Page 50 - Troubleshooting Analog Circuits

Protect Transistors From Voltage Kick 37

voltage high enough to damage or destroy almost any transistor. You can avoid this
problem by connecting a suitable snubber, such as a diode, an RC network, a zener, or
a combination of these components, across the inductor to soak up the energy. The use
of a snubber is an obvious precaution, yet every year I see a relay dnver with no clamp
to protect the transistor. The transistor may survive for a while, but not for long.
The tiniest inductors are called beads. They are about the same size and shape a5
beads worn as jewelry, they are available in various types of femte material, and they
have room for only one or two or four turns of wire. Beads are commonly used in the
base or emitter of a fast transistor to help keep it from oscillating. A bead not only
acts inductive but also acts lossy at high frequencies, thus damping out ringing. In
general, the choice of a bead is an empirical, seat-of-the-pants decision, but designers
who have a lot of experience in this area make good guesses. This topic is one that I
have not seen treated (except perhaps one sentence at a time) in any book or maga-
zine. You’ll just have to get a box of femte beads and experiment and fool around.
Transformers usually are susceptible to the same problems as inductors. In addi-
tion, the turns ratio may be wrong, or the winding polarity might be incorrect. And. if
your wire-handling skills are sloppy, you might have poor isolation from one
winding to another. Most femte materials are insulators, but some are conductive.
So, if you’ve designed a toroidal transformer whose primary and secondary windings
are on opposite sides of the toroid and you scrape off the core’s insulating coating,
you could lose your primary-secondary insulation. If the insulating coating isn’t good
enough, you might need to wrap tape over the core.
Fortunately, it’s easy to establish comparisons between a known-good transformer
and a questionable one. If you apply the same input to the primaries of both trans-
formers, you can easily tell if the secondaries are matched, wound incorrectly. or
connected backwards. If you’re nervous about applying full line voltage to measure
the voltages on a transformer, don’t worry-you can drive the primary with a few
volts of signal from a function generator (preferably in series with a resistor and/or a
capacitor, to prevent saturation and overload) and still see what the various windings
are doing.
Two general problems can afflict power transformers. The first occurs when you
have large filter capacitors and a big high-efficiency power transformer. When you
turn the line power switch on, the in-rush current occasionally blows the fuse. You
might install a larger value of fuse, but then you must check to make sure that the
fuse is not too high to offer protection. As an alternative, you could specify the trans-
former to have a little more impedance in the secondary: Use smaller wire for the
windings or put a small resistor in series with the secondary.
Another approach, often used in TV sets, is to install a small negative-TC ther-
mistor in the line power’s path. The thermistor starts out with a nominal impedance.
so the surge currents are finite. But then the thermistor quickly heats up, and its resis-
tance drops to a negligible value. Thus, the efficiency of the circuit is quite good after
a brief interval. If the circuit is a switch-mode power supply, the control IC should
start up in a “soft-start” mode. In this mode, the IC makes sure the switcher won’t
draw any extreme currents in an attempt to charge up the output capacitors too
quickly. However, you must use caution when you apply thermistors for in-rush
current limiting: Beware of removing the input power and then re-applying it before
the thermistors have had a chance to cool. A hot thermistor has low resistance and
will fail to limit the current; thus, you are again likely to blow a fuse-or a rectifier.
The second general problem with a line transformer occurs when you have a small
output filter capacitor. In our old LM3 17 and LM350 data sheets, we used to show
typical applications for battery chargers with just a 10 FF filter. Our premise was that
when the transformer’s secondary voltage dropped every 8 ms, there was no harm in