Multimeter Problem

Discussion in 'General Electronics Chat' started by hazim, Jul 17, 2009.

  1. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Hi

    I lately measured the frequency of the output of a UPS with 220V output. I was adjusting the frequency to 50Hz... the multimeter measures frequency with range 0-10MHz. That was the last time I used the multimeter.

    Now I found that the multimeter is not measuring anything right.... The voltage, current, resistance, capacitance... the multimeter even gives values without connecting it to anything, for example when I put the range switch at 1000V AC, it gives me some reading about 200V!

    I opened the multimeter and found the fuse broken, I changed it, and the problem still the same...

    Here is a link to the multimeter I am using with it's specifications
    http://www.multimeterwarehouse.com/vc9808.htm

    Any Ideas?
     
  2. AlainB

    Active Member

    Apr 12, 2009
    39
    0
    Hi,

    I dont have the same multimeter but mine is getting crazy when it is time to change the battery.

    Alain
     
  3. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    The problem is not with the battery... I tried a new battery.. the same
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    Hi Hazim:

    Was the fuse physically broken or merely burned out? If the instrument suffered enough mechanical shock to actually BREAK a fuse, it probably has a crack in the circuit board...at least!

    Eric
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    What voltage did the meter get at the time of the frequency measurement?
    Is it a controlled 220 Volts as stated?
    Where there no voltage spikes during the measurement?
    The meter can handle 250 Volts DC or rmsAC during frequency measurement as stated on the site you provided.

    Greetings,
    Bertus
     
  6. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    broken wasn't the correct word:).. What I mean is that the fuse was burned out...
     
  7. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    The voltage was 220V. It is controlled yes. There was no spikes during the measurement...
     
  8. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    It sounds like you may have a broken multimeter, based on your checks. But something's not right with having a blown fuse. Typically, the only time a fuse gets blown is when you're measuring current and there is too much current. The fuse blows to protect the meter. The most reasonable hypothesis for the blown fuse is that you had the leads plugged into the current jacks and touched a signifcant voltage. The fuses will blow quickly and you won't see anything special other than you can't measure current anymore.

    Unless you exceeded the ratings of the meter (250 VDC or Vrms AC) while measuring frequency, it's hard to know what happened. Typically, well-designed meters are more resistant to problems than there specs would indicate (this is because they're used in industrial environments where accidents can happen).
     
  9. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    Although the fuse might not appear burnt, I would just replace it anyway...just to make sure. Also, look for cold solder joints or frayed wires. Make sure that nothing is shorting.
     
  10. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    The fuse was burned out and I changed it. The manual says that there is protection for frequency measurement if the measuring time exceeds 10s... it seems that after about 10 seconds while I was measuring the frequency, the fuse burned out. But if this is true then the meter shouldn't become broken...
    I think that an IC burned while I was measuring the frequency...
     
  11. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    hazim, I believe you, but the manual makes no sense. Why in the world would measuring a frequency cause a fuse to blow after 10 seconds? The designer of this thing would have had to deliberately been handed a gun, pointed it at his foot, and pulled the trigger. It's nominally the same as measuring a voltage and should be able to withstand the input voltage indefinitely, as long as it was within specs. (My Fluke 83 meter behaves in this expected way.)
     
  12. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    You are right..
     
  13. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    So what do you think? burned IC or what? How could I fix the multimeter?
     
  14. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Unless you can find some obvious damage (burned trace, damaged IC, etc.), it's going to be difficult to troubleshoot the thing without a schematic. And on the inexpensive meters I've seen, the ICs are not encapsulated -- they've got a drop of epoxy over them on the board. I'm no expert, but I can't see any easy way of replacing the IC. It would probably be cheaper to replace the whole unit.

    Does the manufacturer have a repair facility? If it's a $50-$75 multimeter, I can't see any manufacturer willing to repair it for less than it costs for a new one.

    My Fluke DMM had a design or manufacturing problem with the display and Fluke wanted nearly the cost of a new meter to fix it. I was fortunate to find a $20 aftermarket kit on ebay that let me fix it myself. No thanks to Fluke -- I had to find this fix on my own.
     
  15. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
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    10s is WAY too long! Fuses shouldn't last more than a second or two over the maximum ratings. Indeed, you should see if you can find anything that looks damaged.
     
  16. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    I have something to say... While I was measuring the frequency at 220V, I turned the multimeter OFF and I turned it ON after a while, where the probes were still connected. What would do??
     
  17. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Hazim, you're asking people who have neither the meter nor the schematic about what would happen. Most of us would just be guessing.

    Here's my opinion: a well-designed multimeter should work fine after turn-on hooked to a 220 V supply.

    I would still be concerned about that fuse you had blown. On the multimeters I have seen, there are typically two fuses. One is a relatively low value to protect the voltage input circuitry and the other is a large value to protect the 10 or 20 A current range. If the low current fuse was blown, I'd assume the input leads at one time had an excessive voltage (beyond the ratings of the meter). If the high current fuse was blown, you had the probes connected to the 20 A input and touched too large of a voltage. It is easy and relatively common to blow the high current fuses by an accident. I've done it myself and the fuses for the Fluke meters are $4-$5 each (and you NEVER have a spare on hand :p). It would be much nicer if DMM manufacturers used common automobile fuses; however, I believe they don't because the fuses they use blow faster and more precisely than the auto fuses.

    All this boils down to two things I can think of: you made a mistake connecting your meter or the meter was defective. Either way, it is probably unrealistic to expect that these inexpensive meters can be fixed unless you do it yourself.
     
  18. Nightsurfer

    New Member

    Aug 5, 2009
    1
    0
    I have calibrated meters for a few years. First, the limit for the freq measurement is 200V. The fuse is probably bad. There is most likely an open, low value, precision resistor directly behind the common or the ACV input that is open. You will probably need to take it to a place that does repair and calibration on digital multimeters to have it repaired and tested. The resitor is probably a 1 or .1 ohm about 1/4 watt .5%. It should be visibly broken in the middle. Fuseable resistors are widly used in digital multimeters in addition to fuses behind the inputs. I suspect there is probably nothing else much wrong with the meter other than this. It sometimes helps to use a 100K precision resistor in series with the test leads to make measurements of this type. Adjust voltage and frequency seperately.
     
  19. Radiocrazy

    New Member

    Aug 5, 2009
    3
    0
    even though the meter had a fuse, it may not have opened fast enough. A case of having the meter set to measure current and you think you are measuring voltage is one case. A meter will read strange numbers when not connected to a circuit because of stray electrical fields. Short together your probes and your reading should read close to zero. If not the internal circuit could be bad in your digital meter.
     
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