I'm trying to read AC current w/ a differential ADC connected to my embedded processor (via I2C), and I'm having trouble.
Here is an image of my setup: http://imageshack.us/photo/my-images/59/currenttransformer.png/
I'm connecting a 3000 turn current transformer around one leg of my AC power.
I placed a 220 Ohm burden resistor across the leads of the current transformer.
When I measure that voltage w/ my multimeter (Fluke 177), I get a RMS reading of around 150 mV, which seems to coincide w/ my 2.0 amp Killawatt reading of the source AC circuit.
I then connect the burden resistor across my CH1- and CH1+ pins of my MCP3424 differential ADC. I sample the ADC repeatedly, compute the RMS, and I get values consistently around 90 mV RMS.
Is this hooked up correctly?
Here is what I've tried so far:
- determining if my ADC is fast enough (it's just 240 SPS). I've done a bit a math, and read a lot of forums online, and it sounds like it should be fast enough. It might not be perfect for this application, but I haven't determined that it's speed is to blame.
- grounded one ADC input (hardware friend recommended. It didn't help.)
- added a per sample fudge factor. If the ADC was averaging its sample, instead of sampling an individual value, then the reading would be low (by .1 according to my math). But, the compensation needed was much greater than this, and wasn't constant across different currents.
- analyzed the source samples: I made a histogram of the raw ADC samples, and they have a 'hump' around zero, which I doesn't sound right for a histogram of a sine wave.
Is there something fundamentally wrong with my setup?
Thanks,
Randy
Here is an image of my setup: http://imageshack.us/photo/my-images/59/currenttransformer.png/
I'm connecting a 3000 turn current transformer around one leg of my AC power.
I placed a 220 Ohm burden resistor across the leads of the current transformer.
When I measure that voltage w/ my multimeter (Fluke 177), I get a RMS reading of around 150 mV, which seems to coincide w/ my 2.0 amp Killawatt reading of the source AC circuit.
I then connect the burden resistor across my CH1- and CH1+ pins of my MCP3424 differential ADC. I sample the ADC repeatedly, compute the RMS, and I get values consistently around 90 mV RMS.
Is this hooked up correctly?
Here is what I've tried so far:
- determining if my ADC is fast enough (it's just 240 SPS). I've done a bit a math, and read a lot of forums online, and it sounds like it should be fast enough. It might not be perfect for this application, but I haven't determined that it's speed is to blame.
- grounded one ADC input (hardware friend recommended. It didn't help.)
- added a per sample fudge factor. If the ADC was averaging its sample, instead of sampling an individual value, then the reading would be low (by .1 according to my math). But, the compensation needed was much greater than this, and wasn't constant across different currents.
- analyzed the source samples: I made a histogram of the raw ADC samples, and they have a 'hump' around zero, which I doesn't sound right for a histogram of a sine wave.
Is there something fundamentally wrong with my setup?
Thanks,
Randy