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Using a multimeter as an ammeter

  1. Jan 20, 2017 #1
    1. The problem statement, all variables and given/known data
    When I connect just the multimeter (as shown in the diagram) to the power source by inserting the lead into the mA connection and change the setting to read the current on the multimeter, it reads 0.

    When I change the lead connection from mA to A, the power source circuit breaker breaks.

    2. Relevant equations
    V=IR

    3. The attempt at a solution
    So then I tried connecting the ammeter in series with a resistor it started showing some current.

    I later set up the new circuit with a rheostat, as shown in the second picture (with a diode and resistor in series with each other and another resistor parallel to it), and I tried changing the resistance using the rheostat, the current wouldn't change. Whether the diode was forward or reverse biased, it didn't seem to make a difference.

    (I really hope that makes sense)

    Thank you
     

    Attached Files:

  2. jcsd
  3. Jan 20, 2017 #2

    gneill

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    Staff: Mentor

    You've described what you've done and observed, but I don't see a question.

    Presumably you worked from lab instructions that provided circuit diagrams. Can you show us the schematic diagrams for your setups? It might be useful for helpers to compare the schematics with the pictures of your implementations.
     
  4. Jan 20, 2017 #3

    Merlin3189

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    Gold Member

    First: Never connect a current meter as you have shown in your first picture.
    An ideal current meter is a short circuit (zero resistance) and will allow the maximum current to flow from the source. This may damage both the meter and the source.
    If that were my meter, it would have blown the fuse for the mA range and that would explain why no current flows.

    When you change to the A range, there is a larger fuse in the meter, so that did not blow and the supply was allowed to provide it's maximum current. Fortunately your supply seems to have overload protection and it's circuit breaker tripped.

    When you use a current meter (or multimeter on current range) try to think, what is the maximum current that could possibly flow? Then start on a range greater than that. If the current shown is low enough, then you can switch to a lower range, but still greater than the maximum indicated value. So for example, if your milliamp range has a maximum value of 1999mA, and the Amp range shows 1.567A, then you can switch to the more sensitive mA range. But if the Amp range shows 2.123 A, then you cannot use the mA range. (In some cases you may find that the max current is more than your meter can handle - so don't use your meter, find one that can handle the max current, or don't do the experiment.)

    Your first experiment seems to be intended to measure the short circuit current provided by the supply. It is labelled "max current # Amp", which I can't read but may be 2A, 3A or 5A? So you should have started with a meter (set on a range) that could handle that current, probably plus a decent margin (25%) for tolerances. This is not really an experiment for inexperienced people to do! Applying a short circuit (ie. amp meter) across a power supply is risky.

    Your second experiment is hard to see through the tangle of wires. Perhaps. as I see gneill has now suggested, you could show the diagram of the intended circuit?
    It looks to me as if the rheostat is not controlling the current through your diode. The rheostat seems to be in parallel with it, so the PSU is applied directly across the diode circuit. Also, it would help to know the values of your resistors.
     
  5. Jan 20, 2017 #4
    Thank you for your replies. It makes a lot of sense now. I've attached the schematic for you to have a look at.

    The experiment asks us to use resistors of 100 and 220 ohms but we only have 4.7 kilo-ohms and 5, 10, 20 and 50 ohms.

    So we used the 25 ohms resistor in series with the diode and the 50 ohms resistor in parallel to the two. We could attach these in series to give the 100 and 220 ohms they want but the connection was getting much crazier that way.

    My question basically was what am I doing wrong with the rheostat and the multimeter which is acting as an ammeter?
     

    Attached Files:

  6. Jan 20, 2017 #5

    gneill

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    Staff: Mentor

    @Merlin3189 covered the use of ammeters and why it's not a good idea to place one directly across a voltage source.

    For the rheostat, you need to understand the connections on the device and match them to your circuit diagram. You've only used two of the three available connections, whereas the circuit requires the use of all three.

    upload_2017-1-20_13-15-58.png

    The rheostat consists of two main components, the cylindrical coil of resistive wire and the sliding contact. The connectors I've labelled A and B connect to either end of the coil (resistor) while connector C connects to the slider. I've placed a schematic representation is below the image.

