ac reading on multimeter

Discussion in 'General Electronics Chat' started by jody, Nov 12, 2012.

  1. jody

    Thread Starter Member

    Nov 12, 2012
    i have a simple step down transformer from 220v to 12v ac this is then rectified with a full wave bridge rectifier and the output smoothed with a capacitor. the no load voltage is approx 16v .
    i was interested to see if i could measure any ac ripple voltage on my dmm but when i measure the output with the dmm set to ac i get 34 v showing. ive looked at the output on a silly scope and it looks like there are no nasty ripples in fact it look very smooth so why does the dmm show such a large ac voltage when it doesnt seem to appear on the oscilloscope. ive tried another dmm i have and that gives the same readings.
  2. MrChips


    Oct 2, 2009
    Try putting a 100nF capacitor in series with the AC meter.
  3. crutschow


    Mar 14, 2008
    Some inexpensive multimeters measure the sum of the AC and DC voltage since they don't have a built-in DC block capacitor.
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    I think to see the ripple you must have some load. And perhaps put your scope in AC mode also
  5. jody

    Thread Starter Member

    Nov 12, 2012
    the silly scope on ac shows a very very fine ripple with a load been drawn . im starting to think its the multimeter. ive tried it on my friends windmill 12v alternator he uses to charge batteries to see if we could measure the ac ripple on that and again it read in the 30volt ish range. the alternator works ok so i think a better quality multimeter is needed. is there anyway i can tell from the specification of a multimeter that it would be able to measure low level ac ripple on a dc line?
  6. ransiluj

    New Member

    Nov 14, 2012
    first you put a capacitor series and check.