Need a cheap and easy way to sense 120-240VAC, and make a charge transformer

[viper]

Working on a project and need to sense when a device is turned on.  The device will be either 120 or 240VAC.  I would rather not directly splice into the wire though. 

obviously simple clamp meters prove this is possible, but I am further wondering if I can find a slick way to extract a small amount of power while the device is running to both sense when the device is running AND charge a small battery! 

Basically wanting to create a small transformer!  I was thinking since there is slack in the wire, I could make a few wraps around a pencil to create a "coil", then install a type of iron core with my secondary coil on the other side?  I am looking for cheap though! 

I was also trying to dream up any other way I could do this in a simpler way?  Less parts and complexity?  Or maybe a device already exists?  I will be working with nothing larger than a single 10ga wire so diameter can be small. 

[Zero999]

What current?

[Doctorandus_P]

Put one of the wires through a toroidal inductor, and you've made a current transformer, with a primary winding of one turn and a secondary winding of the number of turns on that inductor.

current transformers are usually terminated with a low-Ohm resistor, and often have warnings that the load resistors should never be disconnected, as that could lead to high output voltages, so that is ideal for your application.

You will need to have an inductor that works with 50Hz (or 60Hz).

Also, keep a bit of safety in mind. Use the "double isolated" approach by starting with wire that is already double isolated and fit for your mains voltage, and then just use one of the cores of that wire and do not connect the other wire at all. Use a separate piece of wire for the other conductor of the mains voltage and do not put that wire through the inductor core.. When in doubt, add another layer of heat shrink and use decent connectors for anything mains related.

With this you can probably extract enough energy to keep a LIR2032 or similar topped up, but it depends on the other side also. You only have an output when there is current flowing through the inductor, so if the thing you want to monitor is only turned on for brief amounts of time you have less energy for your gadget, and good power management becomes a more important issue.

If the thing you want to check only draws a low current, then use multiple primary windings for the primary winding of your transformer. If the wire you use is not rated for the Amperage of your mains socket, you also need to add a fuse.

[viper]

Thank you!  To add details, we are primarily interested in waking up a sensor monitoring device when we know the device under test is actually running.  So my device would usually need more power when things are running, which is good. 

the devices to be monitored will typically pull 7-10A at either 120VAC or 240VAC, all at 60hz. 

I am used to working with this stuff, but because I have some interest in producing this design, I want to make it easy to install, thus the interest to not have to remove any mains wires.  I have seen separable square iron cores, but never a toroid.  Do they make them?  As far as insulation, the mains wire is already insulated, but I would target a coated core, and use insulated secondary wiring. 

Doctor, I was not clear on your explanation about 'not connecting one side'.  Obviously I would need to connect both sides of the secondary to extract power from it? 

As for charging, technically this device should survive many months without charging and we are looking at either 1 or 2 18650 batts.  It is sort of a variable to determine how much the device will run to get appreciable charging but I might target max reasonable charge current so maybe as much as 3A at 5VDC.  Obviously we can run a higher voltage off the little transformer so 30VAC might be reasonable.  We will need to add a bridge and reg obviously. 

[TheMG]

Is this a device with a normal plug-in mains cord?

Just making sure you're aware that you will need to (physically) separate the live wire from the neutral (or other live wire). If you pass both of them (such as the entire mains cord) through the current transformer, they will cancel out and you'll have zero output.

[viper]

No, this is a hardwired device with separated single conductors.  Even some slack in them.  The main issue I am running into is the inductance needed to generate any meaningful power on the secondary without going nuts on the amount of turns.  I installed a quick CT that I had on a single wire known to be moving 10A and I did not obtain any useful V or I on it. 

[David Hess]

Power can be extracted from a current transformer within the limits of its volt-second rating.  Tektronix did this in their early switching power supplies to power the switching regulator controller.

[boB]

Maybe you could just change the switch on the device you are switching to add another pole to turn on the charger ?

Or put the charger on/off switch next to the device so you can just switch them both at the same time.

Making a current transformer to switch things might be a lot harder than you think ?


PS....   I see you want to "produce" these things.  Well then I guess the question might be how expensive can it be ?