I would like emulate the current output of a CT clamp using an Arduino in the simplest way possible. I found the MCP4725 Breakout Board but my understanding of AC current generation is poor and I'd like some advice.
The CT clamp I'm trying to emulate is a "TOP 90-S10/SP4"
The physical clamp would normally measure grid export current for a 240V AC system. And the output would feed into a solar inverter to trigger the export control AKA stop it sending excess power to the local power grid. I want to control this process digitally so need to emulate the clamp's output.
A similar question has been asked previously, but reading that thread, I don't think any of the answers addressed the original question.
I'm comfortable with soldering component level circuits and I have built 20 or so projects around the ESP8266 platform using breadboards, sensors, relays, modbus interfaces etc. After prototyping I've been able to finalise my work on perfboards which I've deployed with success. I've not worked on anything involving DACs and my AC knowledge is currently limited to switching / detecting / house level 240V AC projects.
Software wise I have found the Arduino C, Micro-python and ESPhome platforms to be very intuitive (I come from a network engineering background).
That being said, if there's an off-the-shelf solution I'd be happy to go with that too.
Analog electronics is quite different from digital one. I have no simple idea of generating a constant AC current. Have you already been searching for current source circuits in general and for AC current sources in detail?
These CT clamps are current transformers. They are not sensitive to DC. They have a magnetic core surrounding the high-current high-voltage conductor with (in the case of the one you describe) 1000 turns wound on the core. With 90A flowing in the conductor, if you terminate the secondary with say a 100 ohm resistor you get an alternating voltage of 9v rms. This can easily be rectified to DC and measured to give a current reading on the main conductor. For 9v rms the peak rectified voltage would be 9 x root-2 = 12.7 volts. Or scale as you want - use a 39 ohm resistor to get 5v max.
So basically you need to generate a voltage which in this case can be varied from 0 to 5v to correspond to 0 to 90A. The easiest way is just use a potentiometer and measure the voltage with a DMM. Or use a DAC and drive with a processor.
Yes, it will emulate a CT clamp provided with a 39R load and full-wave rectifier with smoothing. The CT gives an AC output at line frequency and you can't readily deal with that directly using a processor. You can emulate this voltage in may ways, but a DAC is easy if you've got one spare. Obviously the voltage will increase only in 256 finite steps. (The ESP32 DAC may have only a 3.3v rail so its maximum voltage might be limited to that, but still 256 steps.)
If/when you get to building an interface to the CT, for this type of application if you are measuring a high current and therefore get quite a lot of volts out of the CT you could just use a stamdard low-current bridge rectifier. There are other ways - I use a CT in a slightly odd way in a touch sensor that detects AC being diverted through a tool on contact with a workpiece. For that I'm using an op-amp full-wave rectifier because the signal is quite small, but same principle.
CTs are wonderful devices, very simple but very useful, for example in solar power systems but also used widely in other power engineering systems. You can see that you can get quite a lot of voltage and indeed power from a high current cable without making a galvanic contact, and some wireless sensors for solar power or smart meter applications use this to charge a small battery to power the electronics including a Zigbee transmitter.
@jhaine I've enabled the DAC on my ESP32 and connected it to my multi-meter for testing. In DC mode I can now set the DAC output to anywhere between 0v and ~3.2v - this is basically magic to me Do I need to do anything else or is that it?
The Sun has gone down now so I can't test this until tomorrow.
@anon57585045 do you think this will be a requirement? I was kind of hoping that the inverter would just sample the voltages I'm supplying via the DAC.
Do you think testing it is likely to damage anything or just not work?
Starting this project has made me appreciate how useful an oscilloscope would be for electronics development! Seeing the output waveform from the DAC would be so helpful.
Try it. I doubt that will work for the reason that the inverter is designed to monitor an AC signal. As far as damage, any modifications that you make without full knowledge of the system, are obviously risky.
Actually, the mere act of over-riding a power change over feature, might be dangerous unless the entire system is fully understood. Even if you do succeed in "fooling" the sensing input.
How do you know that it's designed to monitor an AC signal? I don't think the OP specified what the inverter sense input is. If he could either link to or provide a copy of the relevant spec we would know. My assumption was that he is building a controller of some sort.
Clearly the CT is designed to monitor an AC signal. What is unclear is just what the OP intends to do with the signal, to what will it be connected. I was maybe assuming here that the intention was to monitor it with an Arduino in some way, after all this is an Arduino forum.
However if what he wants to do is feed an AC current into the inverter that will emulate the CT clamp then we need to know what its input characteristics are, in particular its input impedance, i.e. what terminating resistance will be on the CT output. A link to a spec of the inverter or copy of the manual would be useful. If the terminating resistance is low, say of the order of the 39R I gave as an example, then an AC signal of 5v peak would be needed, or 10v peak-to-peak. This is outside the capability of an Arduino D/A output without an external amplifier considering that 90mA peak current is also needed.
Somewhere I have a link to a site that gives technical details of a system for controlling power to an immersion water heater that may have some useful information. I'll hunt it down tomorrow. It does use Arduino.
Note that the isolated output is floating and that the resistor RL will convert the output current to voltage. Mid-point of your DAC or ADC range should represent zero volts AC. The black wire would connect to a reference voltage that's at this mid-point value. White/black wire is your AC voltage output. RL and CL values need to be determined (see datasheet).
Hi,
In the case of a current transformer, the resistor, RL is very important.
It is referred to as the "Burden Resistor" and needs to be fitted to the output of the transformer to prevent large voltages that can develop at the coil output, as well as provide an output voltage proportional to the mains current.
Do I need to do anything else or is that it?
