"Jim Wilkins" wrote in message
...
"RogerN" wrote in message
...

I have main propane heat but I also have some of the electric oil
filled radiators. I like the oil filled radiators for cold spots
and to give temperature adjustment to individual rooms.

The problem is that the heaters use up most of the power available
on the circuit. We quit using a heater that was plugged in to the
same circuit as the kitchen, every time we'd run the microwave,
toaster, or other high current appliance, it would kick the breaker
out.

So it seems like a good project to use a current transformer to
measure amp draw on the breaker and automatically turn off the
heater when the amps draw is too high. I don't care much for the
thermostats on the oil filled radiators, it seems to more control
the temperature of the heater, not so much the room temperature.
So I'm thinking an microcontroller, use the analog inputs to read
temperature and amps, and a digital output for a solid state relay
to the heater.

Anyone here mess with signal conditioning for current transformers?
Seems that I need a "precision rectifier" circuit and a capacitor
to charge up to the peak, then scale to RMS amps.

RogerN

I've been monitoring the backup electric heat in my house with a
clamp-on current probe, a TP4000ZC serial-output meter and a laptop.
The Windows datalogging program doesn't include a digital output but
I can read the meter in QBasic under DOS and could control a relay
with an LPT port output bit.

If you are only comparing the CT output to a trigger value the
output doesn't have to be linear.

I'll finish this later. You can't compete with Gwyneth's tango on a
Glee rerun.

John Stamos can sing too.

Anyway, here is a cheap current transformer you might be able to
modify to turn on at a higher current, and detect the LED so you don't
have to worry about open-circuit fault voltage :
http://www.crmagnetics.com/Products/CR2550-P13.aspx

Another approach is to plug the heater into a remote-controlled switch
so you can shut it off -before- turning on the high-current load.

I don't know of a cheap current clamp for a DVM. Mine is an old Fluke
and IIRC came from an auction.

jsw