by Bryan Bergeron, Editor by Bryan Bergeron, Editor
Designing For The
On a recent business trip to the Middle East, I had the pleasure of spending a few nights in the desert
with a camel herder. Base camp consisted of a
makeshift trailer with an old generator a few dozen
yards away for power and lights. In the quiet of the
desert, the generator was piercing. We voted to go
without the floodlights to enjoy the silence.
The solar panels simply overcharged the batteries.
The designers of the solar power system apparently
hadn’t considered the best-case scenario of days filled
with blue skies and intense, uninterrupted, sunlight for
weeks on end. They were probably more concerned
with performance on overcast days and, at best, a day
or two per week of uninterrupted sunshine. This
oversight in design got me thinking about electronic
design in general.
How often do you design for the best-case
scenario? I have to admit that I normally design for the
typical scenario with an eye to the worst case. For
example, in RF links, I don’t assume the transmitter and
receiver are in immediate proximity but may be at or
just past maximum range. When it comes to switching
power supplies, I’m constantly trying to squeeze the last
bit of energy out of a battery before the supply shuts
Design for the best-case scenario is fundamentally
different from over-engineering in which the goal is to
produce a more robust circuit or system. Examples of
over-engineering include using a transistor heatsink
significantly larger than necessary to keep the junction
temperature below maximum rated temperature, and
building a power supply with diodes rated at four or
five times the PIV expected.
Over-engineering requires knowledge of what’s
expected at the extremes of circuit operation, as well as
deep pockets. The return is often increased reliability
and enhanced performance.
Designing for the best-case scenario seems
fundamentally different from and at odds with over-engineering. Take a wind generator. Designing for the
worst-case scenario (as I see it) means designing the
wind generator so that it requires virtually no wind. In
contrast, a wind generator design that considers the
best-case scenario would have provisions for operating
in excessively high winds.
Do you intentionally over-design your circuits, or do
you go for the cheapest solution that should work? Cost
is an important, unavoidable consideration in today’s
economy, especially if you’re building a circuit for
educational purposes. There usually isn’t much at stake
if a resistor or transistor in one of your projects
overheats and has to be replaced. That’s how you
develop an intuitive feel for what you can get away with
in terms of power and voltage ratings.
The take-home message is to consider the best-case
scenario when you’re designing your next circuit. You
don’t want your circuit to fail because of too much of a
good thing. NV