Page 83 - Troubleshooting Analog Circuits

70 6. Understanding Diodes and Their Problems

factor of four, even though the diodes may have only been forward or reverse biassed
by a millivolt. When we substituted collector-base junctions of transistors for the
diodes, the gain went back up where it belonged.
Thus you cannot safely assume that the impedance of a diode at zero bias is high if
the junction’s saturation current is large. For example, at 25 “C a typical IN914 will
leak 200 to 400 pA even with only 1 mV across it. Therefore, a 1N914 can prove
unsuitable as a clamp or protection diodwven at room temperature-despite
having virtually no voltage biassed across it, in even simple applications such as a
clamp across the inputs of a FET-input op amp.
How can diodes fail? Well, if you were expecting a diode to turn ON and OFF, but
instead it does something unexpected-of the sort I have been mentioning-that
unexpected behavior may not be a failure, but it could sure cause trouble.
Further, you can kill a diode by applying excessive reverse voltage without lim-
iting the current or by feeding it excessive forward current. When a diode fails, it
tends to short out, becoming a small blob of muddy silicon rather than an open cir-
cuit. I did once see a batch of 1N4148s that acted like thermostats and went open-
circuit at 75 “C, but such cases are rare these days.
One of the best ways to kill a diode is to ask it to charge up too big a capacitor
during circuit turn-on. Most rectifiers have maximum ratings for how much current
they can pass, on a repetitive and on a nonrecurring basis. I’ve always been favorably
impressed by the big Motorola (Phoenix, AZ) books with all the curves of safe areas
for forward current as a function of pulse time and repetition rate. These curves aren’t
easy to figure out at first, but after a while they’re fairly handy tools.
Manufacturers can play tricks on you other than changing processes. If you expect
a diode to have its arrow pointing toward the painted band (sometimes called the
cathode by the snobbish) and the manufacturer put the painted band on the wrong
end, your circuit won’t work very well. Fortunately reverse-marked diodes are pretty
rare these days. But just this morning, I heard an engineer call the “pointed” end of
the diode an anode, which led to confusion and destruction. Sigh . . . .
Once I built a precision test box that worked right away and gave exactly the right
readings until I picked up the box to look at some waveforms. Then the leakage test
shifted way off zero. Every time I lifted up the box, the meter gave an indication; I
thought I had designed an altimeter. After some study, I localized the problem to an
FD300 diode, whose body is a clear glass DO-35 package covered with black paint.
This particular diode’s paint had been scratched a little bit, so when I picked up the
test box, the light shone under the fixture and onto the diode. Most of these diodes
didn’t exhibit this behavior; the paint wasn’t scratched on most of them.
To minimize problems such as the ones I have listed, I recommend the following
strategies:
Have each manufacturer’s components specifically qualified for critical applications.
This is usually a full-time job for a components engineer, with help and advice from
the design engineer and consultation with manufacturing engineers.
Establish a good relationship with each manufacturer.

Require that manufacturers notify you when, or preferably before, they make
changes in their products.
Keep an alternate source qualified and running in production whenever possible.

My boss may gripe if I say this too loudly, but it is well known that having two
good sources is better than having one. The argument that “One source is better than