Prev: opposite of an RF Modulator?
Next: Followup re: USB malfunction ? / Ezbus digital mixer /Ez-usb USB chip
From: mike on 20 May 2010 17:44
Geir Holmavatn wrote:
> Hi,
>
> I'm looking for non-contact current sensors for 0-60/70A AC connected to
> some digital interface (1-wire, GPIB, RS422, ethernet etc). The
> measured cable should run through a hole in the sensor and not be
> galvanically connected.
>
> I need to monitor and register AC current with a computer from different
> appliances...
>
> Thanks for comments and tips on where I can find such sensors..
>
> regards Geir

You don't say exactly what you're trying to accomplish,
so this may be tangent to your requirements....but I'll try
not to let that stop me.

I set out to instrument my home for the purpose of
energy conservation. I intended to use a current probe
and PIC processor to acquire the current data and
send it via RS232 either directly or via RS232-Bluetooth
adapter.

I started with Mike's metrology rule #1:
Never ask a question unless the answer will modify
your future behavior.

And a corollary: Real-time data without real-time
control is overkill. All it gives you is worry.

I went around the house with a RMS power meter measuring
everything that would unplug.
The bad news is that a bunch of things like a wireless
telephone at 5W each really add up.
The worse news is that I'm unwilling to give them up.
I did unplug infrequently used vampire devices.

Since most things are either on or off. If you know
the consumption, you don't have to measure it in real-time.

A Kill-A-Watt device for under $20 will measure and log
data for you. But you have to interrupt the circuit.

Moving on to the harder devices, I started with the
biggest energy consumers.

First stop was the furnace. It's gas. Hmmm.
I determined from watching the gas meter go around
that the 60KBTU/HR rating was pretty accurate. So, it
was sufficient to measure the on-time. For that,
I used a hunk of cardboard on a microswitch that
opens when the furnace fan comes on placed over a vent.
I used an old Palm III to plot the on-time. Calculate
the gas cost plus the fan cost determined using a
hand-held current clamp meter completes the calculation.

For most things, they're either on or off. If you use
'em less, you save. Knowing exaclty how much you're saving
TODAY isn't useful. If your furnace thermostat is already
as low as you can tolerate, you've done all you can.
And you didn't need a single measurement to reach that
conclusion.

Next, I tackled the electric water heater. The wires
are accessible, so I used a clamp-on probe from an old
Triplett analog meter. It's much better than more modern
sensors because it puts out 500mV/Amp.
Plugged that into a Radio-shack
DVM with RS232 output and used a laptop to log the result.
Now, I know how much it costs to take a hot shower or
wash a load of clothes. The thermostat is already as low
as I can tolerate. Not much use for additional
measurements. The plan to use a PIC to measure the sensor
output and transmit it via RS232 or bluetooth has been aborted.

Kitchen appliances weren't worth measuring. I'm not
gonna stop reheating my coffee just because I have
real-time data on the microwave. See Rule #1 above.
I did buy an energy-efficient fridge.

The air conditioner wiring wasn't accessible, so I
took an alternative approach.
Our neighborhood just got those newfangled digital
power meters from the power company. Mine puts out
an infrared pulse for every watt-hour consumed.

WalMart sells individual cake slices in a plastic
box that is a nice friction-fit over the power
meter cylinder. It's just the right size to put
in a PDA. The handspring visor variant of the
Palm III works better because the IR sensor is
on the side.

A little bit of code and a lot of hair-pulling
later, I have a program that graphs and logs
power consumption. And since it measures
exactly what the power company charges for, I
don't have to worry about power factor, etc.

If you control what else comes on at the same
time, you can determine the "signature" of various
major power consumers. That done, you can
usually tell what's going on/off from the log.

The PDA cake-boxed onto the power meter can do anything
I need to monitor electricity consumption.

Now that I have the data and have modified
my behavior, the measurements stuff in in a
drawer. Refinement plans have been aborted.
I'm really glad I didn't invest a lot of $$$
in permanent measurement and control facilities.

Rule #1 rules...

YMMV.



First  |  Prev  | 
Pages: 1 2
Prev: opposite of an RF Modulator?
Next: Followup re: USB malfunction ? / Ezbus digital mixer /Ez-usb USB chip