This is only a preview of the November 2011 issue of Silicon Chip. You can view 26 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Build A G-Force Meter":
Items relevant to "The MiniMaximite Computer":
Items relevant to "Ultra-LD Stereo Preamplifier & Input Selector, Pt.1":
Items relevant to "2.2-100V Zener Diode Tester":
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Got a bunch of unknown diodes and zener
diodes? Check ’em all with this . . .
Zener Diode
Tester
This zener diode tester plugs
into your digital multimeter
and you can directly check
any zener diode rated from
2.2V up to 100V. You can also
check the forward voltage of
diodes and test low-voltage
Schottky diodes.
By JOHN CLARKE
W
HILE MOST DIGITAL multimeters
(DMMs) do include a diode test function,
they do not test zener diodes. So how many
zener diodes do you have stashed away which
are not used because their value is unknown?
In many cases, the type number will be missing
or partially rubbed off or it is difficult to read
because the print is so small. And even if it can
be read, the type number will not directly give
you the voltage rating. So unless you can look
up the data for that type number, you are still
“in the dark”.
This Zener Tester is the answer. It plugs
directly into your DMM, so that you can easily
read the breakdown voltage of the zener being
tested. The unit can measure all the common
types, from very low values of around 2.2V right
up to 100V. It’s best for 400mW and 1W power
devices, although it will also provide reasonably
accurate measurements for 3W zener diodes.
The Zener Tester can also measure the breakdown voltage of other diode types such as tran82 Silicon Chip
siliconchip.com.au
sient voltage suppression (TVS) diodes,
as well as standard and Schottky
diodes with PIV (peak inverse voltage) ratings below 100V. That makes
it suitable for testing many Schottky
diodes that break down at 20, 30 or 40V
depending on the type (eg, 1N5819 or
1N5822).
As with a standard diode tester, you
can also measure the forward voltage, which is typically in the range
of 0.2-0.8V.
Fig.1: the typical zener characteristic. In the reverse
direction, there is very little current flow until the
“knee” is reached, at which point the zener breaks
down and the voltage remains reasonably constant
over a wide current range.
siliconchip.com.au
(FORWARD
CONDUCTION)
KNEE
Vz
VOLTAGE
–Vr
0.7V
VOLTAGE
+Vf
Idmax
10
10% OF MAXIMUM POWER
How zener diodes work
Zener diodes are manufactured to
provide a specified breakdown voltage where current will flow in the
reverse direction. This is known as
the “zener” voltage, after Clarence
Zener who discovered the effect. The
zener diode effect is the predominant
operating mechanism for zener diodes
with breakdown voltages up to 5.6V.
Above this voltage, the “avalanche”
effect is more predominant. However,
avalanche effect diodes continue to be
called zener diodes regardless.
Zener diodes (breakdown below
5.6V) have a negative temperature coefficient and avalanche diodes (breakdown above 5.6V) have a positive
temperature coefficient for their break
down voltage. Zener diodes with a
breakdown of around 5.6V have a
zero temperature coefficient and so
the breakdown voltage does not vary
with temperature.
Fig.1 shows the typical zener characteristic. In the forward direction, the
zener behaves as a diode and begins
to conduct at about 0.7V. Conversely,
in the reverse direction, there is very
little current flow until the “knee”
is reached. At this point, the zener
breaks down and the voltage remains
relatively constant over a wide current range.
However, the voltage does increase
with increasing current and the slope
of voltage against current is the zener
impedance (or resistance). This impedance can range from 10Ω for lowvalue zener diodes to above 350Ω for
100V zener diodes.
Fig.1 highlights three operating con
ditions for a zener diode and the two
of particular interest are maximum
power and 10% of maximum power.
These define the normal operating
range of the zener. Note how the current/voltage slope is almost a straight
line between these points.
At less than 10% of rated power,
CURRENT
Id
Idmax
4
25% OF MAXIMUM POWER
I
ZENER IMPEDANCE = SLOPE (V/I)
V
MAXIMUM POWER (100%)
Idmax
the zener voltage is much less than
its rated value. On the other hand,
operation at or above the maximum
power rating will destroy the device
(unless it is subjected to brief pulses
of current).
In any case, zener diodes are not
normally operated at maximum power
since they must be de-rated for ambient temperatures above 25°C.
Note: some zener diode types have
a very sharp “knee” which enables
the diode to operate at very low currents, well below 10% of maximum
power, while maintaining their rated
breakdown voltage.
