Issue
142 May2002
You're
Not Alone
Dealing
with Isolation
by
Jeff Bachiochi
Jeff hasn’t turned into a self-help writer, but
you can help yourself by following along as he outlines
the challenges and effects of isolation. Knowing what
the options are will help keep your next project on
track.
Start
•
Magnesolation • Power
Isolation • Ground Loops
• Sources
& PDF
When
I think of isolation, I think of Robinson Crusoe and
Friday or in more recent times, Tom Hanks as Chuck Noland.
When I think isolation in the electronics context, I
think of the transformer. For years, transformers have
been used to isolate signals. Magnetic properties transfer
information without an electrical connection between
primary and secondary windings. On most transformers,
it’s the plastic bobbin that holds the primary and secondary
windings separated that creates the isolation barrier.
More recently, opto devices have become a popular method
of obtaining isolation.
As
you can see in Figure 1, opto devices rely on light
(or the absence of light) to transfer information between
the physically separated primary (input) and secondary
(output). Although the transformer is inherently a bidirectional
isolation device, most isolation application requirements
are unidirectional.
|
(Click here to enlarge)
|
Figure
1—Optoisolators rely on physical separation between
the LED transmitter and photo detector for isolation.
Data is passed via photons (or the lack of). |
Lightsolation
Opto
devices have the advantage of operating down to DC whereas
transformers are specified at some AC bandwidth. From
an isolated control point of view, transformers can’t
be used directly for steady-state isolation. Opto devices
work well in this scenario. An opto device does require
a chunk of current to operate and the transfer ratio
of input current to output current will usually require
a trade-off of speed for drive current. This trade-off
is not a problem until you want to transfer information
at high speeds. Most opto devices have minimum TON/TOFF
times in the microsecond range.
A
new device in the isolation flurry takes a step back
into the magnetic domain. NVE Sensor Engineering uses
giant magneto resistive (GMR) sensors to transfer information
using the magnetic fields produced by current in a conductor.
