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How does a digital ammeter work?

  1. Oct 13, 2018 #1
    I am a student at a trade school, majoring in HVAC. My electricity for HVAC textbook has a chapter on electric meters. However, my textbook does a poor job of explaining how a digital ammeter works.

    My textbook has the following description of how digital ammeters work: "To make meters useful in line-current measuring systems, a shunt is installed within the meter. The shunt is a low resistance current path that allows a proportional amount of current to flow through the meter movement.....Suppose that a 1-ampere full-scale meter is designed using a meter movement that requires .001 amperes (1 milliampere, mA) full scale deflection. The meter movement resistance is 100 ohms. According to Ohm's Law, there will be a 0.001 A X 100 ohm = 0.1 V across the meter movement at full-scale deflection. The scale of the meter would be changed to indicate 1 ampere instead of 1 mA, but it would still take only 1 mA through the meter movement to cause full-scale deflection."

    I understand that most of the current goes through the shunt, and a minority of the current goes through the "meter movement". However, I don't know exactly what "meter movement" means in this context. I know that analog ammeters have a coil that is caused to move by magnetic forces when a current flows through the coil. The coil is attached to the dial of the analog ammeter in such a way that when a magnetic field moves the coil, the dial is also moved, calibrated in such a way as to indicate the current going through the analog ammeter. Perhaps a digital ammeter also has a coil that measures current by being deflected by magnetic fields from current. However, my books never mentions that a digital ammeter also has a coil that is deflected by magnetic fields from electric current.

    Does a digital ammeter measure current by having a coil that is deflected by magnetic forces from currents in a way similar to how analog ammeters measure current by having a coil that is deflected by magnetic forces from currents? If not, how do digital ammeters measure current?
     
  2. jcsd
  3. Oct 13, 2018 #2
    I think the digital multi meter uses a microchip to detect a voltage/amperage which is then converted into an output to a numerical display based on the magnitude of the input. No mechanical parts.
     
  4. Oct 13, 2018 #3

    berkeman

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    There is no coil in a DMM -- coils were used a couple centuries ago for current measurement with analog meters (I've read about them, but I'm way too young to have ever seen one). :wink:

    The DMM measures current by measuring the voltage across a known, internal shunt resistance. For the more sensitive measurements (like 1mA full scale, it may use a 100 Ohm shunt. For higher current measurements, a lower resistance shunt like 0.1 Ohm would be used. You don't want the shunt resistance to interfere with the measurement, but it can if the DMM user doesn't understand how the current meter feature is implemented.

    Most DMMs also have a fuse in series with the current measurement input, and the fuse will blow if you try to use the DMM to measure too high of a current. Please do not ask me how I know this...
     
  5. Oct 13, 2018 #4
    Hah! Then you've missed the fun of "pinning the meter", driving it so hard that the needle hits the little pin at the top of the scale and wraps itself around it.

    Note to self: don't put several amps through a milliammeter.

    It sounds like the author cut and pasted a description of an analog meter from an old edition of the textbook and then just added a couple of words about "digital" up front. As @berkeman said, a digital ammeter is a digital volt meter calibrated to determine the current from the voltage across a known resistor.
     
  6. Oct 13, 2018 #5

    berkeman

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    You turned it up to eleven! :wideeyed:
     
  7. Oct 13, 2018 #6

    anorlunda

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    :biggrin: There's gotta be a good story hidden there. :biggrin:
     
  8. Oct 13, 2018 #7
    And Fluke DMM fuses are not available at the local Radio Shack either. I also know this.

    And if you put AC into a DC milliammeter, the needle vibrates so fast that it disappears. Amazingly enough, that stunt did not let the smoke out of the meter.
     
  9. Oct 13, 2018 #8

    berkeman

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    Mostly this:
    I still haven't been able to find a fuse that fits to replace the mA scale fuse. Sigh.
     
  10. Oct 13, 2018 #9
    First of all, I am asking about a digital ammeter, not a digital multimeter. I know that a multimeter can measure amperage. However, a multimeter might measure amperage differently than how an ammeter measures amperage.

    If an ammeter measures amperage using a microchip to detect an amperage which is then converted into an output to a numerical display based on the magnitude of the input, that still does not really explain to me exactly how the ammeter works. I mean, how does the microchip measure the amount of current running through the microchip? It's not like the microchip can count each individual electron flowing through the microchip.
     
  11. Oct 13, 2018 #10

    berkeman

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    What is a "digital ammeter" and how is it different from a DMM used with its current input? Can you post some links or datasheet info?
     
  12. Oct 13, 2018 #11
    Remember that I asked about digital ammeters, not digital multimeters.

    Your answer assumes that I know how digital multimeters measure voltage. The problem is that I don't know how digital multimeters measure voltage. How does a digital multimeter measure the voltage across a known, internal shunt resistance?

    I'm a student at a trade school, not an electrical engineer.
     
  13. Oct 13, 2018 #12

    berkeman

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    Do you have access to Google?
     