Testing zener diodes
Testing zeners might seem simple;
just apply current so that it operates
between 10% of maximum power and
maximum power. That’s done by supplying a voltage that’s greater than the
zener diode breakdown voltage and
by limiting the current. However, in
practice, it’s not that simple.
Some zener testers apply a constant
5mA to the zener and then read off
the value of breakdown voltage. That
fixed current is suitable for the BZX79
series of zener diodes (or similar) that
are specified for zener voltage at 5mA.
That current applies for zener diodes
ranging from 2.2V to 25V. A 2mA
specification applies to zener diodes
from 25V to 60V.
Other zener diodes are not characterised for 5mA and the current needed
to test a low-voltage zener is vastly
different to that required for a higher
voltage type. In other words, a fixed
5mA is unsatisfactory, as we need to
ensure that the test current runs the
zener somewhere between the 10%
and 100% power conditions.
The 1N5728 (4.7V) to 1N5757 (75V)
series of 400mW zener diodes and the
1N4728 (2.2V) to 1N4764 (100V) series
of 1W zener diodes are designed to
operate at their specified zener voltage
at a current that is 25% of maximum
power. For a 3.3V 400mW diode, this
equates to 30.3mA while for a 75V
400mW zener, the 25% condition is
achieved at 5.3mA.
It will not matter too much if the
current doesn’t precisely give the 25%
full-power rating since the breakdown
voltage will only change slightly due
to the zener impedance. But it is imNovember 2011 83
S1 POWER
+
9V
BATTERY
REFERENCE
(LED1, IC1a)
ERROR
AMPLIFIER
IC1b
+
PULSE
CONTROLLER
(IC2)
K
CONVERTER
(Q1, T1, D3)
METER
A
–
ZENER
UNDER
TEST
–
CURRENT FEEDBACK
Fig.2: block diagram of the Zener Tester. It uses a DC/DC converter to step up the voltage from a 9V battery so that
high-voltage zener diodes can be tested. The error amplifier and pulse controller ensure that a constant power is
delivered to the zener diode under test, for a wide range of zener voltages.
portant that we do not drop below the
zener knee.
Fig.8 (later in the article) shows the
curves for both 1W and 400mW zener
diodes for voltages from 2.5V to 100V.
The lower two plots show the 40mW
(10% of 400mW) and the 100mW
(10% of 1W) power curves. The upper
two traces show the maximum power
curves for 400mW and 1W.
To properly test both 400mW and
1W diodes, we must have the zener
diode operating between the 100mW
and 400mW curves. In this way, we
will be above the 10% power point
and below their maximum limits for
both wattage types. For our Zener
Tester, the current typically follows
the 200mW curve.
The constant 5mA current zener
test is also shown on the graph. This
reveals that in this condition, 400mW
zener diodes below 8V operate at less
than 10% of maximum power (ie,
40mW) while the maximum power
rating is exceeded above 80V. For 1W
zener diodes, the test power is below
the minimum 100mW for any voltage rating below 20V. So the constant
current method does not work well
in practice.
means that at high zener voltages,
the output current is low and at low
voltages, it is higher. A standard digital
or analog multimeter can be used to
read the zener voltage.
Block diagram
The full circuit for the Zener Tester
is shown in Fig.3. IC2 is a 7555 timer
configured as an astable oscillator to
drive Mosfet Q1 with a square wave.
This in turn drives step-up transformer
T1. The output of the transformer is
rectified by fast-recovery diode D3 and
the resulting DC voltage is applied to
the zener diode under test.
Error amplifier IC1b monitors the
current through the 1Ω source resistor
for Mosfet Q1. IC1b has a gain of 470
and it amplifies the difference between
the feedback voltage at its pin 6 and
the reference voltage at pin 5 to generate an error voltage. IC1b then drives
pin 5 of the 7555 (its control voltage
terminal) to modulate the output pulse
width. The operating frequency of IC2
hovers around 67kHz.
If the current through Q1 is too high,
IC1b pulls pin 5 of IC2 slightly lower,
so that the width of the gate pulse fed
to Q1 is reduced. This pulls the current
back to the required level. Conversely,
if the current is too low, IC1b pulls pin
5 of IC2 higher. This increases the duty
cycle of the drive to Q1’s gate and thus
increases the current.
The reference voltage at the noninverting input of IC1b (pin 5) is derived from a red LED via IC1a. Note
that LK1 is installed if the power
pushbutton switch used has no LED,
in which case the reference voltage is
provided by LED1 instead.
