Malfunctioning Mastech MS8221C

Discussion in 'General Electronics Chat' started by the_thief, Apr 28, 2010.

  1. the_thief

    Thread Starter New Member

    Mar 7, 2009
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    I have one of these and recently started experiencing something weird. Whenever I set it to read DC voltage the meter will display an odd number like -49.1 mV at other times it will display -178.3 or some other negative number. The number will fluctuate a bit up and down but won't read zero. It was working fine before as I was able to take measurements from the simple circuit, I put together to learn how to use the meter.

    The AC voltage setting does get close to zero, approximately 0.002V. The resistance is also working fine. I'm not sure about the ammeter as it appears to be off by one decimal place. According to my calculations and Pspice I should have a current of roughly 20 mA or 0.02A based on a 9V battery; 9.36V according the multimeter before it started acting up, and two resistors in series with a total resistance of 436Ω. When ever I hook up the meter in series to take a current measurement however it will only read when set to A but not to mA and I get an initial reading of 0.001A and 0.002A max.

    I'm new to this so I'm not sure if I've done something to damage the multimeter. I've only used it with this little 9V circuit.

    Any input is appreciated.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    What happens when you hold the leads together so the voltage is zero?
     
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  3. the_thief

    Thread Starter New Member

    Mar 7, 2009
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    It reads zero.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Then those odd DC readings are nothing to be concerned about, assuming they only show up with the leads open. You are seeing a combination of stray voltage induction, capacitive coupling from your body, and who knows what else? The high input impedance lets that stray charge rise high enough to show as input voltage.

    As long as the reading is zero with the leads shorted and DC voltage reading appear reasonable, your meter is fine.
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
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    It depends on the meter. For example, my DMM will read open-circuit DC voltages near 0 mV on the mV scale, but not quite zero. My bench DMM will read 0.1 μV when the input is shorted, but will float to a few volts negative on the front terminals, but a few volts positive on the rear terminals.

    The only meaningful measurement is when the leads are connected to a circuit, even a circuit with an impedance much larger than the input impedance of the meter.
     
  6. the_thief

    Thread Starter New Member

    Mar 7, 2009
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    I'm still getting -28.4mV across one of the resistorsbut sometimes I get 1.1V to 1.3V. I don't know perhaps the leads are loose or something.

    I tried plugging the battery clip leads directly into the meter and I consistently get 9.27V but when I try to use the meter leads I will sometimes get odd readings. Sometimes -8.99V or -9.12V, 17.1V and sometimes 9.27V.

    Is it safe to assume that I need to replace the leads?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Try shorting the test leads together when you're getting those odd readings. If the meter then reads nearly 0, your test leads are likely OK.
     
  8. the_thief

    Thread Starter New Member

    Mar 7, 2009
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    I tried holding the leads in different positions and I'm able to get good reads and bad reads depending on which way I hold them. I've isolated it to the black lead.

    I guess I'm in the market for new test leads. It was starting to drive me crazy. This thing pretty much brand new. I bought it last year but I've only used it a couple of times.


    Thanks for the help.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Poorly made test leads are a reality of today's market. Copper prices have gone through the roof, and manufacturers are always pressured to keep prices down.

    If you want good quality test leads, buy Fluke.
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    You cant really play with bad leads.

    All you need is one 0v reading when the circuit is actually 220v and your toasty.

    Get yourself a good pair of leads.

    You can test your leads by plugging in one at a time and plugging the tip into the other outlet.

    Using the continuity and the ohm settings you can see if they have failed or have too high of resistance from the wires failing.

    If you typically wrap your leads around the meter, you should stop. Using a little velcro or zipper pouch that can fit your leads and a few alligator clip lead attachments is a good way to go.

    You may want to look into Flukes lead kit. It has a few different sets of leads as well as many very useful attachments.

    http://www.amazon.com/Fluke-Corporation-TL220-Industrial-Test/dp/B000FKBZFO

    And if youve got the scratch:
    http://www.amazon.com/Fluke-TL81A-Test-Deluxe-Electronic/dp/B000NI2B6A/ref=pd_cp_hi_2

    real nice..
     
  11. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    I strongly agree with retched and SgtWookie -- buy or make yourself some good leads. Don't scrimp -- good industrial quality leads can cost a fair chunk of change, but they will last and be reliable.

    In my opinion, the best leads are heavily strain relieved by molding in plastic and use high quality silicone-insulated flexible wire (make sure it's silicone because test leads almost always have unpleasant encounters with soldering irons). And the tips are sharp to help penetrate oxides on conductors. The tips have to be hardened to stand up to long use.

    Pomona is one brand to buy -- they make good stuff (and I've seen a number of things from Fluke over the years that they OEM'd from Pomona). My 20+ year old Fluke DMM is still using the original leads it came with and they have seen lots of use -- and they still perform perfectly.

    Another brand I'd recommend is Cal Test. I've bought a variety of stuff from them (it's all made in China, but some of the stuff is assembled in the US) and been pleased with the things I've gotten. You can either buy the components and wire and make your own stuff or buy leads already made. The stuff comes in various IEC safety ratings.
     
  12. the_thief

    Thread Starter New Member

    Mar 7, 2009
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    Thanks for the recommendations. I definitely plan on getting new leads, I never gave it a second thought.

    The place where I get my stuff doesn't have a lot of selection so I might just pick up whatever they have to get going again. Making them might be an interesting little project though. I am just starting to work with circuits so I'm limiting myself to things that run off a 9V battery as a safety precaution.

    I'll probably end up placing an order with Either Digikey or mouser in the near future to get some decent stuff. I won't be looking for fluke stuff, not even leads, as it would end up costing more than the price I paid for my DMM. Perhaps in the future when I start working with bigger voltages or need more reliability and accuracy I will look into getting a better DMM.

    Thanks again.
     
  13. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    You can make some good ones yourself, assuming you like to make things. Get some good quality heavy-duty sewing needles and solder some wire to them. Then pot most of the needle and the wire into a tube, such as the body to a ball point pen, straw, etc. Make sure you know how to solder properly, as you won't be able to fix it if the solder joint isn't any good.

    You can solder or clamp on 4 mm banana jacks to the other ends of the wires. Here's a type that clamps on and has a plastic shroud for the conductor; I like this design.

    Here's another tip (see attached photo). Those Velcro "tie-wraps" (I get mine quite cheap at the local office supply store) can be used to hold the test leads together. Put them on tight enough to stay put, but then they can be slid around on the wires as needed. This keeps the leads together, as sometimes in the heat of battle it can become frustrating to find where you put a lead down momentarily.
     
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