Q&A
■ WITH RUSSELL KINCAID
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,
or suggestions.Send all questions and comments to:
Q&A@nutsvolts.com
WHAT’S UP:
Join us as we delve into the
basics of electronics as applied
to every day problems, like:
CAT 5 Cable Tester Revisited ✓
● 400 Hz Gyro Drive
LED Chaser Circuit
✓
✓
A CURRENT INDICATOR
CIRCUIT
CAT 5 CABLE TESTER
QI have a new spa that has an automatic maintenance cycle; it runs every 12 hours. It is powered from a
208V three-wire 30 amp circuit. I
would like a simple LED indicator in
the house to tell me it is running or
that I left it running after using it. I
was thinking in terms of something
starting with a small toroid with a few
turns on it to slip over the red or
black hot lead with a completely
floating sensor circuit running off a
couple of AA cells. Suggestions
would be appreciated.
— Bill Hosking
QI do quite a bit of CAT-5/6 cable installations for telephone and data networks. Although I teach
my crew to be very careful when
attaching a RJ- 45 plug or jack,
invariably we end up with a few
cables with crossed, open, or shorted
circuits.
There are quite a number of
LAN/telephone circuit testers on the
market which are able to identify
which pair or wire is shorted,
crossed, or open, but the biggest
problem and time waster is knowing
which “end” needs to be fixed. One
of Murphy’s Laws states that we will
spend 20 minutes going to and
inspecting the wrong end. Sometimes
it’s so difficult to ID the conductor
colors in a plug that we end up
cutting off and reconnecting both
ends. Sometimes the mistake even
gets repeated.
No one seems to make a tester
that can ID which end of the cable
needs fixing. It seems to me that a
circuit tester with a little more
processing power and smarts could
also determine if, say, wire 1 is on pin
Yes, there are
considerations for “
cross-over” and data “A”/“B”
circuit connections, but most
good technicians know how
to keep that in perspective
for the type of circuit being
connected and tested.
AI believe that you will find the spa draws about 10 amps when running; more at startup. All that is
needed is a current transformer and
back-to-back LEDs for a load. The
LED shown in the schematic in
Figure 1 is a dual red/green which
will probably look yellow because of
the color mixing. The part numbers
are Mouser; current transformer:
$6.11, LED: $0.20.
■ FIGURE 1
AAs I said in January, a time delay reflectometer is needed. I won’t have time to do a complete design,
but I will show a partial schematic
and explanation of how it should
work; see Figure 2.
The CAT5 cable is specified to
be no more than 100 meters long
and the nominal time delay is 5 nS
per meter, so the round trip time
delay is about one microsecond,
worst case. When a pulse is applied
to the properly terminated line, it
travels down the line and is absorbed
by the load; there is no reflection. If
the line is open circuit, the current
has no place to go and produces a
high voltage at the end. As the
voltage collapses, a current is sent
back to the transmitter. If the transmit
end is properly terminated, the pulse
is reproduced minus losses.
It is convenient to have a short
pulse otherwise it is necessary to
differentiate between the normal
pulse height and the extra high pulse
that occurs when the reflection
comes back. With a short pulse ( 10
or 20 nS), it is only necessary to see
if there is a pulse after the main pulse
(for an open circuit).
22
February 2011
