Question about measuring DC output ripple

[moro696]

Hi i've recently repaird a smps wich gives 50V output, my concern is when i mesure whit osciloscope in AC mode i get somewhere arround 2Vpp ripple, but if i connect the scope in DC mode i get somewhere at 100mVpp.
Wich is the corect measurment mode?
I must mention that i monted the scope probes at the output after the rectifiers, coil and capacitors.

 

[DF96]

You should see the same waveshape in either mode, so something else is different. Did you use 10:1 probe for DC but 1:1 probe for AC?

 

[Mooly]

50 volts DC will push the trace off screen, or at settings where it doesn't 2 volts ripple will be hard to see accurately.

Also if using a 10x probe, radiated noise from the PSU can be picked up confusing the reading. AC coupling is the correct method.

 

[AP2]

Hi,
Mooly said right (10:1 probe), this not influence input stage of instrument.
Also measures in AC two ripple at output dc. one is ripple typical at 100hZ (minim load required eg. 100mA), other is residual high frequency of smps.

Regards