tape just as
but also secured to the PC board.
This is to provide a thin magnetic ‘gap’ in the
you did with the first layer, so that when all
Once this has been done, all that
four layers have been wound and covered pot core when it’s assembled, to prevent the remains as far as the transformer is
pot core from saturating when it’s operating. concerned is to cut the start, tap and fineverything will be nicely held in place.
The washer is very easy to cut from a ish leads to a suitable length, scrape the
The ‘finish’ end of the wire can then be
brought out of the bobbin via one of the piece of the thin clear plastic that’s used enamel off their ends so they can be tinned
slots (on the same side as the start and for packaging electronic components, like and then pass the ends down through
tap leads) and your wound transformer resistors and capacitors.
their matching holes in the board so they
This plastic is very close to 0.06mm thick, can be soldered to the appropriate pads.
bobbin should be ready to fit inside the
which is just what we need here. So the idea
two halves of the ferrite pot core.
Don’t forget to scrape, tin and solder
Just before you fit the bobbin inside is to punch a 3-4mm diameter hole in a piece BOTH wires which form the ‘tap’ lead –
the bottom half of the pot core, though, of this plastic using a leather punch or similar,
if they are not connected together, the
there’s a small plastic washer to prepare. and then use a small pair of scissors to cut transformer won’t produce any output.
And if you press button S4 on the
Adaptor to switch down to the 100A
range, you should be able to see the
DMM reading fall down to zero.
That’s if the capacitor is not faulty,
of course.
On the other hand if the capacitor
is one with a tantalum or aluminium
oxide dielectric with inevitable leakage, the current reading will drop more
slowly as you keep holding down the
Test button.
In fact it will probably take up to
a minute to stabilise at a reasonably
steady value in the case of a solid tantalum capacitor and as long as three
36 Silicon Chip
minutes in the case of an aluminium
electrolytic.
(That’s because these capacitors
generally take a few minutes to ‘reform’
and reach their rated capacitance level.)
As you can see from the guide table
earlier the leakage currents for tantalum and aluminium electrolytics also
never drop down to zero but instead
to a level of somewhere between about
4.1mA and 1A depending on both
their capacitance value and their rated
working voltage.
So with these capacitors, you should
hold down the Adaptor’s test button
to see if the leakage current reading
drops down to the ‘acceptable’ level as
shown in the guide table and preferably
even lower.
If this happens the capacitor can be
judged ‘OK’ but if the current never
drops to anywhere near this level it
should definitely be replaced.
What about low leakage (LL) electrolytics? Well, the current levels shown
in the guide table are basically those
for standard electrolytics rather than
for those rated as low leakage.
So when you’re testing one which
is rated as low leakage, you’ll need to
make sure that its leakage current drops
well below the maximum values shown
siliconchip.com.au
CAPACITOR LEAKAGE
MEASUREMENT ADAPTOR
POWER
TEST VOLTS
10mA RANGE
+
–
+
PRESS FOR
100A RANGE
25V
35V
16V
63V
10V
Fig.6:
same-size
front panel
artwork.
in the guide table. Ideally it should drop
down to no more than about 25% of
these current values.
A final tip: when you’re testing nonpolarised (NP) or ‘bipolar’ electrolytics,
PRESS TO
APPLY VOLTS
50V
OUT
TO
DMM
–
100V
SELECT TEST
VOLTAGE
these should be tested twice – once
connected to the terminals one way
around and then again connected with
the opposite polarity.
That’s because these capacitors are
essentially two polarised types, internally connected in series, back-to-back.
If one of the dielectric layers is leaky
but the other is OK, this will only show
SC
up in one of the two tests.
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siliconchip.com.au
April 2010 37
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