READER-TO-READER TECHFORUM
than a silicon photodiode as you stated. CdS sensors are generally used in
consumer products since they are
cost-effective and their relatively slow
response time (compared to silicon) is
acceptable. CdS sensors are light-dependent resistors and can be identified by a serpentine zig-zag sensor
pattern. They have a high resistance
(in the megohm region) in the dark
and a much lower resistance (less than
10K) when illuminated. They are available in various sizes to provide a selection of sensitivity and dynamic range.
You can prevent your clock from
going silent at night by simply bridging
a resistor (try a value between 1K and
10K) across the CdS sensor in your
clock. You could even put a switch in
series with the resistor so that the
clock could operate either normally or
in a modified nighttime mode.
Bob Kovacs
Barnegat, NJ
[#6105 - June 2010]
Vacuum Cleaner Soft Start
I have a Hoover EmPower vacuum
cleaner that causes the breakers in my
home to trip almost every time I first
power it on. After a few minutes of use,
I can turn the machine off and back
on with no problem. I am thinking
that the initial inrush current is too
much for the breakers (they are GFCI
protected). Does anyone know of
some sort of soft start circuit or any
other trick that may help?
#1 The real problem is that the
bearings in the vacuum cleaner motor
are seizing, or that the lubrication is
dried out. This makes the motor draw
more (it's stalled) when starting cold.
When the motor heats up, it spins
more freely and draws less current.
The motor needs repair. This may be
as simple as taking it (the motor) apart,
cleaning out the old dried oil and dirt,
relubricating it, and reassembling. You
might need to drill out some rivets to
take it apart and replace with screws
when you put it back together. I don't
think a soft-start circuit will solve this
problem.
80
Len S.
Los Altos, CA
October 2010
#2 I doubt it's the inrush current,
but a faulty GFCI. Older GFCIs were
prone to tripping with motor loads
and/or motors with brushes. I'd
suggest replacing the GFCI breaker
with a newer one. The specs do list
1,440 watts which does translate to
12A, so I would be attentive to what
else is on the circuit.
A metal oxide resistor combined
with a delay on make timer such as
those made by www.ssac.com would
work as a current limiter. The delay on
make relay shorts the resistor out after
a specified time delay. Unfortunately,
the circuit would have to be at the
vacuum cleaner. If the circuit worked,
I'd almost be tempted to use a remote
AC switch and place the device at the
outlet. One example is the remote
controls used for woodshop vacuums.
I've used the technique before and
had to experiment with resistor values
and time.
Ron Dozier
Wilmington, DE
#3 There are at least two problems
that can cause your circuit breakers to
trip when using the vacuum. The first
may be a worn out circuit breaker. As
circuit breakers age, their springs
weaken which causes them to trip on
less than their rated current. Frequent
tripping can accelerate the aging
process. If that's the case, all you need
to do is replace them with new breakers. Another possibility is a dusty
motor. If you have GFCIs, they will trip
on leakage current. Dust is hygroscopic which means it will absorb water
vapor from the air. That slightly moist
dust will leak current which can trip
your GFCI. After the motor has run a
bit, it warms up and drives off the
moisture. The leakage current goes
down and the GFCI won't trip. You
didn't state what was tripping — the
overcurrent part or the GFCI part.
Either could be the culprit, and the fix
should be fairly easy. I do not believe
it is from too much current draw. The
vacuum is rated for 12 amps which
means that it should be okay on a
standard 15 or 20 amp circuit. Motors
do draw more on startup, but consider that this is a widely-used consumer
product and a Google search shows
nobody else having a problem with it.
#4 Ground Fault Interrupt (GFI)
breakers are only installed in kitchen,
bath, and outdoor outlets. Assuming
that you are plugged into a bedroom
or living room outlet, the GFI breaker
is not the source of your problem. Do
you have a relatively new house with
arc fault breakers? The arc fault
detecting breaker is known to
sometimes trip on brushed motors
such as used in vacuum cleaners. Try
the vacuum cleaner on the outlet serving your refrigerator. It will not have an
arc fault breaker because the reliable
operation of the refrigerator trumps
safety considerations. If the vacuum
cleaner does not trip the refrigerator
outlet, it is not the starting surge current; you need to plug into something
other than an arc fault breaker.
Dennis Crunkilton
via email
[#6104 - June 2010]
Driveway Sensor
I want to set up a home driveway
sensor similar to a traffic light actuator
by using a single wire loop in the
pavement that senses vehicles
by changing the frequency of an
oscillator. I need to know how it works
and how to build it.
The unit works very much like a
large metal detector, although the
traffic signal units have more features.
I have access to many of these
units if you have an interest. The traffic
units have the ability to pulse (pulse
the relay if a vehicle passes over or sits
on the loop) a built-in relay, or they
can place a constant signal to the relay
(if a vehicle is sitting on the loop).
Some have the ability to delay or
extend the detected vehicle. Your loop
will have to be about 4-6 turns of 14
Ga. wire in a 6' x 6’ or smaller area.
The detecter also has frequency and
sensitivity, and will let you know of an
out of range or faulty loop.
Ray
via email
