I have a box with a Variac in it. I want to put in some control circuitry that turns the AC power to the Variac off if the current gets above an adjustable threshold. Thus, I need to measure the AC current.
One method is to use a low side shunt, but I don't like that because it's not insulated from the line. I suppose a small isolation transformer could be used, but I haven't tried that yet (and I don't know how well it would work, as the voltage dropped across the shunt would need to drive some current through the transformer, dropping the shunt voltage).
The method I first decided to focus on was to use a current transformer. I've not used one before; hence my question (I'm a hobbyist, not an EE).
I want to measure AC currents between 0.1 and 10 A. To do this, I put around 49 turns of wire around a small ferrite toroid that was in my junk box (see attached picture). I ran the AC line through the toroid and measured the output voltage with a scope (see attached waveform, which was for a current of 8.6 A(rms)). I was surprised to see this nonlinear waveform when I expected a sine wave (the load was a resistive heater with a power factor of 1).
My question: is this the normal behavior for current transformers? Here, I'm referring to this non-sinusoidal output for a sinusoidal input current.
My suspicion is it's not because I can tell by the peak's phase shifting as a function of current that this is likely due to the shape of the BH curve of the ferrite. From the little I've read, typical current transformers are made with steel toroids, not ferrite.
Second question: would you recommend I just use a commercial current transformer (I found one at Digikey for under $10) instead of trying to make my own?
I like to make my own stuff, so if you think I could e.g. take a stack of steel washers and make a suitable current transformer, lemme know...
BTW, I characterized the 0 to peak voltage as a function of line current (see attached graph) and things are crudely linear from 1 to 10 A or so. Thus, I might be able to use some kind of sample and hold circuit to generate a DC voltage needed to compare with the trip current pot.
One method is to use a low side shunt, but I don't like that because it's not insulated from the line. I suppose a small isolation transformer could be used, but I haven't tried that yet (and I don't know how well it would work, as the voltage dropped across the shunt would need to drive some current through the transformer, dropping the shunt voltage).
The method I first decided to focus on was to use a current transformer. I've not used one before; hence my question (I'm a hobbyist, not an EE).
I want to measure AC currents between 0.1 and 10 A. To do this, I put around 49 turns of wire around a small ferrite toroid that was in my junk box (see attached picture). I ran the AC line through the toroid and measured the output voltage with a scope (see attached waveform, which was for a current of 8.6 A(rms)). I was surprised to see this nonlinear waveform when I expected a sine wave (the load was a resistive heater with a power factor of 1).
My question: is this the normal behavior for current transformers? Here, I'm referring to this non-sinusoidal output for a sinusoidal input current.
My suspicion is it's not because I can tell by the peak's phase shifting as a function of current that this is likely due to the shape of the BH curve of the ferrite. From the little I've read, typical current transformers are made with steel toroids, not ferrite.
Second question: would you recommend I just use a commercial current transformer (I found one at Digikey for under $10) instead of trying to make my own?
I like to make my own stuff, so if you think I could e.g. take a stack of steel washers and make a suitable current transformer, lemme know...
BTW, I characterized the 0 to peak voltage as a function of line current (see attached graph) and things are crudely linear from 1 to 10 A or so. Thus, I might be able to use some kind of sample and hold circuit to generate a DC voltage needed to compare with the trip current pot.
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