76 Silicon chip Australia’s electronics magazine siliconchip.com.au
I wrote into Silicon chip (Mailbag,
November 2018) to whinge about the
fact that very few (if any) DMMs avail-
able these days include a zener diode
testing function, despite this being a
very handy thing to have. The circuit
presented here shows just how easy it
is to add such a function.
I even managed to fit it in the free
space inside a Digitech QM-1500 DMM
I purchased from Jaycar, despite it be-
ing built on a piece of veroboard. It
makes you wonder why the manufac-
turers can't do this!
It's a great Saturday arvo project
since it's pretty quick and easy to build
and doesn't use any exotic parts. You
probably already have them all in your
junk box, which means it will cost you
little-to-nothing to build it.
The first step is to figure out how
much space you have inside your
DMM. In my case, I found I could fit
a board about 55 x 18mm behind the
LCD screen. That equates to a piece of
veroboard with seven copper strips.
The circuit only uses about a dozen
components and is powered from the
Simple zener diode tester fits inside a DMM
DMM's internal 9V battery.
When externally accessible push-
button switch S1 is held down, current
flows from the battery through LED1
and its 4.7kW series current-limiting
resistor, so LED1 illuminates to in-
dicate that the zener testing mode is
operational.
Current also flows through the 100W
resistor and through one half of trans-
former T1, to the collector of Q1. At
the same time, current flows to Q1's
base through the 100W and 470W se-
ries resistors and the other half of T1.
As Q1 switches on and the current
through the top half of T1 increases (at
a rate limited by its 220μH of induct-
ance), current induced in the other half
of the transformer opposes the base
current to Q1, cutting it off and caus-
ing it to switch off. Then, as the cur-
rent through the top half of T1 drops,
the base current flow resumes and Q1
can switch on again.
Thus, it forms an oscillator, which
oscillates at around 85-90kHz.
This allows the transformer, in com-
bination with diode D1 and the 1μFcapacitor, to form a simple DC/DC
boost converter, which provides a lit-
tle over 50V (at light loads) for testing
zener diodes.
This is sufficient for testing most ze-
ner diodes you will come across. The
zener clamps this voltage at a level
depending on its type, and the DMM
is used in DC voltage measuring mode
to show that voltage.
The choice of a BC337 for Q1 is a
compromise. A BC546 or BD681 will
get a higher output voltage at the ex-
pense of much higher (double!) the
current drain on the battery.
The 68W resistor limits the maxi-
mum current through the zener being
tested to about 2mA but the circuit can
only deliver around 3mA continuous-
ly to a low-voltage zener, decreasing
to around 0.1mA for a 28V+ zener. De-
spite this variation, the results should
still be pretty accurate.
D2 is provided merely as a safety
feature, to prevent a voltage that is ap-
plied to the meter leads from damag-
ing the circuit. Note that D2 will still
conduct if you apply a negative or ACThe zener diode tester board (55 x 18mm) fit in the free space at the top of a Digitech QM-1500 DMM. The zener is
connected with cathode to positive lead. If connected the wrong way around, you will measure the zener’s forward voltage.
CIRCUIT NOTEBOOK
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