Current Transformers

Or find a more sensitive current transformer...

"RogerN"

A few turns of mains rated wire around large size reed relay can be the sensor - adjust the position of the reed to get the trip level you want. The relay contacts can then trigger a 555 monostable to drive a 12 volt piezo beeper.

There may be a short beep when the microwave comes on due to inrush surge.

.... Phil

A current transformer is just that - a *current* transformer. In normal operation, it creates a fixed current regardless of the voltage drop required, up to a given maximum voltage. In theory, if you put a bridge rectifier on the signal, you still get the same *current* so you can drop it across a load resistor and still get reasonable outputs.

For example, a 1000:1 current transformer on a 15 amp line gives 15 milliamp output, not millivolts. You need to run that current through a load resistor to generate a voltage.

However, most uses of current transformers use some form of sensitive ADC to measure the signal, so they wouldn't rectify it anyway.

You could, however, use a comparator on an AC signal if you use it to drive a 555 one-shot with a long enough period to stay "on" between cycles. Look up "missing pulse detector".

If you do this, you'll find that you've created a huge oscillator.

Put it on another circuit, (i.e., from a different breaker in the panel) and you'll have no problem. If you'd need to rewire the house to do this, then find another outlet that _is_ on a different circuit, and get a heavy- duty extension cord, preferably one that's the same color as the baseboards.

Good Luck! Rich

I wanted to tell when my water heater was actually pulling current (heating water) since I installed a switch on it to save power and still wanted to take a hot shower now and then.

I had a small power transformer with side by side windings on the split bobbin. I cut the secondary off and wound a coil tapped at 5 turns, 10,15,20,25 turns on it and ran one side of the power line from my 40 watt soldering iron through the taps and rigged two switches with 12 positions - one with 47 ohm resistors and one with 470 ohm resistors in series - giving me 50 load (called "burden" in current sensing speak) resistors from Zero to 2303 ohms and open.

What was the 120 VAC primary, on the transformer is the secondary for my current sensing transformer.

Then I wired a voltmeter and AC ammeter across and in series with the load (respectively) and collected data. The data went into a spread sheet and I found my peak power at ~1250 ohms - but anywhere from 1000 to 2000 ohms was close enough.

I got ~7 volts @ 6 milliamps into 1175 ohms, with the 25 turn winding and my 40 watt load. The 25 turn winding is the highest peak power (naturally) and ~42 milliwatts.

On the water heater it's pulling much more current through a single primary turn and gives about the same voltage out. I use the current it generates to sound a buzzer when the heater turns on or off, and flashes an LED while drawing power. A neon lamp tells me if I have the water heater switched on.

I usually set the heater to the 900 Watt setting, that should give me less than a 10A draw. I can put a time delay in the circuit incase the inrush current is too high. I already use heaters on most of the other area circuits. I use the main heat to get the house close to temperature and the heaters warm the individual rooms a few degrees for even heat and comfort. I guess this is more of a hobbyist way of having a thermostat in each room, except my thermostat is a PID digital temperature controller (thermocouple, soon to be RTD) firing a solid state relay. I know this is overkill but I'm just having fun and learning too, in the summer the temperature control is put to work on my Brinkman electric smoker so I don't have to rely on the water pan to hold temperature.

RogerN

Thanks for the excellent replies from all. The current transformers I saw the spec of 1mV per Amp was the clamp on kind you use with your meter to read higher currents. With a current output and enough voltage available to get through a rectifier, I should be able to get a reasonable voltage output to use for current indication.

As many mentioned, there are better and more practicle ways to solve the problem but one of my goals is to learn about current transformers and I may use this with a comparator circuit and/or I may connect it to the analog card in my PLC. After I learn to use the current transformer I may end up monitoring the current draw for my house or something.

RogerN