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Thread: Simple guide to using a DMM for measurements

  1. #91
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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by Bullzeyebill View Post
    Interesting the differences in mOhm measurements of tailcap using the DMM only, and the DMM, resistor, and power supply. Fluke alone read 20mOhm resistance, and DMM, resistor, and power supply setup ended up showing 34mOhm resistance.
    No, my direct measurement did show 0.2 ohm, that is 200 mOhm and 0.13 ohm (130 mOhm) with shorted probes. This gives a result around 70 mOhm and includes connection resistance between probes and tailcap.
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  2. #92

    Default Re: Simple guide to using a DMM for measurements

    Hi, Had a question about "light dimming" once in a while my light will "dim" after a few minutes, battery is not dead... will still have bout 3v sometimes even 3.5v, these are Recahrgeables... what would I need to check to see if theres a problem? could it be "bulb" requiring too much in the way of "amps", bad batteries, bad connection in light?? any comments are appreciated...
    Thanks...

  3. #93
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    Default Re: Simple guide to using a DMM for measurements

    HKJ - very nice guide
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  4. #94
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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by aingel2k1 View Post
    Hi, Had a question about "light dimming" once in a while my light will "dim" after a few minutes, battery is not dead... will still have bout 3v sometimes even 3.5v, these are Recahrgeables... what would I need to check to see if theres a problem? could it be "bulb" requiring too much in the way of "amps", bad batteries, bad connection in light?? any comments are appreciated...
    Thanks...
    3 and 3.5 volt is empty for LiIon batteries at least with no load, maybe you battery is just about dead.
    Try another battery and see if the problem goes away. It would also be a good idea to clean all connections.
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  5. #95
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    Default Re: Simple guide to using a DMM for measurements

    Added more measurements to the guide:

    Current consumption, using resistor.
    Power consumption.
    Charge voltage: How to measure charge voltage in a charger.

    And a table of contents.
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  6. #96

    Default Re: Simple guide to using a DMM for measurements

    Thanks, I have been wondering about the output current on a few of my chargers. The tin foil and paper sandwich is a nice idea. Makes getting reading easy.

  7. #97
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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by HKJ View Post
    Added more measurements to the guide:
    Current consumption, using resistor.
    Power consumption.
    Charge voltage: How to measure charge voltage in a charger.
    And a table of contents.
    Gosh HKJ , could it get any better, i think you have outdone yourself excellent work.

  8. #98
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    Default Re: Simple guide to using a DMM for measurements

    Cheers HKJ...

    For another newbie like me, that's a great guide, particularly with the pics. Thank you for putting so much time and effort into producing it.

    —Jack.
    "Why do today what you can do tomorrow?"

  9. #99
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    Default Re: Simple guide to using a DMM for measurements

    Thanks for the great guide. I wasn't aware the meter turning off would cause no problems to measuring current. Am I right that no power is consumed from the internal battery either when the meter is off even if still part of a circuit?

    Cheers

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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by Nil Einne View Post
    Thanks for the great guide. I wasn't aware the meter turning off would cause no problems to measuring current. Am I right that no power is consumed from the internal battery either when the meter is off even if still part of a circuit?
    The battery is used for two purposes in a DMM:

    1. To run the internal electronics and LCD display;
    2. To feed current through the circuit when measuring resistance.

    In advanced meters it might have some other purposes such as signal injection, but in ordinary meters that is all the battery is used for. So if the meter is turned off the battery is not being consumed.
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  11. #101
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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by Nil Einne View Post
    Thanks for the great guide. I wasn't aware the meter turning off would cause no problems to measuring current. Am I right that no power is consumed from the internal battery either when the meter is off even if still part of a circuit?

    Cheers
    Turning the meter off might affect the current measurement, if the power is controlled with the range selector switch. But letting the meter turn itself off will usual not change range, because the range selector stays in the same position.

    When the meter turns itself off, it will probably still use a small amount of power, this has nothing with being part of the circuit to do.
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    Default Re: Simple guide to using a DMM for measurements

    May a non-technical guy please pose a question? After many years of faithful service, my budget DMM is acting strangely. When I first turn it on, it measures a cell's voltage too low, and then gradually measures it more accurately as a few minutes go by.

    I changed the battery, but that hasn't solved the problem. Just now, it seemed worse---it won't show a fully charged AAA NiMH cell at more than ~1.23v, when I know it should be closer to 1.35v.

    Any advice would be appreciated. Thanks.

    Andrew
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    Default Re: Simple guide to using a DMM for measurements

    I have inexpensive meters that sometimes behave a bit like that after a period of non-use. In my case I put it down to oxidation on the contacts of the rotary switch leading to poor contact. Sometimes I can fix it a bit by rotating the switch vigorously between settings a few times before using the meter. I don't know if there is a really good cure -- you tend to get what you pay for, and such things may be more likely with budget meters.
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  14. #104
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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by AFAustin View Post
    May a non-technical guy please pose a question? After many years of faithful service, my budget DMM is acting strangely. When I first turn it on, it measures a cell's voltage too low, and then gradually measures it more accurately as a few minutes go by.

    I changed the battery, but that hasn't solved the problem. Just now, it seemed worse---it won't show a fully charged AAA NiMH cell at more than ~1.23v, when I know it should be closer to 1.35v.

    Any advice would be appreciated. Thanks.

    Andrew
    My first thought was flat battery, but you sort of spoiled that (Are you sure the new battery is working).

    Next point is oxidation on the rotary switch, if you can, try opening the meter and clean the switch, maybe use some deoxit.

    The best solution is to buy a good meter (My preference is Fluke), but that is a bit expensive.

    The next best solution is to use a few $ on a new meter and maybe a cheap calibration unit/voltage reference (The best/cheapest way to verify that a meter works).
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    Default Re: Simple guide to using a DMM for measurements

    Thanks, Mr Happy and HKJ.

