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Build this useful test accessory
A zener diode tester
for your DMM
Plug this simple adaptor
into your DMM and you can
directly read the values of
zener diodes. It covers the
range from about 2.2V right
up to 100V.
By JOHN CLARKE
32 Silicon Chip
H
OW MANY ZENER DIODES do
you have stashed away which
cannot be used simply because
their value is unknown? In many cases, the type number will be missing
(rubbed off) or will be very 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
value you anyway – instead, you have
to look it up in a data book.
This Zener Tester is the answer to
this problem. It plugs directly into
your DMM, so that you can directly
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
a reasonably accurate measurement
for 3W zeners.
Testing zener diodes
Testing zener diodes has always
been difficult. This is because the current needed to test a low-voltage zener
is vastly different to that required for
a higher voltage type.
In the past, many zener testers
tried to circumvent this problem by
applying a constant 5mA and then
reading off the value of breakdown
voltage. Thus, for a 5V zener, the
power dissipated would be 25mW
and for a 30V zener, 150mW. While
these values may appear OK, let’s see
why the constant current idea does
not work in practice.
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 (as
in a normal diode), until the “knee”
is reached. At this point, the zener
breaks down and the voltage remains
essentially constant over a wide range
of currents.
Note the maximum power position
(the power rating of the zener) and the
10% maximum power location. These
two power limits set the operating
range of the zener.
If the current is taken below the
10% maximum power position, the
zener voltage will drop markedly as
it follows the knee in the curve. This
means that if we read the zener voltage
below the 10% position, the reading
will be well under the correct zener
voltage which can only be obtained
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 virtually constant over a wide
range of currents.
at higher currents.
Note: some zener diode types have
a very sharp knee, which enables the
diode to operate at very low currents
Features
•
Tests 400mW and 1W zener
diodes
•
•
Test range from 2.2V to 100V
•
Connects to a multimeter for
zener voltage reading
•
Battery powered
Constant power testing at
200mW
while maintaining its rated breakdown
voltage.
Fig.2 shows the curves for both 1W
and 400mW zener diodes for voltages
from 3-100V. The lower two traces
show the 40mW (10% of 400mW)
and the 100mW (10% of 1W) power
curves, while the upper two traces
show the maximum power curves for
400mW and 1W.
To properly test 400mW and 1W
diodes, we must have the zeners operate between the 100mW and 400mW
curves. In this way, we will be above
the 10% power point for both types
and below their maximum limits.
The trace (dotted) for a zener tester
using a constant 5mA current shows
Specifications
Zener diode test power �������������������� 200mW
Test power linearity �������������������������� within 10% of 200mW for zener
diodes from 4V to 100V; less than
3.5% change for battery supply
variation from 6-9V
Battery current drain ������������������������ 35mA <at> 9V; 47mA <at> 6V
Open circuit output voltage �������������� 112V nominal
Overall efficiency ������������������������������ 63%
Converter efficiency ������������������������� >90%
March 1996 33
Fig.2: voltage vs.
current curves for
both 1W and 400mW
zener diodes, for
voltages from
3-100V. The lower
two traces show
the 40mW (10% of
400mW) and the
100mW (10% of 1W)
power curves, while
the upper two traces
show the maximum
power curves for
400mW and 1W.
that while zeners from 20-80V fit
between these limits, the maximum
dissipation is exceeded for 400mW
diodes above 80V. At the other end,
the 10% limit prevents 1W diodes
from giving accurate readings below
20V (for 400mW diodes, the limit is
extended to below 8V).
One way around this is to use a
fixed resistor tester operating from
a 110V supply. This will enable all
400mW and 1W zener diodes to be
34 Silicon Chip
tested down to about 3V. Note, however, that this type of tester will go
close to the 400mW limit at about
66V.
At the same time, the tester will also
need to provide up to 1.42W of power
to dissipate 40mW in a 3V zener. This
represents an efficiency of just 3%.
While efficiency may not appear to
be a problem, it does present a strain
on a small 9V battery when it is called
upon to deliver 160mA.
