I have a SCT-013-000 100A Non-invasive AC Current Sensor Split-Core Clamp Current Transformer that I want to connect to the Raspberry Pi 3b+. I am thinking of using an A-to-D converter (ADC 3008) as the interface, but wondering if a relay might work; i.e. with a 3.3v source turned on/off based on transformer opening or closing the relay?

Does anyone have any other suggestions?

Note that I do not need to know the value of the power current, but only if it is ON or OFF.

  • So, all you need to know is whether there is greater-than 0.00mA of current in the clamp? – stevieb Oct 28 '18 at 20:38

Your question omits some specifics. Consequently, this answer may be insufficiently specific, but I'll make a guess or two, and try to give you some ideas to begin with. If you'll edit your question and add some details (e.g. what is it you're trying to do?), then I'll try to edit my answer to be more informative.

Your SCT-013-000 sensor produces a "constant current" output proportional to the current flowing through the coil. But it's an ac current, meaning that it will flow alternately in one direction, then the other, in sync with the ac current that you are measuring. That is, if your measured ac current is 50 Hz, then your sensor output will also be at 50 Hz.

One thing you should understand about a current source: It tries to supply a certain current, and the voltage will vary according to Ohm's law (E = I*R). In other words, if you place a high impedance load across the output of a current source, the voltage across that load will satisfy Ohm's Law, which could be a very high voltage - even a lethally high voltage. Your sensor appears to have a voltage limiting device built in, but I wouldn't depend on this to keep me (or my equipment) safe. Be careful.

As far as using an ADC: This seems like a bad idea based on the fact that you've described your application as "Binary ON/OFF..." in the subject. An ADC would be appropriate if you were trying to measure the amount of current flowing through your sensor instead of an "ON or OFF" state.

As far as using a relay: Again, your objective is not clearly stated, so it's difficult to give you a detailed answer. However, as you mentioned "3.3V" which is the I/O voltage of RPi's GPIO pins, I will hazard a guess that you want to interface your current sensor with your RPi. IF that's the case, you could use a relay, but you might need a rather specialized relay with an ac coil... or a rectifier to convert your ac sensor output to DC, and perhaps a clever interface circuit design to ensure you don't over-drive the relay. This is all "do-able", but it may not be the best or easiest way to do it.

Instead, consider using a 3.3V Zener diode across the output of your current sensor, and then tying that to a GPIO pin as shown in the (grossly simplified) schematic below:


simulate this circuit – Schematic created using CircuitLab

Please note that this is not a "ready to build" schematic! I've included it only to show a design concept. I'll add the details once you've edited your question with some details.

Hope that helps.

  • Seamus, I have been playing with this project for several months. The project is to detect and alarm if a 220v (120v/two phase, 50am) power cord for a boat loses power. The cord powers a Charles ISO Boost transformer that then feeds two inverters. I want to put the Current Transformer around one leg of the power cord and then either use an Arduino or Raspberry Pi with ADC to send a SMS/email message. I have been looking at a MOSFET to work as a switch but so far have smoked two....ops. – HenryD Jul 12 at 2:13
  • Following up...trying to determine the specs needed for a Zener Diode – HenryD Aug 8 at 5:13
  • @HenryD: You know the Zener voltage must be less than or equal to 3.3 V to protect the GPIO. The current rating of the Zener will have to match the output of your current sensor, as modified by the "TBD" interface circuitry. – Seamus Aug 11 at 10:01

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