IC1a monitors the battery voltage via
a voltage divider comprising 100kΩ
and 1.2kΩ resistors, connected to its
pin 2. The 100kΩ feedback resistors
The Zener Tester is based on a 9V
to 125V DC-DC step-up converter. The
block diagram is shown in Fig.2. It
has four sections: a voltage reference,
error amplifier, pulse controller and
the converter itself.
Error amplifier IC1b monitors the
current supplied to the converter and
adjusts pulse controller IC2 to maintain a constant current to the converter
from the 9V battery. The reference
circuit also compensates for falling
battery voltage as it discharges, so the
power delivered to the converter and
thus to the zener diode under test is
also constant.
With the power being constant, this
Features & Specifications
Main Features
•
•
•
•
•
Tests 400mW and 1W zener diodes
Test range from 0.6V to 100V
Constant power testing (about 200mW)
Reading displayed using a digital multimeter
Battery powered (9V)
Specifications
Diode test power: typically 200mW from 3.3V up to 30V, tapering to 150mW at 75V
and 2.2V; 70mW at 100V.
Test power variation with supply voltage (6-9V): 0% (8.2V zener); 21% (3.3V
zener); 12% (75V zener)
Battery current drain: from 51mA (9V) up to 84mA (6V)
Open circuit test voltage: ~125V
Short circuit output current: 100mA
84 Silicon Chip
How it works
siliconchip.com.au
1k
2
K
A
K
A
7
1
IC2
7555
D1–D3
1.5nF
6.8k
6
3
5
10nF
470k
IC1: LM358
4
IC1a
2011
SC
K
* ALTERNATIVE TO
SWITCH LED
K
A
9V
BATTERY
LED1*
1.2k
4.7k
LK1
100k
A
SWITCH
LED
ZENER DIODE TESTER
3
2
100k
1k
S1
K
A
D1 1N5819
ZD1–ZD3
A
K
ZD1
10V
10
D2
UF4003
4
8
7
IC1b
6
8
5
1k
1
100k
100nF
100k
Voltage limiting
Fig.3: the complete circuit diagram of the Zener Tester. IC1b is the error amplifier and this controls the duty cycle of oscillator IC2. IC2 in turn drives
Q1, and this switches the primary winding of step-up transformer T1. The secondary output of T1 is then rectified by D3 and applied to the zener diode.
S
D
K
A
LED1
S
1
Q1
STP16NE06
D
G
10
K
A
A
ZD2
27V
K
100 F
IC2 is configured in a slightly unusual way; the 1.5nF timing capacitor
is charged and discharged directly
from the output. Normally this results
in a fixed 50% duty cycle. However,
because IC1b overrides the control
voltage, the oscillator ramp voltage is
not necessarily symmetrical any more.
For example, if IC1b pulls the control voltage below the normal 2/3VCC,
the 1.5nF capacitor charges faster
than it discharges, because the voltage across the 6.8kΩ resistor is higher
than usual when the output is high and
lower than usual when it is low. As a
result, the output duty cycle is lower.
The reverse is also true and hence IC1b
controls the duty cycle at Q1’s gate.
siliconchip.com.au
D
G
10
K
A
17T
ZD3 27V
100nF
100 F
Timer configuration
Zener diodes ZD2 & ZD3 limit the
voltage spike which occurs at the drain
of Mosfet Q1 each time it switches
off. What happens is that as the drain
voltage rises above about 54V, zener
diodes ZD2 and ZD3 begin to conduct
and pull the gate of Q1 above 0V.
This switches on Q1 to suppress any
excess voltage and so the drain voltage
is limited to a value which is the sum
of the voltages across ZD2, ZD3, diode
D2 and the gate on-threshold voltage.
STP16NE06
–
–
+
METER
ZENER
UNDER
TEST
+
10M
10nF
250V
40T
K
D3 UF4003
A
T1
connected to pins 1 & 2 give IC1a a
gain of -1 for this signal path.
Similarly, the 1.8V across LED1
is divided using 100kΩ and 4.7kΩ
resistors to give about 80mV at pin 3
of IC1a. IC1a then amplifies the difference by a factor of 2 (1 + 100kΩ /
100kΩ) to give 160mV.
