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Whats wrong with my multimeter?

  1. Feb 22, 2017 #1
    I am very new to electronics. On 200M a resistor is "00.9". I go to 20M it is "0.00" and on 2M it is " .000" I thought that "00.9" on 200M means 0.9 MegaOhm so it will fit in the 20M and 2M range - so I get something like 0.945 on 2M but I do not.. What do I do wrong?

    Details: Multimeter type: SMA64 Mastech, bought 2 weeks ago.
    85551396.sma-my-64.jpg
     
  2. jcsd
  3. Feb 22, 2017 #2

    BvU

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    Hello Bill, :welcome:

    On 200 M a short-circuit may well indicate 0.9 too. You can switch to a lower range for this resistor until the reading becomes significantly non-zero.

    And :smile: it is a resistor you are testing, right ?
     
  4. Feb 23, 2017 #3
    Thank you very much for your answer. It is a 0.66 kilohm resistor :)
     
  5. Feb 23, 2017 #4

    Averagesupernova

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    I would think most meters would improve with accuracy when you switch the range closer to the actual resistance. I would have expected the opposite. Not surprising a $25 meter would do that on a 200M range.
     
  6. Feb 25, 2017 #5
    Have seen voltage dependent resistors (VDR) do this before.
    IIRC they have zinc oxide inside, and you can sometimes find them in old equipment as a low tech version of a Zener.
    Also relevant, you probably already know this but LEDs can be damaged if you test them on a multimeter, even with as little as 2V in the reverse direction.

    I once made an RF oscillator using a single red LED picked from my junk box, run in the reverse direction with a current regulator and it "hooted".
    This particular unit was removed from a board at my employer which jammed the radio when it was being tested, turns out it was indeed this part!!!
    Amateur radio folks have known this since the days of selenium rectifiers, even Sinclair himself used "seconds" for RF components because he needed
    a specific set of characteristics that wasn't available in a stock part.
     
  7. Feb 25, 2017 #6
    Looking up that model online, I couldn't find it as the SMA64 on the Mastech web site that I would go to as a U.S. customer; but I did an image search and found web pages I couldn't translate that said a similar model is the MY64. I was able to look that up; casing looks different but at a glance the controls look quite similar: http://www.p-mastech.com/product/detail/462

    So this meter is pretty much close to the bottom end of low-priced DMM's (digital multimeters) these days; so certain functions may be a bit flakey, e.g. don't expect it to read AC ripple on top of DC very well - you'll probably want to rig up a lead with a cap in it, to do that; and of course it won't give you true RMS voltage for AC. But other than that, it should be okay for most non-high voltage tasks. I also have a low-end Mastech, in my case the auto-ranging MS8268. I've had it for more than 2 years and it has done very well for me. So even an inexpensive meter can work okay if you understand its limitations.

    As for your unexpected readings -

    1) Have you read the manual yet? You ought to have gotten a copy when you bought the meter. Read the manual cover to cover to understand all the functions, any warnings, etc. Digital meters can seem very strange until you get used to their ways. In some situations they behave differently than analog meters. Also you want to know exactly what the ranges represent, any limitations they have, and when/why to select a particular range; it might seem that this should be ridiculously obvious, but don't assume anything; just read the manual to see what it says.

    2) How many different resistances have you checked? Just checking one resistor and getting what may be a funky reading doesn't tell you much. You need to check a bunch and see how you do across the ranges; keeping in mind of course what the tolerances are for those resistors, e.g. 5, 10, 20 percent? I would also do some test readings for the continuity function, where the intention is to check resistances so low they are in effect negligible; and check what reading you get on various ranges for just touching the leads together.

    3) I would also suggest Googling for "how to use your digital multimeter"; the better sort of articles may be helpful for you, e.g.. http://learn.sparkfun.com/tutorials/how-to-use-a-multimeter/introduction

    4) Are the batteries fresh? Make sure. Batteries are inexpensive, so if it shipped with possibly stale batteries and those are what are still in it, go ahead & replace them.

    5) Learn about other things that can cause funky component readings; e.g. poor connection possibly due to oxidized leads. If you're doing any soldering, which I assume you will be, you can address two issues at once by stocking up on some rubbing alcohol & appropriate tissues for cleaning off leads before soldering; cleaning them will also go a ways towards aiding accurate readings if the leads really are quite oxidized, which can happen.

    6) Consider acquiring some additional probes if you haven't already, including alligator-clip type probes, both the mini-and regular-sized versions; these can be helpful when taking readings of resistors or other small components, as they eliminate the "fumbling fingers" problem.
     
    Last edited: Feb 25, 2017
  8. Feb 25, 2017 #7
    Thank you for all the answers. Liked them all :)
     
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