On Dec 20, 7:10*pm, wrote:
On Sat, 17 Dec 2011 11:50:16 -0800 (PST), nesesu



wrote:
On Dec 17, 10:59*am, klem kedidelhopper
wrote:
Our area suffers from frequent blackouts, and many people including
myself use generators, (manually) during these periods. It would be
really helpful to know when the utility side of the circuit is once
again live so that I can put the generator away. Basically I need to
monitor the entrance cable side ahead of the main.
I thought about the simplest way which would be to wire a small NE2
neon lamp across the 240, ahead of the main, (with the appropriate
series resistor of course). The lamp would be lit all the time there
is utility supplied power and off during a blackout. The plan is to
not have to uselessly be running my generator after power unbeknown to
me is restored. The down side of this if it is really any concern
would be that this lamp, small as it as well as its associated wiring
would be would not be protected by a breaker.
The lamp is of course the simplest way, but I was wondering if there
is some sort of inductive circuit someone may know of that that would
not require that current be flowing through a conductor, basically a
circuit *that would indicate the presence of voltage.
For instance I carry a *little "pen" shaped device *in my tool box.. I
press and hold an on button and the unit chirps when brought near a
hot AC circuit. It's a great time saver when trouble shooting a job,
but can something like this be implemented as a full time monitor
circuit? It could be powered off small rechargeable batteries and
always indicate the state of the entrance cable. Thanks, Lenny

Lenny, try taking a plain NE-2 with no resistor and connecting about a
foot of insulated #22 solid insulated wire to one lead and wrapping
that around a piece of insulated wire the same size as your service
entrance wire and connecting the other lamp lead to neutral. Energise
the heavy wire with 120V and see if the NE-2 glows just from the
capacitance of the large wire to small wire. That is about as simple
as it gets and is adequately insulated from the service cable to not
need any current protection.

Neil S.

I did some rough calculations with this, and it doesn't look good.

The recommended resistance for an NE2 on a nominal 220V line is 220
Kohm. *You are suggesting using capactive reactance instead of a
resistor. *That is valid for a fixed frequency signal like a power
line.

Using the equation for capacitive reactance, you need 0.01µF - not a
very big capacitor. *I've got a bag of .01µF 1KV ceramic caps sitting
on my desk (for another project I STILL haven't gotten to), they are
about 3/8" in diameter and 1/16" thick.

Now the calculations get a little dicey. *First some big assumptions.
200A service entrance, whiich requires 000 gauge wire, and a couple of
real SWAG guesses of insulation 0.1" thick, said insulation having a
dielectric constant of 1.

000 guage wire has a diameter of .4", so a diameter of 1.25". *That
means caovering a 8" long piece of wire with foil will give a
capacitance of 22pF. *You would need to cover about 300' of wire with
aluminum foil.

PlainBill

So then if you are capacitively coupling to the main it will require
some type of amplification to activate a signal of some kind. My
little "pen" I mentioned earlier blinks and beeps when held near a
live line. I have never opened it but it runs off a small button cell
of some kind. I haven't tried the inductive, (wrapping the probe wire
around one leg of the entrance cable) method yet.

Years ago I designed a little circuit to monitor compressor run time
on a refrigerator. A small coil was wound around one of the wires to
the compressor. That pickup coil, through a step up transformer
introduced sufficient signal into the input of a UA709. The output
turned on a relay driver which ran an elapsed time indicator. I
thought of trying to modify that circuit to include rechargeable
battery back up, modify the pickup, different turns ratio on the
transformer etc. However depending on what was on in the house when
power failed, the total load would always be different. I never try to
run my electric stove, well pump or electric hot water heater from the
generator. However once utility power is restored they would need to
be turned back on. Only one thing would be certain though. After an
extended blackout the chip would be blasted with an overload. So
trying to implement this old circuit doesn't seem feasible. I suppose
I probably ought to look more at the capacitive "pen"approach too.
Does anyone know exactly what is inside that little "pen"? Lenny