Is It Dead or Not?
Great editorial by Bryan Bergeron
in the July 2017 issue on a grab-and-go
kit, but I'm flummoxed regarding the
inclusion and use of a clamp-on
ammeter to "verify that the circuit is,
indeed dead." I'm sure you must have
meant something else.
As I'm sure Bryan must know, the
ammeter will only indicate the
presence of current; it won't tell you if
the circuit is 'dead' or if any potential is
present. That's where a DVM would be
used.
Put simply, if you're replacing an
electrical outlet, a reading of zero from
your clamp-on ammeter around any or
all of the wires is no guarantee at all
that the breaker is off.
Ira Wexler
You are, of course, correct Ira.
Thanks for pointing out the error.
Readers like you help make it a better
magazine for all.
Bryan Bergeron
Values MIA
I was reading the June Q/A
column and realized Figure 1 has no
component values except for transistor
types. Do you have an updated
schematic?
Also, tying the emitters of Q3 and
Q4 together looks very odd! Should
the collector and emitter connections
of Q4 be swapped?
Dan NE7JN
My bad Dan! Below is an updated
schematic that has the values I used
when I built it. They must have been
buried in the metadata somewhere
when I pulled the schematic out from
the board layout.
As for Q3 and Q4, that’s quite
intentional. It makes it into a
comparator. Only when Q3’s emitter is
below Q4’s by ~1.2V does the circuit
begin to do anything. In other cases,
they are reverse-biased and the fan
doesn’t turn on at all because there is
no current flowing in R4.
With no current flowing in R4, Q2
doesn’t begin to be a current source.
Q3’s emitter is lowered when Q1
becomes warm, shrinking it’s Vbe, which
is then multiplied by the gain around
the potentiometer connected to its
collector.
You’ll notice that I’m pretty boring
about the component values I choose. I
like to make things easy on myself.
Unless something needs a precise
amount of current, I usually don’t worry
too much about more than the order of
magnitude.
In the same way, D1’s voltage
could be different, as long as there’s
about 20 mA of current available. Even
that current is a guess. It’s enough
current to keep the zener diode stable
and far more than the attached
transistors will need.
One other thing of note is that Q1
is a pretty hefty NPN only because
those TO-220 packages are easy to
mount as a temperature sensor. Mine is
extended by a shielded three-conductor
cable to reach the radio. It’s shielded to
lessen RF incursion.
I think this is a fun circuit since it’s
a bit of a puzzler. It uses transistors in
both conventional (class-A amplifier,
current source) and unconventional
ways (Vbe multiplier, comparator,
thermal feedback loop). Try it out on
your friends the next time you’re at a
cocktail party!
Kristen McIntyre
Q&A Editor
READER FEEDBACK
Continued on page 54
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September 2017 7