digital ammeter with no voltagedrop, do those exist?

Discussion in 'General Electronics Chat' started by bartgrefte, Aug 24, 2012.

  1. bartgrefte

    Thread Starter New Member

    Aug 24, 2012
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    Hi :)

    I think the title says it all. I'm looking for a digital ammeter that does not cause a voltagedrop. Do those exist?
    It's because I've got 2 types of ammeters here right now and both cause such a significant voltagedrop that the attached load can no longer function.

    So... any suggestions?

    edit:
    Some info might be useful. I'm talking about 0-10 amps, preferably with a 10mA resolution.
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    You basically have 2 methods to measure DC current.
    Using a shunt and a hall effect sensor.
    A shunt is installed inline with your circuit and the voltage across that shunt is fed to the ammeter. (Typically 50mV shunts..mayne 60mV or 100mV)

    A hall effect sensor is placed near the current carrying wire/trace,etc.. and measures the magnetic field and is non-contact/isolated so it does not effect the circuit in anyway.


    Now you saying significant voltage drop/causing no longer function is sort of strange.. How do you have it hooked up exactly.
     
  3. bartgrefte

    Thread Starter New Member

    Aug 24, 2012
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    Okay.

    Well, I hooked both models up exactly as specified in the (online) documentation, see the links in my first post. Both have the shuntresistor at the negative side of the load.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    How much current are you measuring?

    What is the load?

    The Ebay device has a 75mV, 50A shunt that you put in series with the load. That's only 1.5mΩ of resistance, which should have little effect on most loads.
     
  5. bartgrefte

    Thread Starter New Member

    Aug 24, 2012
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    12vDC power supply, load varies, but no more then 10 amps.

    In my test setup, I'm using a ATX powersupply for power and to make hooking up load for testing easier I hooked up a 12vDC->230vAC converter so that I can hook up any device.

    For testing I took a couple of light bulbs, one of them has a dimmer. When the load increases, I can see a drop in voltage. The drop is so high that the voltage converter stops working for a short time. After that it seems to be working again but when the load and voltagedrop get high(er), it stops working again.
    And yes, I already ruled out the voltageconverter, without ammeter hooked up in the circuit there's no problem what so ever.

    The one from Ebay works better than the other, with the other the circuit loses power around 2 amps, the one from Ebay can go further but ends up showing the same problem.
     
    Last edited: Aug 24, 2012
  6. crutschow

    Expert

    Mar 14, 2008
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    I'm confused about how everything is connected. :confused: Post a schematic of your setup including where you place the meter shunt.
     
  7. wmodavis

    Well-Known Member

    Oct 23, 2010
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    There is also a clamp-on type ammeter which I suppose like the hall effect current sensor would have little or unmeasurable effect on the current in the circuit being measured.

    This is a common problem - that of the measuring instrument causing a change in the total circuit operation such that it affects the parameter being measured and therefore giving false readings or reduced accuracy.
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Clamp-on IS a hall effect sensor.. The clamp is just a slit iron donut with the hall effect sensor located in the gap between the cores.
     
  9. bartgrefte

    Thread Starter New Member

    Aug 24, 2012
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    [​IMG]
    Does my quick (MS)paint job clarify my test setup?
     
  10. crutschow

    Expert

    Mar 14, 2008
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    The Ebay device should only drop about 15mV across the shunt at 10A. It's hard to imagine how that could be a problem.

    What powers the ammeter?
     
  11. bartgrefte

    Thread Starter New Member

    Aug 24, 2012
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    The ammeter is powered by the 12v power supply, the same one the 12v->230v converter is hooked up to.

    edit: Hooked one of the ammeters up to a separate 9v power supply and I wrote down some numbers.

    ATX power supply without anything attached to it: 11.86v
    With the 12vDC->230vAC converter: 11.53v, drawing 0.1A according to the meter.
    1A load: 11.27v
    3A: 10.94v
    4A: 10.76v
    5A 10.56v, around this voltage the converter gives up and stops delivering power for a few sec.
     
    Last edited: Aug 26, 2012
  12. Wendy

    Moderator

    Mar 24, 2008
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    There is a third method I am surprised was not mentioned. Using the virtual ground concept of an op amp, it is possible to make a current to voltage convertor. The higher the gain on an op am the less interaction it will have. It can be of limited use, but it can be used for some jobs.

    [​IMG]
     
  13. crutschow

    Expert

    Mar 14, 2008
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    That method can only be used for measuring small currents going to ground. It can't be used in series to measure currents.
     
  14. crutschow

    Expert

    Mar 14, 2008
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    The 10A ammeter you have uses a .47Ω series resistor so will generate a large voltage drop (.47V per amp). The ebay device should drop very only 1.5mV per amp (unless its specifications are incorrect).

    Edit: Measure the voltage drop directly across the shunt.
     
    Last edited: Aug 26, 2012
  15. bartgrefte

    Thread Starter New Member

    Aug 24, 2012
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    I just found out the voltage drop came from the power supply I was using....:eek:

    Apparently the one I was using for testing (Codegen, 300w) isn't too good, swapped it for one made by Cooler Master. With a 7 amp load and the ammeter attached it only dropped to about 11.6v, which was 12v with nothing connected.

    Did loose power for a few sec again though. So I checked the wires, I noticed one part (a cut up molex->sata adapter) got hot. Think that was a bit to much current for that adapter and I think that part might be the culprit.

    Guess I'll have to test again with some decent wires. School starts tomorrow so could take a few days before I report back ;)
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    I think the phrase limited use covered that.
     
  17. crutschow

    Expert

    Mar 14, 2008
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    From a lawyer point-of-view, yes. :rolleyes: But I wanted to emphasize to the OP that the op amp circuit won't work to do the measurement he was trying to do.
     
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