Q&A
with TJ Byers
Electronics Q&A
In this column, I answer
questions about all aspects
of electronics, including
computer hardware,
software, circuits, electronic
theory, troubleshooting, and
anything else of interest to
the hobbyist.
Feel free to participate
with your questions,
comments and suggestions.
You can reach me at:
TJBYERS@aol.com
What's Up:
E
l
e
c
t
r
o
n
i
c
s
NUTS & VOLTS
F
o
r
In-depth look at
relay and transistor
design/selection. For
you valve audiophiles,
a circuit for pure DC
filament voltage. On
the fun side, we have a
garden train throttle
and two sun chasers.
Look for more relay
stuff in reader Mailbag.
Finally, the Fourth of
July is D-day for NASA’s
E
v
e
r
y
t
h
i
n
g
comet Deep Impact.
16
Relay Diodes
Explained
Q. I read your April 2005 column
about EMF suppression. I work
with pinball machines, so I’m familiar
with the diode across the solenoid
coils to prevent high-voltage kickback.
What I fail to understand is where the
current flow is when the field is collapsing. I understand that no current
flows through the diode when the
switch or transistor is closed. But when
the switch opens or the transistor
turns off, why exactly does the current
go through the diode? Where does
that current flow? How does this
prevent it from flowing back towards
the transistor or switch? When I try
and grasp this, all I can think of is the
current going around in a circle!
— Terry Cumming
A. Like a capacitor, an inductor is
an energy storage device. When
you apply voltage across an inductor,
a current starts to flow and slowly
rises to a steady level (actually, an
Figure 1
exponential curve that levels off after
about five time periods expressed as
5 x (t = RL)). The relationship of voltage to current verses time gives rise
to a property called inductance. The
higher the inductance, the longer it
takes for a given voltage to produce a
given current.
The changing current produces an
increasing magnetic field — Figure 1,
which, in turn, stores energy in the
inductor. When the voltage is removed,
current ceases to flow and the magnetic field collapses. This magnetic-field
movement cuts through the windings
of the coil and generates a voltage
across the inductor — with a reverse
polarity to the “charging” voltage. The
magnitude of the voltage is proportional to the rate of the field collapse. The
faster the magnetic lines cut through
the windings, the higher the voltage —
which can reach voltage spikes 10
times that of the operating voltage.
Unless this high voltage is tamed,
it will exceed the voltage rating of the
driving circuitry (transistor, IC,
mechanical switch). A good way to
dissipate this high-voltage energy is to
place a diode across the coil. When
the switch is on, the diode is reverse
biased (doesn’t conduct) and the
relay engages. When the switch is off,
voltage is removed and the field collapses. This forward biases the diode,
which now conducts.
Where does that energy go? It’s
dissipated as heat through the resistance of the coil’s windings at a rate
determined by t = RL (notice the symmetry?). In essence, the current does
go around in a circle. The problem is
that the reverse current flow sustains
the magnetic field, which prevents the
relay from dropping out until all the
energy is spent. Hence, the alternative solutions I published in the April
column.
JULY 2005
