hi
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.

 

Try putting a 100nF capacitor in series with the AC meter.

 

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

 

I think to see the ripple you must have some load. And perhaps put your scope in AC mode also

 

hi
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?

 

first you put a capacitor series and check.