Measuring AC Current with Arduino/CT/Rectifier

Discussion in 'The Projects Forum' started by BenFranske, Apr 5, 2011.

  1. BenFranske

    Thread Starter New Member

    Apr 5, 2011
    Hello! I'm trying to create a circuit to measure current used by a motor with an Arduino using a current transformer and a bridge rectifier. I'm afraid it's been too long since I've done AC circuits or calculated RC values so I'm having some difficulty making sense of the numbers I'm getting, any help would be greatly appreciated. I took a look through some of the old posts here and found some help but I'm still having a little difficulty.

    My specific hardware, with links to the datasheets where applicable:
    CR Magnetics CR8410-1000 Current Transformer
    Fairchild DF02M DIP Bridge Rectifier
    Currently using 200 ohms of burden/load resistance on the AC side of the rectifier for the CT
    Currently using 100uF capacitor from the positive output of the rectifier to ground
    Currently using 10K resistor in parallel with the capacitor
    Measuring across the dc side of the rectifier, needs to be 0-5vdc for Arduino

    Based on the datasheet for the CT I thought I should be using a 100 ohm burden resistor but the voltages across it that I'm seeing when the motor runs (it's on a sump pump either 1/4 or 1/3 HP I think) are lower than I expected based on my calculations (all well below 1V) so I bumped the burden resistance up to 200 ohms to raise the voltages. With a multimeter I'm now measuring voltages around 1.4 Vdc when the motor is running, this still seems a little lower than I'd expect...

    One of the things that's somewhat important to me is getting an accurate run duration (being able to detect when the motor starts and stops with reasonable accuracy, say 1/2 second or less) and I'm somewhat concerned that the capacitor is causing a delay in detecting the stop time. If I'm thinking correctly I could reduce this lag by reducing the capacitor size however the only other capacitors I have on hand are a couple non-polarized 1uF and some tiny ceramics. If I were to order another size capacitor what would be ideal.

    The other thing I'm trying to do is to convert the 0-5vdc measured back into the original current being used to track changes in current draw as the motor ages so I need to come up with an equation to turn the measured voltage back into a current.

    The circuit itself is a dedicated 20A circuit so ideally I would be able to measure up to 20A but realistically I expect the motor draws somewhere around 10A. I'm located in the US so this is 110/120v. Again, any help is greatly appreciated!