    The circuit in your lab drawing shows both ends of the "resistor" part of the rheostat connected to the power supply, while the image of your setup shows the power supply connected to one end of the coil and the slider. Effectively you've built this:
    upload_2017-1-20_13-42-29.png

    As you can see, the rheostat is not controlling the voltage delivered to the load.
     
  7. Jan 20, 2017 #6
    I thought connect all three connections of the rheostat then it behaves as a potential divider. So it this particular experiment, it has to behave as a potential divider?

    Is this my only mistake?
     
  8. Jan 20, 2017 #7

    gneill

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    Staff: Mentor

    From what I can see from the provided circuit diagram it's the only connection error.
    I'd be concerned about using resistance values that are much smaller than those recommended, particularly the one in series with the diode: It's meant to protect the diode from excessive current. Without seeing the rest of the lab instructions it's hard to say if you'll run into any other issues due to the change in resistor values.
     
  9. Jan 20, 2017 #8
    We're trying to get the resistors they recommended. When we used the 4.7 kilo ohms resistance, the multimeter showed us a current of 0 (it was the only other option we had).
     
  10. Jan 20, 2017 #9

    gneill

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    Staff: Mentor

    If you have multiples of the smaller resistor values, use series connections to create the desired values. Yes there will be a lot of connections, but you can build these separately first and check their values with the multimeter.
     
  11. Jan 20, 2017 #10
    Yes we did that too as mentioned but the circuit just looked even crazier and harder to manage since we only had one 50 ohm resistor and 20 ohms and quite a few smaller ones such as 5 and 10. Thank you, we'll try it out tomorrow and give you the update regarding what happens.

    So we have to connect A and B to the source and C to the multimeter
     
  12. Jan 20, 2017 #11

    gneill

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    Correct.
     
  13. Jan 20, 2017 #12
    Can you just verify this for us, when we want to use the rheostat as a variable resistor we only make the two connections but when we want to use it as a potential divider we have to use all three connections?

    Thank you for your help
     
  14. Jan 20, 2017 #13

    gneill

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    That's true.
     
  15. Jan 20, 2017 #14
    Thank you, do you mind if I ask you another lab question regarding capacitors? It's a really small question regarding the time versus resistance graph when a capacitor discharges so it doesn't make sense making a separate post for it.
     
  16. Jan 20, 2017 #15

    gneill

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    Actually, it would be preferable to make a separate post. If the question is not described in the initial post of the thread, a new post is required.
     
  17. Jan 20, 2017 #16
    Alright great, thank you!
     
  18. Jan 21, 2017 #17
    Hi, so we tried it today and it works! But there is one slight issue, when we connect the diode as reverse biased we shouldn't pick up changing current. When we slide the rheostat, the current changes from 0-2.4 mA for reverse biased! And 0-4.2 mA for forward biased.

    And the even bigger confusion is when we connect our diode as forward biased it shows the one with the smaller range.
     
    Last edited: Jan 21, 2017
  19. Jan 21, 2017 #18
    It wasn't clear to me what the values of the resistors are within "Component M". But you have two parallel paths within Component M. One contains a diode in series with a resistor, and the other path contains a resistor. Isn't it true that, even if the diode is reverse-biased, the current through the single-resistor path will vary as you vary the rheostat? But like I said, I don't know what the resistor values are in Component M.
     
  20. Jan 21, 2017 #19
    Ah yes that makes sense, the resistors they recommended are 100 and 220 ohms but we don't have those so we are just testing with a 10ohms and 20 ohms, we don't even have enough of those otherwise we could have attached them in series to get the recommended resistance. We've tried to get them to order some so we're just waiting now.

    So when the diode was actually forward biased, it showed the smaller range values and when it was reverse biased, it showed the larger range values.
     
  21. Jan 21, 2017 #20
    What is your DC voltage set to?

    Edit: And as @gneill pointed out, you should be careful of using small resistor values - such as what you are using. It sounds like there is a problem if the maximum current is higher for the reverse-biased case.
     
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