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I need a method to control an AC load
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rvalente:
Would this work for you?

The mosfet shorts the bridge, the fast pwm is mixed with the sinusoidal wave.

This configuration acts as a fast dimmer.
jackbob:

--- Quote from: Ian.M on October 22, 2019, 12:48:07 pm ---Isolated, high bandwidth current sensors that can handle very non-sinusoidal waveforms shouldn't *be* expensive if you buy bare chips and mount them on your own PCB.
Output is a voltage that reproduces the current waveform, centered on 1/2 Vcc.

--- End quote ---

The problem with these sensors is that I would like an output voltage proportional to the RMS, not reproduce the current waveform. With these, I would have to sample them extremely fast and compute the RMS with a microcontroller or something similar. This is tough to do for 16 sensors and we are a little strapped for computing power as is just by sampling the 16 analog voltages proportional to the RMS current.

The current sensors I have now appear to be extremely simple devices which I can tell operate with an RC filter based on their response to transients. I believe they have the current transformer rectified, then load a burden resistor and store the peak voltages in a capacitor across that resistor.

I did try controlling the AC waveform with a 1KHz PWM, although I realize the frequency isn't very high, I just wanted to see how the sensor would respond, I varied the duty cycle quite a bit but the sensor output remained relatively constant. I believe this is because of my description of the current sensor above (it stores the peaks of the current waveform in the capacitor voltage). I have a picture of the waveform attached which looks rather neat but doesnt do the job. This was created by feeding the transformer through a full wave bridge and then taking the output and putting it through a mosfet and applying a PWM signal to the gate. Similar to what rvalente drew but with the resistor on the DC side and snubbers added to the FET to handle voltage spikes due to high di/dt through the transformer. The picture below is the current measured through the transformer before the rectifier. It was mentioned by some to add filter caps but i'm not sure where it would make sense to place them. If I place them across the load resistor, then this creates large current spikes to charge the caps once the input voltage passes the voltage on the capacitor (just like a simple linear power supply circuit) these current spikes due to the filter capacitors increase the measured current by the sensor significantly as the sensor stores the peak currents.

jackbob:
Also when operating a FET as a resistor, it seems to chop off the top of the sinusoid current waveform (see the picture below). Am I misunderstanding this mode of operation? I simply applied a variable voltage to the gate with a power supply like the simple schematic shown below and it doesn't seem to behave linear like a resistor.
jackbob:
I figured out the answer to my last question. I am able to use the FET as a linear resistor and it does behave quite linear if the voltage to the gate is also sinusoidal with the source voltage. I used a simple constant resistance circuit with an op amp and feedback from the source going to the inverting input and the non inverting input from a potentiometer with one side ground and the other side the rectified sineusoid. The outcome is exactly what I was looking for, it regulates the load quite linearly (with the exception of some small zero cross distortion). The FET is acting as a resistor so it does dissipate a bit of power BUT its a very simple solution.

If I had more time for the project I would dig deeper into a more power efficient solution and after looking into the back to back configured FETs that topology is one I find quite useful and it will definitely stick in the back of my mind for future projects. I just know floating gate drives opens a whole new can of worms!

Thanks for all the help folks!!! This community is invaluable!
Zero999:

--- Quote from: jackbob on October 23, 2019, 03:57:52 am ---The problem with these sensors is that I would like an output voltage proportional to the RMS, not reproduce the current waveform. With these, I would have to sample them extremely fast and compute the RMS with a microcontroller or something similar. This is tough to do for 16 sensors and we are a little strapped for computing power as is just by sampling the 16 analog voltages proportional to the RMS current.
--- End quote ---
You might not need to sample as fast as you think. It's possible to measure and compute the RMS voltage or current by sampling at a lower frequency than the waveform. It depends on how often you need to read the RMS value. For example if you sampled the mains voltage at 1Hz, over a few minutes you could quite accurately calculate the RMS voltage, as long as the sampling occurs at different parts of the mains cycle, which can be ensured by introducing some jitter into the sampling.


--- Quote from: jackbob on October 23, 2019, 06:10:41 am ---I used a simple constant resistance circuit with an op amp and feedback from the source going to the inverting input and the non inverting input from a potentiometer with one side ground and the other side the rectified sineusoid. The outcome is exactly what I was looking for, it regulates the load quite linearly (with the exception of some small zero cross distortion). The FET is acting as a resistor so it does dissipate a bit of power BUT its a very simple solution.
--- End quote ---
Being picky but that sounds like a constant current source circuit, rather than constant resistance. Please post the schematic.

I'm glad you got it to do what you want.

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