To understand how this all works
in practice, let’s assume that the battery supply is 9V. In this case, the
voltage across the 1.2kΩ resistor will
be 106.7mV and so the output (pin 1)
of IC1a will be at 160mV - 106.7mV =
53mV. However, if the power supply
falls to 7.5V (for example), then the
voltage across the 1.2kΩ resistor will
be 89mV. The pin 1 output of IC1a
will now be at 160mV - 89mV = 71mV.
Thus, as the supply voltage goes
down, the reference voltage applied to
pin 5 of IC1b goes up. This ensures that
greater current is supplied with lower
voltages, to maintain constant power.
As the accompanying specifications
panel shows, the scheme works well,
with the power remaining constant for
a supply of between 9V and 6V for an
8.2V zener diode.
November 2011 85
SWITCH
LED
RETSET ED OID RE NE Z
LED1
100nF
PRIMARY
17 TURNS
4003
CABLE
TIE
T1
IC2
7555
27V
27V
ZD3
10
4003
D2
10
ZD2
ZD1
10V
6.8k
10
1.5nF
4.7k
100k
1.2k
100k
100nF
100 F
ZENER DIODE TESTER
Fig.4: install the parts on the PCB as shown here, taking care to ensure
that all polarised parts (including toroidal transformer T1) are correctly
orientated. The two ICs can be directly soldered to the PCB.
(LID OF CASE)
REAR OF
TEST
TERMINALS
S1
+
–
T1
SECONDARY
40 TURNS
D3
CABLE
TIE
Q1
1k
1k
100k
10nF 250V
1
K
10nF
IC1
LM358
100k
11101140
5819
470k
9V BATTERY
10M
A
+
D1
LK1
A
100 F
+
K
1k
- -
Fig.6: T1 is wound using 0.25mm
enamelled copper wire with 17
turns for the primary and 40 turns
for the secondary. The winding
direction is important, so follow
the way the windings are shown
for both the primary and the
secondary.
indicated by the brightness of the LED.
If LED1 is dim, then it’s time to change
the battery. The fact that the circuit
works below 6V means that battery
life is good.
An alternative battery check is to
measure the output voltage when the
Zener Tester is plugged into the multimeter, without anything connected
across the terminals. If the output is
above 100V then the battery condition
is satisfactory.
Construction
CABLE TIES
END OF
CASE
- +
K
A
5819
4003
RETSET ED OID RE NE Z
+
BANANA
PLUGS
27V
27V
10V
11101140
4003
9V BATTERY
Fig.5: the switch, binding posts and banana plugs are connected to PC
stakes on the PCB via medium-duty hook-up wire. Use heatshrink tubing
over the PC stake connections and at the ends of the binding posts to stop
the connections from breaking due to vibration.
Typically, this will be just over 60V
and this in turn limits the maximum
voltage that can be delivered by the
transformer’s secondary winding (with
no zener diode connected across the
test terminals) to something less than
145V.
In practice though, the limit appears
86 Silicon Chip
to be about 115V (depending on the
battery condition).
Power supply
Power for the circuit is derived
from a 9V battery via reverse polarity
protection diode D1 and pushbutton
switch S1. The battery condition is
Construction of the Zener Tester is
straightforward, with most of the parts
mounted on a PCB coded 04111111
and measuring 61 x 107mm. This is
housed in a plastic utility box measuring 130 x 68 x 44mm. The PCB clips
into slots moulded into the sides of
the case. Corner mounting holes are
provided on the PCB for other applications.
Check the board for faults and repair
it if necessary. Also check that the PCB
mounting holes and the holes for the
battery leads are the right size (3mm).
Fig.4 shows the assembly details.
Begin by fitting all resistors. The resistor colour code table can be used to
read their values however it’s best to
check them with a digital multimeter
in Ohms mode.
The diodes, including zener diodes
ZD1-ZD3, can then be installed and
must be mounted with the orientations shown. Note also that D1 is a
1N5819, while D2 and D3 are UF4003
or 1N4936 types. There are two different zener diode types (10V & 27V) so
don’t get them mixed up.
IC1 & IC2 go in next. 8-pin DIL
sockets may be used but are not necessary; the ICs can be soldered the PCB.
siliconchip.com.au
In either case, take care to orientate
them correctly, with the notch/dot
positioned as shown.
Solder the PC stakes next, then the
capacitors. The electrolytic types must
be orientated with the correct polarity, ie, longest lead through the hole
marked “+”.
LED1 is mounted flush against the
PCB (it’s there to provide a reference
voltage only). Make sure the anode
(longer lead) is placed in the hole
marked “A”.
