Question about measuring DC output ripple

moro696 · 21st August 2010, 09:17 PM

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 · 21st August 2010, 10:00 PM

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 · 24th August 2010, 06:35 AM

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 · 24th August 2010, 06:44 AM

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

21st August 2010, 09:17 PM	#1
moro696 diyAudio Member Join Date: Jul 2009	Question about measuring DC output ripple 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.

24th August 2010, 06:35 AM	#3
Mooly diyAudio Moderator Join Date: Sep 2007	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. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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21st August 2010, 10:00 PM	#2
DF96 diyAudio Member Join Date: May 2007	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?

24th August 2010, 06:44 AM	#4
AP2 diyAudio Member Join Date: Dec 2009 Location: Italy	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