DMMvac vs. SCOPEpk-pk vac: Pic

JJman

Member
Messages
994
My scope and my DMM match each other perfectly for DC. My scope shows 0.5vac when using it's calibration terminal (square wave.) My DMM shows 123vac on the house line. All those are ok.

When measuring vac, the two differ by a factor of 2-3 times or so. I read that pk-pk vac should be more than what a DMM reports (RMS?)

I'm seeing this large DMM-Scope spread on all ac voltages. The pics shows a reading from my variac set to 100vac. Each vertical line is 50volts. It shows 5-1/2 lines x 50v = 275vpk-pk. So the scope shows ~2.75times the DMM. Is this to be expected? :huh

DMM-vs-Scope.JPG
 

VacuumVoodoo

Member
Messages
1,546
Your DMM is showing RMS value.
Scope shows 5.6 graticule divisions (if you move the waveform using "Horizontal position" so that first negative peak passes the center vertical scale than the positive peak you see it is 5.6 divisions = 280V

280V peak to peak--->140V peak--->Vrms=140/sqrt(2)=99.9! your scope is just fine :AOK

Check if your DMM is true RMS measuring or not. You will have false readings for anything other than sine waves if it measures rectified average only.
 

paolojm

Member
Messages
68
You're DMM reads Vrms which equals 1/2 Vp-p, or Vp X .707 which is the dc euqivalent of the ac value. If you're reading 275 Vp-p on your scope your DMM will show (275/2)X.707 = 97.2125Vrms.
If you took a scope reading of your mains outlet (120Vac in the states I believe) then your scope would show (120 X 2) X 1.414 = 340 (ish) V p-p.

Vrms is the value of ac voltage that would cause the same current through the same resistor as the same dc value.
Take a 120 ohm resistor. 120Vdc will cause 1 amp to flow through it, your mains would also cause 1 amp to flow, even though your scope would read 340 Vp-p.

Hopefully that's clear enough.
 

59Vampire

Platinum Supporting Member
Messages
4,576
i have a tektronix scope and have no idea if it is working can someone tell me the settings to get a wave form or something? thanks
 

skipm45

Member
Messages
292
What model Tek scope? some have an "autoset" feature that sets horizontal sweep and volts per division (vertical) to an "average" value allowing you to see the trace.
Also check the trigger mode. set it to "auto" (free running mode). "Normal" mode alows you to set the trigger level and slope (rising or falling edge) and may prevent you from seeing the trace.

HTH

Skip
www.skipzcircuits.com
 




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