The final trace shows the 200mW
power curve and this fits neatly between the limits specified. The SILICON
CHIP Zener Tester follows this curve
closely. It always provides the same
power to the zener diode, regardless
of voltage. And, as a bonus, battery
drain is much lower at 35mA.
Block diagram
The Zener Tester is based on a high
voltage supply, produced by stepping
Fig.3: block diagram of
the Zener Tester. It uses a
converter to step up the
voltage from a 9V battery
so that high-voltage
zeners can be tested. The
error amplifier and pulse
controller ensure that
the power delivered to
the zener diode remains
constant.
up from 9V using a converter – see
Fig.3. This converter produces up to
about 112V, so that high-voltage zeners
can be tested.
The current supplied to the converter is monitored by error amplifier IC1b
which in turn drives a pulse controller (IC2). This maintains a constant
current to the converter from the 9V
battery. Since the battery voltage is
also constant, the power delivered to
the converter and thus to the zener is
also constant.
In practice, this means that the
converter alters its current output depending on the zener voltage. At high
zener voltages, the current is low and
at low voltages, the current is high.
A LED reference is used to provide
a fixed voltage for the error amplifier,
so that current can be maintained.
Note that this reference is also compensated for input voltage, so that as
the battery voltage falls, the reference
voltage rises and allows more current
flow through the converter. This
maintains the constant power to the
converter, regardless of variations in
the supply voltage.
A standard digital or analog mul-
timeter is used to read the value of
zener voltage.
How it works
The full circuit for the Zener Tester
is shown in Fig.4. It consists of just a
few low-cost components and a stepup transformer.
The step-up circuit uses the two
windings of transformer T1 to produce up to 112V. Mosfet transistor
(Q1) is used as a switch to charge the
primary winding via the 9V supply.
When Q1 is switched off, the charge
is transferred to the secondary and
delivered to a 0.1µF capacitor via
diode D1.
The advantage of using a 2:1 stepup transformer is that the voltage
developed across Q1 is only half that
developed across the secondary winding. This means that a 60V Mosfet can
be used rather than a 200V type.
Q1 is driven by an oscillator formed
by 7555 timer IC2. This operates by
successively charging and discharging
a .0039µF capacitor via a 6.8kΩ timing resistor connected to the output
(pin 3).
When power is first applied, the
.0039µF capacitor is discharged and
the pin 3 output is high. The capacitor
then charges to the threshold voltage
at pin 6, at which point pin 3 goes low
and the capacitor discharges to the
lower threshold voltage at pin 2. Pin
3 then switches high again and so the
process is repeated indefinitely while
ever power is applied.
The current through Q1 is monitored by measuring the voltage across
the 1Ω source resistor. This voltage is
filtered using a 120Ω resistor and a
0.1µF capacitor and applied to error
amplifier IC1b. Its output (pin 7) directly drives the threshold pin (pin
5) of IC5.
If the current is too high, IC1b
pulls pin 5 of IC2 slightly lower, so
that the pulse width duty cycle to Q1
Fig.4 (below): the 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 which switches the
primary of step-up transformer T1.
The secondary output of T1 is then
rectified via D1 and applied to the
zener diode.
March 1996 35
The PC board fits neatly into a standard plastic case,
with room for the battery at one end. Take care to
ensure that the test terminals are correctly wired.
is reduced. This in turn reduces the
current. 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.
IC1b compares the average current
value with a reference at its pin 5
(non-inverting) input. This reference
is derived from the power supply and
LED1 via IC1a.
In operation, pin 2 of IC1a monitors
a voltage dependant reference derived
from a voltage divider (100kΩ & 560Ω)
across the supply rails. This reference
is fed to pin 2 via a 100kΩ resistor,
while a 100kΩ feedback resistor gives
the amplifier a gain of -1 for this signal
path.
Similarly, the 1.8V that appears
across LED1 is divided using 100kΩ
and 2.4kΩ resistors to give about
42mV at pin 3 of IC1a. IC1a then amplifies this signal by a factor of 2 (1 +
100kΩ/100kΩ) to give 84mV.
