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Programmable DC Ammeter

  1. Mar 2, 2008 #1
    Hello All,

    I've began to look all over the net to see if there are any programmable DC ammeters out there. For some of my projects, I want to have something like a current-flow data logger that I can read values directly from using the serial port. I have thought about making my own, but before I jump into that, I thought i would pop in and ask if anyone knows of such a thing (or plans out there to make one). I know that a lot of high-end equipment can perform this function using GPIB and other protocols, but is there a low-cost solution out there that does just this?

    Thanks,
    Jason O
     
  2. jcsd
  3. Mar 2, 2008 #2

    dlgoff

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  4. Mar 2, 2008 #3
    Hi,

    Thanks for the great link. I checked through that site's product finder and most of those data loggers are waaay out of my budget range... I may have to buld my own from scratch. What I'm aiming for is really not that deep. Just a simple DC ammeter that can measure in real time, currents from about 0.5 mA up to 1A for most of the experiments I would use it for. IF I wanted to get fancy, maybe I would enable it to measure negative currents too just for the flexibility.

    But I'm not exactly sure how to most simply accomplish this. My first thought was to use a shunt with an op-amp to amplify the signal and a A/D converter to send the data to a micro and output to a PC. Does anyone know if there are any ICs out there that are specialized for this type of device?

    Thanks,
    Jason O
     
  5. Mar 2, 2008 #4

    f95toli

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    I don't think it would be cheaper to build than to buy in this case, dataloggers aren't that expensive. Also, sending data to a PC without using controller of some sort is somewhat tricky; most AD converters are designed to work with something like a PIC and can't send data directly to a serial port.
    It is possible to do it using a parallel port but if you have a new computer it is not likely to have one.

    If you really want to build something I would suggest using the line-in port of a cheap soundcard instead. You can either just read-put the voltage over a shunt directly, or even better, amplify the signal first with e.g. a differential op-amp. That way there is also less risk of burning the input of the soundcard.
     
  6. Mar 2, 2008 #5
    This would actually be WAY easy. PIC processors (as mentioned in the above post) have built in A/D convertors and serial ports.
     
  7. Mar 3, 2008 #6
    Hmmmm..... actually I like the sound card idea. Do you know what the average sound card's voltage resolution is? I know the sample rate is usually something like 44100Hz but not sure about the rest.

    If I could pull this off, it would definitely be much simpler than rigging up a board with a microcontroller and A/D setups. Currently I'm working with a BASIC Stamp. I would go for the PICs but I'm not ready to go on another learning curve just yet. However, I can program pretty well and could sample the sound card mic input.

    - Jason O
     
    Last edited: Mar 3, 2008
  8. Mar 3, 2008 #7

    chroot

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    Audio is usually about 2V p-p, and sound cards typically record with a resolution of 16 bits (though, admittedly, the last couple of bits are probably mostly noise).

    - Warren
     
  9. Mar 3, 2008 #8
    Thanks for the info Warren,

    I did some more reading on the idea and found out that most sound cards AC couple the mic input (which makes sense). So I just thought of an alternate way to do this. I could use an op-amp to measure across a shunt and run that output into a Voltage to frequency converter such as the LM231, which has an output of 1Hz to up to 100 kHz depending on the voltage input. I could use that to modulate the voltage and send it through to the sound card.

    Then on the software end, I could use something like a FFT algorithm to sample the sound and decompose it into the first order frequency. Since the LM231's frequency output is supposed to vary linearly with the input voltage, I can simply convert the frequency value back to a voltage.

    I was also thinking about how I might implement different current ranges. Assuming that the input bandwidth is not great enough to handle the full range I need, I could use a range knob on the device to set the sensitivity level, and, perhaps superimpose a 'range signal' on top of the LM231 output signal to tell my program what current range I'm reading in. It sounds crazy but it just might work :smile:.
     
  10. Mar 3, 2008 #9
    You can buy cheap DMMs with a serial interface these days...

    I saw an advert for one at £10... though I'd check that the serial i/f is isolated from the measuring side...
     
  11. Mar 3, 2008 #10

    chroot

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    If you want to go the sound card route, you might want to consider simply buying a $10 el-cheapo board, and then hacking it to short its input coupling cap. You'd then be able to measure DC voltages with it. Just make sure you don't throw it signals outside the range 0-2V, with respect to the computer's power supply ground.

    - Warren
     
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