    I will follow your suggestions and report back.

    Andrew
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    Default Re: Simple guide to using a DMM for measurements

    Oxidation on the contacts of the rotary switch must be it---I vigorously rotated the switch a number of times, and voila'---working like a champ again.

    I removed the cover, thinking I'd try cleaning and applying DeOxit to the contacts, but the circuit board is a bit difficult to remove. For now, I will leave well enough alone.

    Thanks again, gents, for your help. I really appreciate it.

    Andrew
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  17. #107
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    Default Re: Simple guide to using a DMM for measurements

    hmm if there is stoll room for DMM trouble shooting what is the reason for one not zeroing....at times it reads (standalone)0.11V or more..some things happens with A measurements.....

    it is not a batetry thing not a calibrating issue (have accessed the rotary dial (a resitor) taht zeros the readings....

    toasted maybe?

  18. #108
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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by kosPap View Post
    hmm if there is stoll room for DMM trouble shooting what is the reason for one not zeroing....at times it reads (standalone)0.11V or more..some things happens with A measurements.....

    it is not a batetry thing not a calibrating issue (have accessed the rotary dial (a resitor) taht zeros the readings....

    toasted maybe?
    What does it read on DC V if you short the probes together?
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  19. #109
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    Default Re: Simple guide to using a DMM for measurements

    still the same number

  20. #110
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    Default Re: Simple guide to using a DMM for measurements

    I had something like that happen after blowing a fuse, but then (fairly obviously) all the functions that ran through that fuse stopped working.

  21. #111
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    Default Re: Simple guide to using a DMM for measurements

    Most DMMs give non-zero indications on the volts ranges while the probes are unconnected due to static charges on the inputs. They will only read zero if you short the probes together. I have never owned a meter that didn't read zero when I shorted the probes.

    It's probably a bad idea to adjust any of the internal calibration pots unless you have suitable calibration references to use and you know exactly what you are doing.

    If you have turned any of those potentiometers since the meter was originally calibrated you might have thrown the readings off.
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  22. #112
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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by Mr Happy View Post
    It's probably a bad idea to adjust any of the internal calibration pots unless you have suitable calibration references to use and you know exactly what you are doing.
    +1

    I got me one of these 1-2 years ago. It is awesome to routinely check my Fluke DMM's:
    highly stable 5Volt reference

    And my Flukes are still very accurate:









    and the same guy now sells another one that also offers current and resistance checks (probably a better buy price-wise):
    DMM DVM Tester Checker Calibrator Voltage Reference

    I have no affiliation with the seller - I am just a happy customer.
    Last edited by wquiles; 07-09-2010 at 09:16 AM.
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    Default Re: Simple guide to using a DMM for measurements

    Will, will that checker for current, if the DMM calibrates to that 1.000A accuracy, hold up when measuring, say, 5A's? So, 5A's exactly?

    Bill

  24. #114
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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by Bullzeyebill View Post
    Will, will that checker for current, if the DMM calibrates to that 1.000A accuracy, hold up when measuring, say, 5A's? So, 5A's exactly?

    Bill
    Bill,

    I don't have the current checker, only the voltage checker, but basically yes. Once you "correct" the calibration factor, which causes the true value to deviate as components age, drift, etc., the meter should then be again accurate to within what is designed to do. These are for example the specs on my Fluke 289:
    http://us.fluke.com/fluke/usen/digit...specifications

    Corporations/ labs/engineering groups routinely send out their measuring equipment to be calibrated (sometimes yearly) to ensure that their results are meaningful. It is no good to have great resolution if the result lacks accuracy.
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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by Bullzeyebill View Post
    Will, will that checker for current, if the DMM calibrates to that 1.000A accuracy, hold up when measuring, say, 5A's? So, 5A's exactly?

    Bill

    The "DMM DVM Tester Checker Calibrator Voltage Reference" delivers 1.000mA, so you only test the range 1mA. A good closeness there does not necessarily imply precision in the 5A range.

    When an instrument is to be calibrated, every range is examined and adjusted individually.

    Wulf

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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by Meterman View Post
    The "DMM DVM Tester Checker Calibrator Voltage Reference" delivers 1.000mA, so you only test the range 1mA. A good closeness there does not necessarily imply precision in the 5A range.

    When an instrument is to be calibrated, every range is examined and adjusted individually.

    Wulf
    Thanks for correction. Yes, I did read the spec wrong.

    Bill

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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by Bullzeyebill View Post
    Thanks for correction. Yes, I did read the spec wrong.

    Bill
    Same here. Sorry about that
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    Default Re: Simple guide to using a DMM for measurements

    Going back to DC current measurements what would be an ideal resistor for measuring 3-8A's?

    Bill

  29. #119
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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by Bullzeyebill View Post
    Going back to DC current measurements what would be an ideal resistor for measuring 3-8A's?

    Bill
    That depends on what you are going to measure, i.e. how big voltage drop you can accept.
    If it is a direct drive led from a single LiIon, without any series resistor, you need a very low voltage drop. A 0.01 ohm resistor would work, but you need to be very careful with the connection from the resistor to the flashlight and battery, they could add considerable resistance to the connection.
    If you are working with 2xLiIon and a buck driver, you do not need to worry as much about voltage drop and a 0.1 ohm resistor (0.8 volt drop at 8 amp) would do fine.
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  30. #120
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    Default Re: Simple guide to using a DMM for measurements

    Quote Originally Posted by HKJ View Post
    It sound like you are measuring volts at the battery, not at the light. The meter and wires can easily drop a few tenths of volts.



    I did things just like on your picture. I'm bit confused, if i'm doing thing just like your pic, I'm I measuring volt battery or current draw?

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