To understand how this all works in
practice, let’s assume that the power
supply is at 9V. In this case, the voltage across the 560Ω resistor will be
50mV and so the output (pin 1) of IC1a
will be at 84 - 50 = 34mV. However,
if the power supply falls to 7.5V (for
example), then the voltage across the
560Ω resistor will be 42mV. The pin
Fig.5: this diagram
shows the winding
details for the stepup transformer (T1)
– see text. Note that
both windings are
wound in the same
direction.
36 Silicon Chip
1 output of IC1a will now be at 84 42mV = 42mV.
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 at lower voltages, to maintain the constant
power. As the accompanying specifications panel shows, this scheme
works well, with the power varying by
only 3.5% for battery voltage ranging
from 6-9V.
Power supply
Power for the circuit is derived from
the 9V battery via switch S1. Note that
the battery condition is indicated by
the brightness of the LED. If LED1
is dim, then it is time to change the
battery. The fact that the circuit will
work down to below 6V means that
battery life is quite good.
Construction
Construction of the SILICON CHIP
Zener Tester is straightforward, with
most of the parts mounted on a PC
board coded 04302961 (56 x 104mm).
Begin construction by checking the
PC board for shorted tracks or small
breaks. In addition, the corners of the
PC board will need filing out so that
it will fit inside the case. The actual
shape is shown on the copper side of
the PC board.
This done, install PC stakes at the
Fig.6 (right): make
sure that transformer
T1 is correctly
oriented when
installing the parts
on the PC board
(ie, pin 1 to bottom
left). Fig.7 (far right)
shows the full-size
PC pattern.
external wiring points – see Fig.6.
These are located at the positive (+)
and negative (-) battery wiring points,
at the positive and negative terminal
connection positions, and at the
switch (S1) and LED1 positions. Once
these are in, install the two wire links
(next to IC1 and next to IC2).
Next, install the resistors, followed
by the diodes and ICs. Table 1 lists the
resistor colour codes but it is also a
good idea to check them using a digital
multimeter. Make sure that the diodes
and ICs are correctly oriented.
The capacitors can now be installed,
taking care to ensure that the 100µF
electrolytic is oriented correctly.
This done, install Mosfet Q1 on the
board (metal tab towards IC2). LED1
is mounted on the end of its leads, so
that it will later protrude through the
front panel. Similarly, switch S1 is
soldered on the top of its corresponding PC stakes.
end on pin 6; (2) wind on 20 turns
side-by-side in the direction shown
and terminate the free end on pin 3; (4)
wrap a layer of insulating tape around
this winding.
The secondary is wound on in
similar fashion, starting at pin 5 and
winding in the direction shown. Note
that the 40 turns are wound on in
two layers (20 turns in each), with a
layer of insulating tape between them.
Terminate the free end of the winding
on pin 4.
The transformer is now assembled
by sliding the cores into each side of
the former and then securing them
Transformer winding
Transformer T1 is wound using
0.25mm enamelled copper wire – see
Fig.5. The primary is wound first, as
follows: (1) remove the insulation
from one end of the wire using a hot
soldering iron tip and terminate this
TABLE 1: RESISTOR COLOUR CODES
❏
No.
❏ 1
❏ 1
❏ 4
❏ 1
❏ 1
❏ 2
❏ 1
❏ 1
❏ 1
❏ 1
Value
10MΩ
470kΩ
100kΩ
6.8kΩ
2.4kΩ
1kΩ
560Ω
120Ω
10Ω
1Ω
4-Band Code (1%)
brown black blue brown
yellow violet yellow brown
brown black yellow brown
blue grey red brown
red yellow red brown
brown black red brown
green blue brown brown
brown red brown brown
brown black black brown
brown black gold gold
5-Band Code (1%)
brown black black green brown
yellow violet black orange brown
brown black black orange brown
blue grey black brown brown
red yellow black brown brown
brown black black brown brown
green blue black black brown
brown red black black brown
brown black black gold brown
brown black black silver brown
March 1996 37
+
+
-
+
Ζ
+
ENER TESTER
POWER
+
Fig.8: this full-size artwork can be
used as a drilling template for the
front panel.
