Page 51 - Troubleshooting Analog Circuits

38 3. Getting Down to the Component Level

letting the regulator saturate. That premise was correct, but we began to see occa-
sional failed regulators that blew up when we turned the power on.
After extensive investigations, we found the problem in the transformer: If the line
power switch was turned off at exactly the wrong time of the cycle, the flux in the
transformer’s steel core could be stored at a high level. Then, if the line power switch
was reconnected at exactly the wrong time in the cycle, the flux in the transformer
would continue to build up until the transformer saturated and produced a voltage
spike of 70-90 V on its secondary. This spike was enough to damage and destroy the
regulator. The solution was to install a filter capacitor of at least lo00 pF, instead of
just 10 pF. This change cut the failure rate from about 0.25% to near zero.
Another problem occurred when the LM3 17 was used as a battery charger. When
the charger output was shorted to ground, the LM3 17 started drawing a lot of current.
But, the transformer’s inductance kept supplying more and more current until the
LM3 17 went into current limit and could not draw any more current. At this point,
the transformer’s secondary voltage popped up to a very high voltage and destroyed
the LM317. The addition of the lo00 pF snubber also solved this problem.

Inductors, Like Resistors, Can Overheat
How do you spot a bad inductor or transformer? I have already discussed several
mechanisms that can cause the inductance or Q of an inductor to be inferior to that of
a normal part. And, as with a resistor, you can smell an inductor that is severely over-
heating. Overheating can be caused by a faulty core, a shorted turn, incorrect wire
gauge, or anything else that causes losses to increase. An open winding is easy to
spot with an ohmmeter, as is a short from a primary to a secondary. If the pattern of
winding has been changed from one transformer to another, you may not see it unless
you test the components in a circuit that approximates the actual application.
However, you may also be able to see such a discrepancy if you apply a fast pulse to
the two transformers. Changes in winding pattern-ven clockwise vs. counter-
clockwise-have been observed to cause significant changes in transformer perfor-
mance and reliability.
Tightly-coupled windings, both bifilar and twisted pairs, have much better mag-
netic coupling and less leakage inductance than do well-separated primary and sec-
ondary windings. As the magnetic coupling improves, the capacitance between wind-
ings increases-but high capacitance between windings is often an undesirable effect
in a transformer. An experienced transformer designer weighs all the tradeoffs and
knows many design tricks-for example, the use of special pi windings and Litz
wire. Mostly, you should know that these special techniques are powerful; if you ask
the transformer designers the right questions, they can do amazing tricks.
I recently read about an engineer who designed an elegant shield made of mu-
metal. However, the shield was difficult to install, so the technician had to tap on it
with a hammer. When the engineer operated the circuit, the shielding seemed nonex-
istent-as if the shield were made of cardboard. After a lot of studying, the engineer
realized that the mu-metal-which costs about $2 per 15 square inches, the same as a
$2 bill-had been turned into perfectly worthless material by the pounding and ham-
mering. In retrospect, the engineer had to admit that the mu-metal, when purchased,
was prominently labelled with a caution against folding, bending, or hammering. So
remember, in any area of electronics, there are problems with inductors and magnetic
materials that can give you gray hair.