by Mark Balch
Just For Starters
Basics For Beginners
Just For Starters
Small oversights can get you into
trouble when you’re working
with electronics. One factor to
consider in every project — big or
small — is ensuring that components
do not overheat. Resistors, transistors,
and other parts can get dangerously
hot, even when you are not dealing
with high voltage or lots of power.
Basic thermal management is not
difficult for most projects. Investing
a little thought early on can keep
you safe and save a great deal of
reworking down the road.
The first step in thermal management consists of evaluating your
circuit for potential trouble spots. Look
for components that regulate the
power supply — like voltage regulators,
Zener diodes, or switching transistors.
Power regulation circuits are common
sources of heat because they pass the
full current required by the rest of
the system. Simple linear regulators
dissipate a quantity of power equal to
the product of supply current and
input/output voltage differential.
A 5 volt circuit that draws just half
an ampere from a 12 volt wall transformer will require a linear regulator
to dissipate 0.5 A x 7 V = 3. 5 watts;
3. 5 watts dissipated in a confined
space with inadequate precautions
can quickly lead to a meltdown or
worse. It is important to understand
how much current your circuit draws
from each voltage rail and what the
power dissipation impact will be on
the voltage regulators in your system.
Other parts of a circuit are candidates for heat trouble. Are you switching
a load on and off? You may drive a
light display. You may drive relays or
solenoids to open doors or close
valves. The switching components —
which often include transistors,
diodes, and resistors — can fall victim
to heat-induced failure without proper
design. High performance digital integrated circuits can run very hot.
Running a state-of-the-art embedded
controller at an internal clock frequency of 100 MHz or more can result in
substantial heat dissipation. The high
density of modern integrated circuits
makes them ideal candidates for shedding lots of power in a small volume.
Thermal management — or cooling — seeks to spread power over
a larger volume to reduce the
maximum temperature in any one
location. Cooling does not prevent
your circuit from generating heat;
cooling simply redistributes that thermal energy to prevent overheating —
just like your car’s radiator disperses
the engine’s heat.
Once you have identified a poten-
tially troublesome heat source, the
next step is determining that component’s operating conditions. What is
the maximum power it will dissipate?
What is the maximum ambient air
temperature that the system will operate in? You must consider the worst
case conditions across all variables
for a reliable solution. Finally, what is
the component’s thermal resistance
to ambient air? This is often a troublesome variable, but one that you have
a degree of control over.
Thermal resistance defines the
ease with which heat is conducted
from a source to the ambient environment. A higher thermal resistance
results in more heat build-up. Thermal
resistance is often designated by the
Greek letter theta — θ — and relates
power and heat: DT = θP. θ is
expressed in units of °C/W. The heat
relationship tells us that a temperature rise results from a quantity of
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