that durn multimeter

Discussion in 'The Projects Forum' started by CaliusOptimus, Mar 7, 2007.

  1. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    sry, i posted this in the wrong section, ill justify it by asking a project question too.....but first:

    ive had a few different multimeters, and ive never been able to figure out the milliamps feature. the one i use now has 2ma, 20ma, 200ma and a 10 amp setting. the 10 amp works great, i dont know what i would do without it. but no matter how i connect the leads to test small current, i dont get a reading. i tried connecting the + supply to the V,ohms,diode,mA jack and the COM jack to my load. i tried connecting it as if i were reading a voltage it just dun make sense! its probally a stupid mistake, maybe i shouldnt have tossed the directions after i opened th box lmao!


    is there a somewhat simple way to sense the amount of current flowing through a load and turn the reading into say a binary or voltage output? ive been thinking of using my OOPic as an automatic battery charger control. eg: on startup the controller would set to charge voltage and after a current threshold was reached it would switch over to float.
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    It sounds like the protective fuse has blown.

    Have you tried going to the manufacturer's website to see it they have a downloadable copy of the owner's manual?

    A common technique for measuring the current to a load is to put a small valued precision resistor 1 ohm, or 10 ohms depending on the amount of current you are trying to measure. You can then use your voltmeter to measure the voltage across this resistor and then use Ohm's Law to compute the current consumed by the load. You want to be careful and choose a small resistor value so that it has a minimal effect on the operation of the circuit being monitored.

  3. thingmaker3

    Retired Moderator

    May 16, 2005
    Put a resistor in series with the load and measure the voltage across said resistor. We call this a "current sensing resistor." Resistors are available in mere fractions of an ohm.
  4. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    yeah! you were right, the fuse was blown. i assumed if the fuse was blown then all the other functions wouldnt work. since im about to do the ol' tin foil trick, is there a right and a wrong way to hook up the leads? i couldnt find a manual, its a cheepo walmart meter.

    so what you're saying, is adding a small value resistor would effectively make a voltage divider, increasing or decreasing voltage with load fluctuations?
  5. mrmeval

    AAC Fanatic!

    Jun 30, 2006
    You lose your geek status if you wrap it in foil. Sorry, turn in your beanie and glasses. :(
  6. Gadget

    Distinguished Member

    Jan 10, 2006
    Foil kinda defeats the purpose of having a fuse. I.E. stopping the meter from turning into a blackened charcoal mess the next time you hook the leads up to the wrong thing while on the wrong scale.
  7. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    yeah, i went out n got one, even though its rated 1A rather than .5A

    i shorted a big lead acid battery through the 10A one time, as my meter read O.L. i decided to tripple check my settings from then on, lol.
  8. ashokcp

    Active Member

    Mar 8, 2007
    It's the fuse. The 10A range has a shunt (solid conductor) and the other ranges have a switchable resistor with a protective fuse. The switchable resistor (you switch by turning the knob for selection of range, say, 2 mA, 20 mA, etc.,). Since the fuse is blown, the lower ranges wont work & 10A range continues to work as it is not fuse protected. Replace the fuse with 3A rating, and should be ok.
    Word of cautiion - Get the Meter back in voltage mode the moment you finish the current measurement. Or else, even if you try to measure a low voltage, since it is in current mode, you will blow the fuse out.