Strange osciloscope measurement - Page 1
Forum section: electronics / beginners

author: charon1985
author_rank: Contributor
timestamp: May 25, 2016, 02:39:32 pm
content: Hi, i have an oscilloscope, Hantek DSO 5072P i have recently bought it, and i-am trying to measure the output from a transformer.I have made the transformer for a project. I have 4 outputs: 6V(8A), 13V(3A), 17V(3A), 25V(3A), volts are measured with the multi-meter and are according to my design calculations. In the secondary between each layer of wire is a layer of paper and electric tape, excepting the primary because there are to many turns and layers... there is no short circuit between coils.When is use a multi-meter to measure the Volts, everything is ok, even with very cheap chinese multi-meter.When i use the oscilloscope, well... i attach some screenshots to see what is showing.I have made a Self-Calibration on the oscilloscope, and the test signal shows ok (there is also a screenshot attached). I have measured some other signals from some PWM devices and they are ok. i have measured direct current and is ok...So what is happening?Thank you.The screenshot are in order: 25 volt, 6 volt, 13 volt, and test signalLater Edit: if i change the coupling from DC to AC the voltage shows even higher.

author: Neganur
author_rank: Unknown
timestamp: May 25, 2016, 02:45:39 pm
content: What measurements are you not happy with?The scope seems to give you the positive and negative amplitudes (the values at the right side of the screen) and your DMM is most likely giving you the RMS value.(25VRMS*1.4 = 35V amplitude, 70Vpp)

author: charon1985
author_rank: Contributor
timestamp: May 25, 2016, 02:59:02 pm
content: I never known what the RMS is or stands for. On my multi-meter it does not write that is measuring the RMS value, but i have read a bit about RMS now and i guess you are wright.

author: jitter
author_rank: Frequent Contributor
timestamp: May 25, 2016, 04:56:33 pm
content: RMS is the abbreviation ofroot mean square. In your language (assuming the Austrian flag means you speak German) it would beEffektivwert.Please note that the RMS value depends on the waveshape, and multiplying the RMS value by the square root of 2 to get peak value (or twice the square root of 2 for peak-peak value) is only valid for sinusoidal waveshapes.Multimeters without anything on it most likely display RMS values correctly only for sinusoidal waveshapes. Multimeters that will display correct RMS values for other waveshapes as well almost always have "True RMS" printed on them. Your scope should also be able to display RMS values.In the attachment is a 1 kHz 5 Vppsinewave with three different measurements all made on this one sinewave: Vpp, Vrmsand Vp(called "Top").5 Vpp/2 = 2.5 Vp2.5 Vp/sqrt2 = 1.77 Vrms.

author: tautech
author_rank: Super Contributor
timestamp: May 25, 2016, 05:33:23 pm
content: Quote from: charon1985 on May 25, 2016, 02:39:32 pmAre those Auto measurements or Cursor measurements? (right panel)Turn the Cursors off and re-check.For absolute voltage measurements with a DSO you want to use DC input coupling.For measurement of AC riding on DC use AC input coupling.1:1 ProbeandDSO channel input settings?If you have the graticules set brighter it also provides a sanity check by matching graticule count to amplitude measurement.And yes as others have said RMS......

author: jitter
author_rank: Frequent Contributor
timestamp: May 25, 2016, 06:00:47 pm
content: They were auto measurements on a sinewave fed from the built in sig gen directly through a coax. Normal aqcuisition.I redid the measurements, same signal, shifted to fit in 5 vertical divisions, auto and cursor measurements, averaging (1024).

author: tautech
author_rank: Super Contributor
timestamp: May 25, 2016, 06:05:39 pm
content: Quote from: jitter on May 25, 2016, 06:00:47 pmSorry jitter, I was addressing the OP.I'll edit my post to make that clear.Yep always good to provide as much as possible info on screenshots as not all DSO settings are always visible.

author: jitter
author_rank: Frequent Contributor
timestamp: May 25, 2016, 06:16:03 pm
content: Ah well, I was going to try and get better matching measured values anyway...And DMMs say:Agilent U1232A: 1.759 VDynatek 9020a: 1.772 VExtech EX830: 1.769 VVoltcraft ME-32: 1.757 V

author: jitter
author_rank: Frequent Contributor
timestamp: May 25, 2016, 06:30:48 pm
content: @charon1985To show the difference between "normal" and True-RMS multimeters:First look at the attachment, then at the measured DMM values and you should be able to tell which meters are True RMS and which are not. Notice that they all agreed that the sinewave was about 1.76-1.77 Vrmsbut now with a square wave, they don't all agree that it's about 2.5 Vrms. "Trap for young players" Dave would probably say.Agilent: 2.486 VDynatek: 2.777 VExtech: 2.450 VVoltcraft: 2.671 V

author: charon1985
author_rank: Contributor
timestamp: May 25, 2016, 06:43:09 pm
content: Thank you Jitter for all the explanations. I have seen in the flee market a "True RMS" DMM but seller asked a price higher than a new ordinary DMM... i didn't knew why he asked that price...

author: jitter
author_rank: Frequent Contributor
timestamp: May 25, 2016, 07:26:55 pm
content: You're welcome.In the past True RMS DMMs were always quite a bit more expensive that regular DMMs, but nowadays True RMS is available in quite affordable (sub €50) DMMs like e.g.thisone.