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Very basic multimeter questions.

Discussion in 'Electronic Basics' started by Simon Bartlett, Dec 29, 2003.

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  1. Hi group.

    I've always used a multimeter on a trial and error basis, till I get a
    reading that looks like what I should get. I think it's time now that I
    try and understand what's going on though. As such, I've composed a
    list of simple questions, the answers to which would be a great help to
    me.

    1. The fused connector is rated to 500v max, 200mA max. Does that mean
    500v at 200mA? If I want to check a circuit running 12v 1A, can I
    safely use the fused connector or do I have to use the un-fused
    connector?

    2. I was trying to measure a current on a circuit of 12v 1A. To be
    safe, I used the unfused (10A) connector. I couldn't get a reading of
    volts at all (perhaps I should have tried amps). Should the 10A
    connector give identical readings to the fused connector where the
    current is within the fused connector's range?

    3. The Volts DC selector scale has points at 200m, 2000m, 20, 200, 500.
    Does the m refer to milli as in 200 millivolts, 2000milivolts? Of not,
    what?

    4. The Amps DC selector scale has points at 200(funny u symbol), 20m,
    200m, 10. What is the funny u symbol? And again, does the m stand for
    milli?

    5. The transistor tester has NPN and PNP markings. By looking at a
    transistor, how do I know if it's a NPN or PNP?

    6. The transistor tester has letters around the circle, e E B c E e c
    b. What's the story there? How do you know which to use?

    If anyone has a good reference that will answer similar questions, that
    would be much appreciated. I had a look at some sites offering
    multimeter basics, but weren't quite basic enough :)
     
  2. I don't know your meter, so can't answer this one (perhaps you should
    post these questions again, mentioning the make and model of your
    meter.)
    To measure current, you must break the circuit, and insert the meter
    in the break, so the current flows through the meter - AND set the
    meter to read current.

    With the meters I've used, you have to move the read meter lead from
    the "Volts" socket to a socket labelled "Amps" or "mA". My meter has
    one socket labelled "10A", and another labelled "mA". The "mA" socket
    is used when measuring currents up to 200 mA, and the 10A socket is
    used for larger currents.

    To measure voltage, you put the red meter lead in the socket marked
    "Volts/Ohms" (or something like that), and connect the two leads to
    the two points in the circuit you want to measure between. You do not
    break into the circuit when measuring voltage.
    Yes - "m" is "milli", or .001
    The funny "u" is a greek letter "mu", which is used to indicate
    "micro", or .000001
    With North American transistors (part numbers starting with "2N"),
    there is no way, other than looking them up in a book (or on the web).
    I think European transistors may indicate the sex and application in
    the letter prefix.
    E, B, and C are the emitter, base and collector leads of a transitory.
    The manual for the meter should give you some indication of how to
    connect a transistor.
     
  3. You will have to use the unfused connector. This is because your circuit is
    drawing 1A and the fused connector is rated at 200mA max. This rating is
    independent of the circuit voltage (as long as it is below 500V).

    Usually the unfused connectors in a multimeter are used only for current
    measurement. If you want to measure voltage you should use the connector
    marked with "V" on the multimeter (usually the fused one). If measuring
    current, the 10A connector should be used for high currents, and the fused
    one for low currents. The 10A connector may in fact only work when the
    multimeter is set to the 10A range. For all other ranges use the fused
    connector.

    Yes, it means milli.

    The "u" symbol means micro (i.e., 200 microamperes). The "m" is as above.

    Have a look at the transistor's data sheet. It will tell you the type and
    the pin layout.

    B: base, C: collector, E: emmiter. Check the transistor's data sheet and
    connect each pin to the corresponding point on the multimeter.

    You probably already know this, but just in case. To measure voltage you set
    the multimeter to a suitable V range and V connectors, and connect the probe
    *in parallel* to the circuit under test. To measure current you select a
    suitable A range and A connectors, and connect the probe *in series" with
    the circuit (i.e., you break the circuit and then insert the probes).

    Hope this helps.

    Regards,
    Costas
    _________________________________________________
    Costas Vlachos Email:
    SPAM-TRAPPED: Please remove "-X-" before replying
     
  4. Hmmm, my reply above is a bit misleading...

    Just to clarify, the above is true only if you want to measure the circuit's
    current. For voltage measurement you should use the fused connector (that's
    probably the only connector you can use for voltage measurements anyway).

    So it all depends on whether you are measuring V or I. For V you use the
    fused connector, no matter how much current the circuit draws. For I you use
    the fused one for small currents (up to 200mA) or the un-fused one for high
    currents (up to 10A).

    Apologies for any confusion caused!

    Costas
     
  5. Thanks Peter and Costas.

    That made a lot of sense. I was trying to read volts with the circuit
    broken, instead of parallel. I now see that one connector is volts up
    to 500v, or Amps up to 200mA, depending on what I'm measuring at the
    time. The other connector is only for Amps (and I was trying to use it
    for volts).

    Again, thanks for all the help.

    Simon
     
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