That done, the 2-way pin header can
be installed, followed by Mosfet Q1
which is installed vertically. Be sure
to orientate Q1 as shown.
Winding the transformer
T1 is wound as shown in Fig.6. It
uses 0.25mm enamelled copper wire
with 17 turns for the primary and 40
turns for the secondary. The winding direction is important so follow
the way the windings are shown on
Fig.6 for both the primary and the
secondary.
When winding is completed, the
transformer can be installed on the
PCB. Use a sharp knife or emery paper
to strip the enamel insulation at each
end of both wires, then tin them and
solder them to the appropriate PCB
pads. The transformer is held in place
with cable ties that pass through holes
in the PCB.
Preparing the case
Use the front panel artwork (Fig.7)
as a guide to drill the holes in the lid
for the switch and binding posts. Start
with a small pilot drill, then enlarge
the holes and ream them out to the correct size. The binding post holes must
be 8mm in diameter but the switch
hole will depend on the switch used.
You also need to make holes in the
This is the view inside the case after the wiring has been completed. The
metal battery holder is secured to the side of the case using an M3 x 9mm
tapped spacer and machine screws.
end of the box for the banana plugs,
with the standard 19mm spacing.
These go 12mm down from the top
edge of the box. Drill the holes out
smaller than the screw thread of the
banana plugs so that these can be
screwed into the plastic box, forming
a thread in the process.
Finally, a 3mm hole is also required
for the battery holder screw support in
the opposite end of the box. This hole
is positioned 13mm down from the top
Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
siliconchip.com.au
No.
1
1
4
1
1
1
3
3
1
Value
10MΩ
470kΩ
100kΩ
6.8kΩ
4.7kΩ
1.2kΩ
1kΩ
10Ω
1Ω
4-Band Code (1%)
brown black blue brown
yellow violet yellow brown
brown black yellow brown
blue grey red brown
yellow violet red brown
brown red red brown
brown black red brown
brown black black brown
brown black gold brown
edge of the box, then countersunk on
the outside.
The next step is to prepare the front
panel label. This can be downloaded
from the SILICON CHIP website (in the
November 2011 downloads section) or
Table 2: Capacitor Codes
Value
100nF
10nF
1.5nF
µF Value IEC Code
0.1µF
100n
0.01µF 10n
.0015µF 1n5
EIA Code
104
103
152
5-Band Code (1%)
brown black black green brown
yellow violet black orange brown
brown black black orange brown
blue grey black brown brown
yellow violet black brown brown
brown red black brown brown
brown black black brown brown
brown black black gold brown
brown black black silver brown
November 2011 87
onto the panel.
Once the label is in place, use a
hobby knife to cut out the holes.
If it isn’t self-adhesive, affix it to the
panel using an even smear of neutral
cure silicone sealant or spray contact
adhesive. For plastic film, if you are
affixing to a black coloured panel, use
coloured silicone such as grey or white
so that the label can be seen against
the black.
Wiring
These waveforms illustrate the operation of the step-up converter. The yellow
trace is the waveform fed to the gate of Mosfet Q1. Each time the gate signal
goes positive, the Mosfet turns on and its drain is pulled low, as shown by the
green trace. As the gate pulse goes low again, the Mosfet turns off and the drain
voltage swings high and rings at a high frequency, producing a peak voltage of
around 60V. This is stepped up in the transformer and rectified by diode D3 to
charge the 10nF capacitor. When the diode stops conducting, the ringing at the
drain continues at a lower frequency until the Mosfet is switched back on by the
next positive gate pulse.
photocopied from this article.
You can either print it onto paper and laminate it, or print it onto
sticky-backed photo paper or plastic
film. When using clear plastic film (ie,
overhead projector film) you can print
the label as a mirror image so that the
ink is behind the film when placed
Begin the wiring by removing the banana plugs, then solder short lengths
of hook-up wire to the rear of each one
(if you solder it with them in the box,
the box will melt). That done, screw
them back in, allowing the wires to
rotate freely as you do so, so they don’t
get twisted.
Fig.5 shows how the wiring is
done. The 9V battery leads are looped
through the holes in the PCB and
then soldered to the PC stakes with
heatshrink tubing over the soldered
joint. It’s important to loop the wire
through the holes provided in the PCB,
to improve retention and to prevent the
wires from breaking off the PC stakes
when the battery is changed.
