Just For Starters
+ 5 V
forward-biased and allows the
coil's stored energy to
handle the small currents involved in
Safety Above All Else
Sensing an AC
circuit with a protection diode. While
the IC output may be limited to a few
milliamps, the transistor can be
selected for higher currents.
Still, very high current
requirements are rare. A 10-amp
relay's coil may require 20 mA at 12
V, which is well within the range of
small signal transistors.
A protection diode dissipates the
coil's energy when the transistor turns
off. Coils, or inductors, resist changes
in current flow. The coil voltage
reverses in an attempt to maintain
current flow when the transistor turns
off. The diode is placed so that it is
reverse-biased under steady-state conditions: the anode is at a
lower voltage than the cathode.
However, when the coil voltage
abruptly changes, the diode becomes
The relay can be turned
around so the control circuit
can sense the presence of AC
n However, provisions must
be made for driving the
coil with an AC signal. One
solution is using an AC relay.
Another solution is rectifying and
filtering the AC signal so that it can
drive the DC coil. The most practical
solution for many situations is replacing the relay with an optoisolator and
a rectifier. The optoisolator provides
electrical isolation and can certainly
e 5:FTigraunresis5to. rTrbaansseidstroerl-abyac so eidl drr eilvayercwoiitlhdprirvoetrectio
with protection diode.
It should go without saying
that safety is the primary design
issue, especially when interfacing
requirements lead to manipulating
potentially dangerous currents and
voltages. Isolation alone does not
guarantee a safe product, regardless
of the interfacing scheme in use. All
components and wires should be
conservatively rated for power,
current, and voltage.
Don't be shy about seeking
advice from others who have
experience solving these types of
problems. You can feel confident in
the result when you take the time to
build safely. NV
About the Author
Mark Balch is the author of
Complete Digital Design (see www.
works in the Silicon Valley high-tech
industry. His responsibilities have included PCB, FPGA, and ASIC design. Mark
has designed products in the fields of
telecommunications, HDTV, consumer
electronics, and industrial computers.
In addition to his work in product
design, Mark has actively participated
in industry standards committees and
has presented work at technical
conferences. Mark holds a bachelor's
degree in electrical engineering from
The Cooper Union in New York City.
He can be reached via Email at
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