The test leads are fitted with banana plugs (red for positive, black for negative),
so that they can be plugged into standard multimeter terminals. The zener
breakdown voltage is the read directly off the multimeter display.
with the clips. This done, insert the
transformer into the PC board, making
sure that it is oriented correctly, and
solder the pins.
Final assembly
A plastic case measuring 64 x 114 x
42mm is used to house the assembled
PC board. This is fitted with a self-adhesive label measuring 55 x 103mm.
Begin the final assembly by affixing
the label to the front panel (lid), then
drill out mounting holes for the LED
bezel, switch S1 and the two banana
plug terminals. You will also need to
drill a hole in one end of the base to
accept a small grommet. This done,
mount the two test terminals (red for
positive, black for negative) and fit
the grommet and LED bezel in place.
Next, fit the board inside the case (it
38 Silicon Chip
sits on four integral mounting pillars)
and secure it using four small self-tapping screws. The lid can now be test
fitted to check that the switch and LED
line up correctly with the front panel.
Adjust them for height as necessary,
then solder the battery clip leads to
their respective PC stakes.
Finally, run short lengths of hookup wire from the PC board to the test
terminals. Additional leads are then
attached to the test terminals and
brought out via the grommet fitted to
one end of the case. Terminate these
leads using banana plugs (red for positive, black for negative). This lets you
plug the leads directly into a standard
DMM or analog multimeter.
Testing
You are now ready to test the unit.
Apply power and check that the LED
lights. If is doesn’t, check that the LED
is oriented correctly. Now measure the
voltages on IC1 using a multimeter.
There should be about 9V DC across
pins 4 & 8 and a similar voltage between pins 1 & 8 of IC2.
If these voltage checks are correct,
plug the output leads into your multimeter and press the Power button.
Check that the meter reads 112V. If it
doesn’t, switch off immediately and
check for wiring errors.
If everything is OK so far, connect a
1kΩ resistor across the test terminals
and check the voltage again (press the
Power button). This time, you should
get a reading of about 14V across the
resistor, which means that the resistor
is dissipating about 200mW. If this
reading is quite different, check that
the voltage across LED1 is 1.7-1.8V
and that about 42mV at present on
pin 3 of IC1.
Assuming a fresh battery, you
should also get about 50mV across the
560Ω resistor. If the latter two reading
are incor
rect, check the associated
voltage divider resistors.
If all is working correctly, you are
now ready to measure zener diodes.
PARTS LIST
1 PC board, code 04302961,
104 x 56mm
1 plastic case, 64 x 114 x 42mm
1 front panel label, 55 x 103mm
1 pushbutton momentary contact
switch (S1)
1 9V battery and battery clip
1 red banana socket
1 black banana socket
1 red banana plug
1 black banana plug
1 EFD20 transformer assembly
(Philips 2 x 4312 020 4108 1
cores, 1 x 4322 021 3522 1
former, 2 x 4322 021 3515 1
clips) (T1)
1 2-metre length of 0.25mm
enamelled copper wire
1 100mm length of red hook-up
wire
1 100mm length of black hookup wire
1 30mm length of 0.8mm tinned
copper wire
8 PC stakes
4 3mm screws
1 small grommet
1 3mm LED bezel
Semiconductors
1 LM358 dual op amp (IC1)
1 7555, TLC555, LMC555CN
CMOS timer (IC2)
1 MTP3055E or A version
N-channel Mosfet (Q1)
1 3mm red LED (LED1)
1 1N4936 fast recovery diode
(D1)
1 56V 3W zener diode (ZD1)
Capacitors
1 100µF 16VW PC electrolytic
2 0.1µF MKT polyester
1 0.1µF 400VDC polyester
1 .0039µF MKT polyester
Resistors (0.25W, 1%)
1 10MΩ
2 1kΩ
1 470kΩ
1 560Ω
4 100kΩ
1 120Ω
1 6.8kΩ
1 10Ω
1 2.4kΩ
1 1Ω
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There’s just one important thing to
watch out for here – be sure to connect
the zener diode to the test terminals
with the correct polarity; ie, cathode
(banded end) to positive, anode to
SC
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March 1996 39
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