CT Sensor help please!

Anyone have success with current transformers and the Netduino? I'm trying to use a small CT sensor (see this SCT-0400-005 and datasheet) and connect to an analog port.

I want to use it to monitor the operation of a 220V pump. I don't need to get any sort of accuracy, I just want to be able to sense when the pump is powered on and when it is off.

I have tried using a voltage divider type circuit from here: OpenEnergyMonitor But I get no change when testing with a 3000W fan heater. I've tried driving from the 3.3 and 5v pins, no different (other than a higher v)

Is there any combination of resistors or easy circuit to get basically 2 readings, lower for off, and higher reading for on, with enough discrimination between each other and the noise? And if not, any other way to sense a motor is running?

Thanks!

You are going to have to put the output through a diode into a resistor/cap constant to get a DC voltage out of the circuit. It might me easyest to create a Timeing based ADC where you groung the cap then let it charge fron the diode into a digital pin. Measering the time it takes to charge will give you the voltage. Im sure there a better explanation on the web somewhere. I think this should work.

Thanks MDS. Do you have any links or an example? From my rather basic understanding, I'm thinking the 5A sensor I've got is not going to work as it only outputs .333v max at 5A. I've just looked at the specs for the pump and it only draws about .17A when running, too low to sense. I may have to think outside the box on this. The fallback plan is to wire the pump through a socket that has a light when powered and use a photo-resistor to sense...

Your Current Transformer will put out an AC signal if you are monitoring an AC device. You need to rectify and filter this to get a DC value proportional to power use. Here’s a simple circuit to do precision rectification, amplification and averaging. http://www.crmagneti...onditioning.pdf

Thanks, I saw that from another thread, but it has no sample values for the components. Have you found this with example values showing the resulting volts?

Hi Emg I did a quick calc, and it looks like you will get 0.33 * 0.17 / 5 = 11 millivolts rms out of you current sensor when your pump is running. By the time you rectify this with a silicon diode, I would imagine you have 5/8 of F all. If it's going to be a permanent fixture you could tap into the circuit somewhere and measure the voltage. With safety being a priority of course. This might be enough for your purposes. The main trouble with the clip on current sensors is the primary being less than one turn. So you dont get much output whatever happens. The other thing that you may or may not realise is that you only put one conductor through the current sensor, otherwise they cancel. ie. leave the neutral and earth out of it. This would involve stripping away the sheath, which is probably illegal in your country as it is in mine. I suggest another approach. Cheers.

Thanks, Magpie, pretty much what I had worked out in the end. I was aware of the phase cancellation but as I was not actually trying to get an accurate reading (basically just a current switch). The plan is to use an inline spur fuse faceplate that has a neon indicator light and use a photo resistor mounted on a blank plate on top of that to sense on/off. Simples. Thanks

I want to use it to monitor the operation of a 220V pump. I don't need to get any sort of accuracy, I just want to be able to sense when the pump is powered on and when it is off.

This might be to simplistic for your needs, but what about mounting a reed switch next to the motor? It should trip when the motor engages.

-dan