  14. Oct 13, 2018 #13
    A digital ammeter is an electronic meter with a digital display that only measures amperage. I don't have any datasheet info.

    My textbook describes digital ammeters, not digital multimeters. After thinking more about this, I suppose a digital ammeter probably measures current the same way that a digital multimeter would measure current.
     
  15. Oct 13, 2018 #14
    This is not a question like "Who was the 16th President of the United States?" or "Is a frog a reptile or an amphibian?" This is not the type of question in which I can get an answer from google.

    It's going to take human help. If information like this could be ascertained from google, this website would not exist.
     
  16. Oct 13, 2018 #15

    berkeman

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    Yes, exactly. I suppose somebody could put together a strange combo retro intrument @davenn ? that would digitize the analog meter movement from the old coil-based ammeters, but other than the novelty, I don't know why they would do that.

    All modern current measurements that I know about with simple meters use the precision shunt resistance and the voltage measurement capability of the DMM core circuitry.

    BTW, you will probably work some with clamp-on current meters at some point in your HVAC training and work. If the current is AC, you can just use a simple clamp-on AC sensing coil attachment for your DMM. For measuring DC currents with a clamp-on meter attachment, the attachment will need to be Hall-Effect based. You can find some interesting reading by using Google with Hall Effect Meter Clamp as the search terms. :smile:
     
  17. Oct 13, 2018 #16
    How does the DMM core circuitry measure voltage?

    I have used clamp on current meters in my HVAC training. But I know how the clamp on current meters measure current.

    BTW, I googled "How does a multimeter measure AC voltage" and I got a lot of links explaining how to measure ac voltage in a circuit using a multimeter, but I could not find anything explaining how the multimeter itself measure the voltage.
     
  18. Oct 13, 2018 #17

    berkeman

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    I can help with some Google search terms if you like. Learning to use Google for technical searches is a valuable skill that will serve you well. It certainly helps me every day in my full-time EE R&D Lab work. :smile:
     
  19. Oct 13, 2018 #18

    berkeman

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    I like to use Google Images to help me focus my searches. Maybe try that? If you can link to a schematic, we can help to explain it.
     
  20. Oct 13, 2018 #19

    davenn

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    irrelevant …. the processes are the same

    @berkeman has already given you the answers but you seem reluctant to accept them

    It doesn't matter if it is a stand alone digital voltmeter or ammeter or both built into a multimeter package … it uses the same principles
    that Berkeman stated

    For voltage …. the voltage being measured gets digitised in a ADC (analog to digital converter) which is then used to drive the display driver and then displayed.
    commonly, these days, the ADC and display driver are in the one package … The ICL7106 was a common chip for doing this even up to 30 yrs ago.

    The current measurement just has an extra stage on the front end, as Berkeman also described in several earlier posts eg ……

    I googles circuit of a digital multimeter and got a bunch of circuit diagrams …. I'm sure you could do the same


    Dave
     
  21. Oct 13, 2018 #20

    Merlin3189

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    Most digital meters (voltmeters, ammeters, multimeters, even temperature meters - though not frequency meters and related ones) work by measuring voltage.
    The ammeter uses a shunt through which the current flows to produce a voltage to be measured by the electronics of the meter.

    For an electronic digital meter, the input resistance is (usually) so high that the input current can be neglected. Just take the full input voltage of the meter circuit, say 1 V, the full scale current you want to measure, say 1 mA, which means you need a resistance of 1 kΩ for your shunt. Connect the digital meter across this.
    When 1 mA flows through the 1 kΩ shunt, you get 1 V for a reading of 1.000. For smaller currents you get smaller voltages and the reading will tell you the current in mA, so 0.3 mA through 1 kΩ gives 0.3 V for a reading of 0.300, etc.

    The 10 MΩ or more input resistance is 10,000 x greater than the shunt, so won't matter unless you're going for more than 4 digit accuracy. Then you need to know your actual input resistance to calculate the shunt. Or more likely, you still use a similar shunt, but can adjust the sensitivity of the meter a little, so that you calibrate it to read exactly what it should.

    A multimeter just uses a digital voltmeter and measures various voltage and current ranges by using combinations of resistors, large ones in series (for voltage) or small ones in parallel (shunts for currents) to get the range required. Most have other functions like; resistance, requiring a current source; frequency using the counter and crystal clock; capacitance and inductance based on the frequency counter; temperature based on the voltmeter or resistance meter.

    The basic voltmeter function can be implemented in several ways, but I think commonly it is done by charging a capacitor connected to the input voltage (or a fraction of it) for a full scale count, then discharging connected to a reference voltage source in the meter and counting how long it takes. If the input voltage equals the reference, discharge takes as long as charge, so you get the full scale count. If the input is only half the reference, it gets discharged in half the time from the full reference voltage, so you get half the full scale count. (It's a lot easier to understand with graphs. Look up "dual slope integration". OIC, Dave has quoted the ICL7106 which has this info)
     
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