I'm building a circuit to measure AC current using a current transformer, which feeds into a microcontroller's ADC for the actual measurement.
As the measurement needs to be fairly accurate, the first version had a burden resistor straight on the CT's secondary, followed by a precision rectifier/peak detector circuit using op amps. It mostly worked fine, but was expensive, so I looked for ways to replace it with something cheaper.
I found a circuit which goes like this:
Basically, the CT's secondary feeds a bridge rectifier, with the burden resistor after the rectifier.
My understanding of a CT is that the secondary acts like a current source. As such, and given that the burden resistor is the only return path for the current, then whichever current is induced on the secondary windings will be the same current that passes through the burden resistor; since the diodes have voltage drops, then the secondary winding will produce whichever voltage is required to compensate for these diode drops and force the required current through the burden resistor.
Unfortunately, this is not what I verified in practice -- the waveform was severely distorted and attenuated for low currents and much better for nominal currents (although still wrong, as verified by comparing with a current probe on the primary). I turned back to SPICE simulations, not actually expecting to see the phenomena there, but to my surprise, the simulations did show part of the problem as well -- at least the attenuation issue is clear. Simulating the circuit above produces the following output for a 5 A current on the CT's primary:
Green line is primary current, red line is secondary current times turns ratio. As can be seen, secondary current shows a 20% attenuation compared to the primary. Now for a simulation with 25 A on the CT's primary, closer to the nominal current of my circuit:
The error is close to 2% now.
Although I was sure this would make no difference, I tried replaced the silicon diode on the simulation with a Schottky diode. This improved the situation considerably, as can be seen on the following two figures, the first with 5 A and the second with 25 A on the CT's primary:
The error is about 4% for 5 A and less than 1% for 25 A.
Next step is to replace the silicon diodes on the circuit with Schottky diodes and confirm that this shows the same improvements as the simulation. I will update the post when I do this.
Clearly the diode drop needs to be taken into account, but according to my qualitative analysis of the circuit (above), this shouldn't happen. I was half expecting for it to happen in practice because of non-idealities in the CT, but I was startled to see it in simulation as well. I'm looking for insights into where my analysis went wrong, and why the diode drop is still a factor in this circuit.