The wiring shown assumes switch
S1 has an integral LED. If not, simply
omit the two additional wires. Use regular hook-up wire for the connections
and as with the battery, heatshrink the
joints to the PC stakes as well as where
the wires join to the binding posts.
Parts List: Zener Diode Tester
1 PCB, code 04111111, 61 x
107mm
1 plastic utility box, 130 x 68 x
44mm
1 9V alkaline battery
2 banana line plugs
1 red binding post
1 black binding post
1 9V battery clip connector
1 9V battery holder (Jaycar
PH-9237, Altronics S5050)
1 momentary push-on switch
with red LED indicator (Jaycar
SP-0706, Altronics S1086)
(S1) OR 1 momentary
pushbutton switch
1 ferrite toroid, 18 x 10 x 6mm
(Jaycar LO-1230 or equivalent)
1 1.3m length of 0.25mm
enamelled copper wire
88 Silicon Chip
1 M3 x 6mm panhead screw
1 M3 x 6mm countersunk screw
1 9mm M3 tapped spacer
1 2-pin header (2.54mm pitch)
1 shorting plug for header (LK1)
10 PC stakes
4 100mm cable ties
1 100mm length of 3mm-diameter
heatshrink tubing
1 30mm length of 5mm-diameter
heatshrink tubing
200mm of red hook-up wire
200mm of black hook-up wire
120mm of white hookup wire
Semiconductors
1 LM358 dual op amp (IC1)
1 7555 CMOS timer (IC2)
1 STP16NE06 60V Mosfet (Q1)
1 1N5819 Schottky diode (D1)
2 1N4936 or UF4003 fast
recovery diodes (D2, D3)
1 10V zener diode (ZD1)
2 27V 1W zener diodes
(1N4750; ZD2, ZD3)
1 3mm red LED (LED1)
Capacitors
2 100µF 16V PC electrolytic
2 100nF MKT
1 10nF 275VAC X2 class MKP
1 10nF MKT
1 1.5nF MKT
Resistors (0.25W, 1%)
1 10MΩ
1 1.2kΩ
1 470kΩ
3 1kΩ
4 100kΩ
3 10Ω
1 6.8kΩ
1 1Ω 5%
1 4.7kΩ
siliconchip.com.au
Zener Diode Power Curves
Zener
Diode
Tester
100
95
90
++
85
80
Press To Test
A
+
K
75
70
+
65
SILICON CHIP
Fig.7: this artwork can be copied and
used as a drilling template for the
front panel. It’s also available in PDF
format from our website, to make a
front panel label.
Once the wiring is complete, secure
it using cable ties as shown.
With the board in place and wired
up, install the battery holder. Use a
machine screw to connect the 9V battery clip to the M3 tapped spacer, then
attach the other end of the spacer to the
box using an M3 countersunk screw.
Testing
If you are not using a power switch
with integral LED, install a shorting
block on LK1. Otherwise, leave it out.
Press S1 and check that the LED lights.
If not, check the LED and switch wiring. The LED may be wired or installed
backwards.
Now plug the unit into a multimeter
and set it to read DC volts. Press power
button S1 and check that the output
produces 115-125VDC. If not, check
that T1 is wound correctly, as shown
in Fig.6. You can swap the two primary
connections if necessary; there is no
need to rewind it if it is wrong.
If it still doesn’t work, check other
voltages on the circuit. The supply for
IC1 (between pins 8 & 4) and IC2 (between pins 8 & 1) should be about 0.3V
less than the battery voltage. Check for
around 80mV at pin 3 of IC1a.
siliconchip.com.au
Zener Current (mA)
60
55
50
45
40
35
1W
30
25
20
400mW
15
5mA
Constant 10
Current
Test
5
200mW
100mW
40mW
0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Zener Voltage (V)
Fig.8: the voltage versus current curves for both 1W and 400mW zener
diodes for voltages from 2.5 to 100V. The lower two traces show the 40mW
(10% of 400mW) and the 100mW (10% of 1W) power curves. Our Zener
Tester typically follows the 200mW power curve.
To check operation of the Zener
Diode Tester under load, connect a
1kΩ resistor across the test terminals.
The multimeter should indicate a
reading of about 14V. This means that
close to 200mW (14V2 ÷ 1kΩ) is being
delivered to the resistor.
Further testing can be done using
zener diodes with known breakdown
voltages.
Note that zener diodes can have a
tolerance of 10%, 5%, 2% or 1% and
that the measured voltage can also
SC
depend on the zener current.
November 2011 89
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