Electrical newbie with a few multimeter questions

Discussion in 'General Electronics Chat' started by Gunmetal, Feb 5, 2012.

  1. Gunmetal

    Thread Starter New Member

    Feb 5, 2012
    3
    0
    Hi, I've been doing most of my automotive work (repairs, maintenance) and have been doing fine with most of that. But now I'm venturing into more electrical issues -- for example, I have been diagnosing some starting issues and have needed to take various electrical measurements and am trying to get a better understanding of circuits and the relationship of voltage, current, etc.

    First of all the guides here are simply amazing. I can't believe this information is all in one place, freely available. Thank you!

    My questions relate to the use of multimeters, I think I am starting to "get it" to some extent and would like to make sure I really understand some things that have happened and that I've read. For the most part I'm trying to answer my own questions but would like validation to see if I really do get it. :)

    For some background, I've got 4 DMMs: a Harbor Freight cheap-o, 2 Craftsman, and an Equus 3320. The Equus I purchased recently, the others I've had for years. Well, after using them for a bunch of measurements recently I opened them all up and found that the fuses were blown in most of them so I think in years past I've probably done some stupid things without knowing what I was doing. I'm pretty sure I know "what not to do" now but would like to validate my understanding as well as test out the DMMs to make sure that they still operate properly and give valid results (once I replace the fuses).

    With that, on to my questions! :)

    1. For what types of measurements should I de-energize a circuit prior to attaching the meter? If I'm correct, it's:
    - resistance
    - continuity
    - diode tests
    Is that right? I can/have RTFM but I haven't found anything that explains "why" and that leads to my next question:

    2. Why must the circuit be de-energized for the above and not for things like Voltage tests? Is it because the meter, in these modes, builds a low-resistance/impedance circuit within itself and essentially creates a short circuit?

    3. Why is it OK to keep the Positive lead in the mA (low current measurement) jack when testing for Voltage even on high-current circuits? Is it because when in Voltage mode it creates a high-resistance circuit which is not the path of least resistance and therefore the majority of the current won't flow through the meter?

    4. When the meter is connected in parallel, such as when measuring voltage, why does any current at all flow through the meter and therefore cause voltage to register? In other words, if the DMM is a high-resistance alternative path, why is it possible to get any kind of voltage reading if the current doesn't pass through the device?

    5. I noticed the DMM still registers continuity and resistance, even though the fuse is blown. Does that make any sense? I would have thought there would be an "open" in these modes and that these modes would no longer work with a blown fuse.

    6. Is the fuse (when in the fused positive jack) only crossed when the DMM is set to explicit current measurements or does it typically protect other "current-style" tests such as continuity, resistance, and diode? (Related to the above question.)

    7. Is there anything other than the fuse that I can/should look at to see if my meter's been damaged by past, stupid errors?

    8. What's the best way for me to validate all the meter functions, should I just check each function against known voltage/current sources and compare against another meter?


    I should add, since I'm new to this forum, that if this is not an appropriate place to ask these level of questions please let me know.

    Thanks!
     
  2. Lundwall_Paul

    Member

    Oct 18, 2011
    220
    19
    High voltage measurements! For safety de-energize the circuit, connect meter re-energize. De-energize before removing test lead.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    The only time you measure with the DUT (Device Under Test) POWER ON is when you want to measure active behaviour: voltage, current, frequency.

    You cannot reliably check resistors and diodes in circuit. Either unsolder the component or at least unsolder and lift one lead of the component away from the circuit. (You can take some initial measurements with the component in circuit to give you an idea of what is happening. It helps tremendously if you have a circuit schematic of the DUT so that you can see what else will influence the reading.)

    Because you will damage the meter.

    NO. Never leave the meter in a current, mA or A range. Standard rule: ALWAYS put the leads back into the VOLTAGE jack. ALWAYS switch the meter to the highest AC voltage setting when you are finished using the meter.

    Current will always flow into the meter. With modern meters the current is very small, in the micro-amps range, that is, 0.000001A.

    On some meters, the fuse protects the meter in the current measuring mode only.

    Same as above.

    If the meter is not functioning properly, look for burnt components such as resistors. Once I was able to repair a meter giving wrong readings but I was able to repair it only because I have a duplicate meter and I was able to compare resistance measurements.

    Use an ordinary battery such as a 1.5V, 9V and 12V battery and check the voltage readings.

    For resistance measurement, firstly, touch the two leads together and check for close to 0.0 ohms reading on all resistance scales. Next find some known resistance values and check.
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    Any further questions, let us know.

    Edit: MrChips beat me to the punch.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    What is the value of the blown fuse in your meter? 10A? 20A?
    How much current flows through the fuse when the meter is set to measure current and is connected to a fully charged car battery that can deliver 500A to a starter motor?
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,415
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    I don't think you get a chance to connect the leads to the battery. As you touch the tip of the probe to the battery terminal the tip of the probe starts to disintegrate pretty spectacularly. I know from experience. Been there, done that.
     
  7. Gunmetal

    Thread Starter New Member

    Feb 5, 2012
    3
    0
    Thanks for all the responses, looks like I'm on the right track for the most part!

    Understood, I was more looking to understand why it would be damaged -- if I'm understanding things correctly, it's because there's such a low resistance thru the meter when in the current ranges, that too much current flows through the meter and causes it to get damaged?

    I didn't mean leaving it in the current range (dial setting), just leaving it in the Positive jack which is used for both current and voltage readings.

    That said, I understand that some DMMs have completely separate jacks for current and voltage and that probably explains your response -- all of my DMMs just have 3 jacks, 1 COM, 1 V/mA, 1 A.


    Audioguru, I'm not sure what you're getting at -- my DMM has something like a 200mA fuse for low current, and unfused for high (<= 10A) current. I would think that the fuse would blow if it were set to measure current and connected in series with the battery (assuming I'm not in the unfused 10A jack) during a starter crank. For that matter, if I put the test leads across the battery (parallel connection) and set the meter to current mode, will I still blow away the fuse and/or meter? I'm thinking that if I'm in current mode then I have a very low resistance thru the meter and the circuit would be completed thru the meter, with a high draw (close to that of a short circuit). Does this sound right?


    Thanks again everyone for the follow-ups, this has been very helpful!
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I have never blown a fuse in a meter.
    Is there a video of it somewhere?
     
  9. Gunmetal

    Thread Starter New Member

    Feb 5, 2012
    3
    0
    One thing I forgot to ask... my Equus 3320 has a Battery Test function for 1.5, 6, 9, and 12 V batteries which puts a small drain on the battery to get a more realistic voltage reading. In the manual for another DMM with a similar function, it warned against using the Battery Test function to test lead-acid automotive batteries. Unfortunately I didn't read this until after having done so with my 3320, but I haven't seen any reason to think it was damaged. Why would something like this be an unsafe practice? It doesn't seem to me like it would put much current into the leads, regardless of the capability of the battery.
     
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