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How to determine a current is AC or DC by use of analog multimeter?

Discussion in 'General Electronics Chat' started by abdulbadii, Sep 16, 2017.

  1. abdulbadii

    abdulbadii New Member

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    How can I determine a current is AC or DC by use of an analog multimeter, or its workarounds in rudimentary tools / equipment ?
     
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    You measure the current or voltage, first with the multimeter set to measure AC and then set to measure DC.
    Which ever gives the highest reading tells you whether it's AC or DC.
     
  3. Ratchit

    Ratchit Well-Known Member

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    First, measure the current or voltage with the meter in series with a diode. Then reverse the diode leads and measure again. If the two reading differ significantly, the current/voltage is DC.

    Ratch
     
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  4. dave

    Dave New Member

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  5. dr pepper

    dr pepper Well-Known Member

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    A meter set to Dc applied to an ac voltage will (so long as theres no dc offset like from a transformer) under read a lot or even show no voltage.
    A 'trick' I've used that works on some analogue meters is to touch the probe to the terminal several times, if the meter is set to Dc and the source is ac then the pointer will sometimes 'flick' upwards a little and other times downwards, not very professional but works.
    You need to be carefull not to overrange the meter, if it doesnt show a reading its tempting to set to a lower range, and if you over voltage a meter irrespective of whether your measuring ac or dc you can easily blow up the meter and yourself.
     
  6. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    With the meter on an AC scale, connect a capacitor in series with the meter and take a reading. The meter can only read AC then so if the signal has AC content it will produce a reading, but if pure DC then it will read zero.

    The reason for using the capacitor is because some meters will read voltage on the AC range even when there is just a DC voltage. The cap ensures that only AC can get through.

    Of course you have to use a cap that is appropriate for the expected voltage, and it must be non polarized and preferably rated for AC operation. I keep a 2uf 200v non polarized mylar cap around for such measurements.

    Note that you'll see a reading if there is any AC component even if it is small, but then the reading will also be small. Thus the ripply from a half wave rectifier for example will show up a little but it should be low.
     
  7. Chris Edwards

    Chris Edwards New Member

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    Both digital and analog multimeters are able to measure current very easily. Analog multimeters are normally based around a DC microammeter. Each DC volt range adds a suitable series resistor to limit the current. AC voltage ranges add an extra rectifier, so try measuring the voltage on an AC range. If in doubt, start at a high voltage and work down, then switch to the equivalent DC range. If the meter reads on both AC and DC, it's DC current. If it only reads on AC, then it's AC current.
     

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