seven_segment
Member level 2
Hi everyone. You might remember a couple of weeks back I made a post about converting the output voltage of a current transformer to DC. In the end I decided to go for the AD736 from analogue devices.
I've breadboarded my circuit and wired the chip to give true RMS - DC conversion, so theoretically 100mV AC in would give 100mV DC out. The current transformer is also linked into the circuit and I've chosen a burden resistor that will give me 200mV @ 5A for testing purposes, this is the maximum input voltage on the AD736 without an attenuator.
With a multimeter on the secondary output of the current transformer and another on the DC side of the RMS-DC converter (and also an ammeter in series with the mains load), I seem to get a true conversion whenever I run a resistive load through the primary of my current transformer. The loads were a 100w lamp and also a 150w floodlight. I get exactly the same voltage reading on both meters, and the ratio between current on the primary of the CT and the readings on the meters seems correct.
However, when I put an inductive load into the circuit (in this case a desk fan) this all seems to go completely out the window. For example, I measured 11.6mV AC directly on the secondary of the CT, but I'm getting 13.0mV DC out of the AD736. The ratio between the current in the primary and the readings on the meters also seems to be a little out, the problem seems to be worse with larger loads.
My first thought is that perhaps this is something to do with power factor - has anyone got any thoughts on this? I really need this circuit to be as accurate as possible, as the output is going to be amplified 25 times and then fed into the A-D on a microcontroller, any small error could take the reading a long way off. As usual, all comments much appreciated!
I've breadboarded my circuit and wired the chip to give true RMS - DC conversion, so theoretically 100mV AC in would give 100mV DC out. The current transformer is also linked into the circuit and I've chosen a burden resistor that will give me 200mV @ 5A for testing purposes, this is the maximum input voltage on the AD736 without an attenuator.
With a multimeter on the secondary output of the current transformer and another on the DC side of the RMS-DC converter (and also an ammeter in series with the mains load), I seem to get a true conversion whenever I run a resistive load through the primary of my current transformer. The loads were a 100w lamp and also a 150w floodlight. I get exactly the same voltage reading on both meters, and the ratio between current on the primary of the CT and the readings on the meters seems correct.
However, when I put an inductive load into the circuit (in this case a desk fan) this all seems to go completely out the window. For example, I measured 11.6mV AC directly on the secondary of the CT, but I'm getting 13.0mV DC out of the AD736. The ratio between the current in the primary and the readings on the meters also seems to be a little out, the problem seems to be worse with larger loads.
My first thought is that perhaps this is something to do with power factor - has anyone got any thoughts on this? I really need this circuit to be as accurate as possible, as the output is going to be amplified 25 times and then fed into the A-D on a microcontroller, any small error could take the reading a long way off. As usual, all comments much appreciated!
