Instrumentation amplifier

Discussion in 'General Electronics Chat' started by garch, Dec 8, 2008.

  1. garch

    Thread Starter Member

    Aug 23, 2008
    24
    0
    Hi,

    I am trying to make a energy monitoring device that is hooked up to my PC. I have decided to go for the Velleman USB Project board, with 2 A/D inputs and 8 bit resolution.

    The input is an inductive sensor device that I have taken from a off-the-shelf energy monitor. The device clamps around the utility (Live) side of the consumer box (at 230V AC).

    I have measured the AC voltage across the sensor at various loads and the output ranges from about 100mV AC (when the house is drawing 0.5 KW from the supply) to 1.5V AC (when the house is drawing 5KW from the supply).

    The A/D converter in the Velleman USB board is rated at 0 to 5V DC with 8 bit resolution.

    I was wondering how to build an instrumentation amplifier, such that the range of the input into the A/D is amplified from (0 to 1.5V AC from the sensor) to (0 to 5V DC to the A/D).

    I have looked at the AD620. However, I am concerned that their is not a common Ground between the Velleman Board (0V/5VDC) and the Ground of the input sensor (assuming that I use a bridge rectifier etc to convert the AC signal to DC).

    Help would be most greatly received.

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Unless there is some overriding reason about the circuit (unknown), adding a link to connect the grounds will give that common reference.

    If you are rectifying the voltage, it will be single ended - a DC voltage relative to ground. An IA is overkill. An op amp with a gain of 3.33 will work for the amplification. What kind of supply voltages do you have to use?
     
  3. garch

    Thread Starter Member

    Aug 23, 2008
    24
    0
    Thanks for the reply.

    The supply voltage is 0V(GND) / 5V DC.

    Since the sensor is indirectly connected to the mains (via electrical inductor) would it be sensible to isolate the Ground of the sensor and the Ground of the USB Module (which uses the common grd of the PC), rather than connect them together? In theory, if a very large surge passed through the 240VAC Utility-side supply cable then this could induce a very large voltage through the sensor and exceed the max voltage rating of the A/D converter on the USB Interface Board.

    Thanks,
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If your sensor is a current transformer, then it is isolated from the power line. A 5.1 volt zener might give adequate surge protection.
     
  5. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    Maybe you could use a transformer to raise the AC voltage before rectifying.

    If you raised the AC signal until the rectified DC is at 5VDC at the maximum load you expect to be monitoring then you could use the Velleman to directly measure the DC without any amplifier at all.

    That would also give a more linear signal rectification at the lowest signal levels, where diode rectifiers start to get nonlinear.

    I would include a surge protection clamping diode; it would be a lot cheaper than having to replace the Velleman board or your PC.
     
  6. garch

    Thread Starter Member

    Aug 23, 2008
    24
    0
    Thanks. Sounds like a good idea. I was concerned about the non-linearity of the rectifier diodes as quite a significant part of the signal will be contained below 1.4V and I would like to keep the signal as linear as possible from 0 to 5V.
     
  7. garch

    Thread Starter Member

    Aug 23, 2008
    24
    0
    Any ideas where such a transformer could be purchased? I can only find mains step-down transformers (240V - 12V, etc). I guess I am looking for a ratio of 1:5 windings or so.

    I guess I could find a bit of steel, bend it into a U shape and wrap some copper wire?

    Thanks
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    A 120 - 24 volt transformer has that ratio built in.
     
  9. garch

    Thread Starter Member

    Aug 23, 2008
    24
    0
    Thanks. Maybe I'll have to order from the US as can't find a 120v to 24v transformer here in the UK from retail electronics outlets.
     
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