March 2017 13
so fast that oscillation could be a problem. The more
gain that a device has at high frequencies, the greater
the probability that some phase shift somewhere will
be sufficient to have poles in the right-half plane of the
transfer function (if you’re familiar with stability plots in the
Poles are discontinuities in that transfer function. A
greater than or equal to 180 degree phase shift — until you
return back to zero shift — with gain will lead to oscillation.
Of course, when building an oscillator, that’s a necessary
So, while I don’t have a large catalog of devices that
are my favorites, I do have a way of looking for what I
want. Let me try to explain. Reading transistor datasheets
can seem intimidating, but I skim them for the most
part. The first thing to determine is whether I need an
NPN or PNP. Sometimes I swap the topology around if
I don’t see the device I want. I really begin the search
by thinking about the category of application: switch,
amplifier, oscillator, sensor (yes, I use them as sensors
too), voltage/current source, etc. Within that category,
I ask how important is current, voltage, and power, and
how important is speed. That sets me up to look at the
datasheet (see Figure 1 for the really important bits).
Most of the time, I’m interested in only a few things.
First, is the maximum collector current Ic more than
sufficient? If so, what about the maximum collector-emitter
voltage, or Vce? If that’s enough, I have to think about
maximum power and whether that fits into the SOA. I do
some mental math and ask myself if the maximum power
I care about is around half of the product of Ic and Vce. If it
is, then that’s probably fine. We want to stay away from the
secondary breakdown region for bipolar transistors.
For FETs, that is often not a worry because they have a
negative thermal characteristic. Then, I ask myself how fast
the circuit will run. If ft is at least a factor of 2 above what
frequency the circuit will run, then that’s generally fine.
I might check the beta, or b, to be sure there is enough
current gain so that I won’t have to care about it very
much. Beta is quite variable with temperature, so it’s not a
good idea to depend on it much in my experience. There
are ways to design circuits to avoid beta dependency.
Another thermally variable parameter is Vbe. Avoid
depending on that too.
Typically, one transistor can cover a wide range of
applications and specifications. Some evidence of this can
be seen by searching the Texas Instruments’ Transistor and
Diode Data Book, available at archive.org for the 2N2222.
You will see there are many transistors that have the
2N2222 as a substitute. Figure 2 shows a small sample.
Unless I’m designing something to run at RF in the
VHF and above range, this is generally sufficient to make
a choice. It’s not uncommon for me to go to the local
electronics parts place, stand in front of the transistor
bins, and use my phone to look briefly at the datasheets
as I make a choice. When I find something that’s in the
ballpark, I’m done! Mostly, it’s that simple.
A Simple Charge Controller
QI would like to build a charger for a battery system. The idea would be to sense when the voltage drops below 13 VDC and when it does, turn a charger on for a period that would be
adjustable from between five and 10 minutes. I’m not
interested in checking the voltage levels during charge, just
the time period of the charge. Once the charge time has
elapsed, I’d like to go back to monitoring for the voltage to
again drop below the 13 VDC threshold in a loop.
AThere’s a bit of missing information here about hings like battery chemistry and what the charge circuit is expected to deliver. Since that is missing, I’ll focus on control signals that
might turn on or off whatever charge circuit might match
the battery chemistry.
As an aside, batteries are all about chemistry. Charging
a battery with the wrong current or voltage limits can have
bad effects. At the benign end of the spectrum of possible
results, the battery’s usable lifetime can be significantly
shortened and charge capacity diminished when the
electrodes are degraded by electrochemical reactions that
are not within design parameters.
At the more unpleasant end, the battery can expand,
QUESTIONS and ANSWERS
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300 25 25
300 30 30
300 45 25
300 50 50
. 6 5
. 5 5
. 8 5
300 20 20
600 30 25
800 30 25
600 45 30
n FIGURE 2. 2N2222 substitutions.