DMM and DS2072 disagree in voltage measurement. Why? - Page 1
Forum section: electronics / beginners

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 24, 2013, 01:15:18 am
content: I measure the output from a LM317 with Fluke 87V (RMS) and it indicates 5.0V DC.I attach the DS probes to same points and it indicates ~3V. Vertical set to 1V/division, with trigger (?) (small yellow arrow on left side of screen) at 0V, the line is display about 3 divisions up. Shouldn't this be 5 divisions up?User error? Normal expectations? Stoopid question?I've skimmed through the Oscilloscope tutorial, but didn't see where I might be going wrong.Please, help! And thank you for your consideration.

author: kg4arn
author_rank: Unknown
timestamp: November 24, 2013, 02:27:41 am
content: DMM is probably correct.Be sure scope horizontal sweep is set to auto trigger.It should be close to right on 5V.An image of the scope screen would help.

author: Lightages
author_rank: Unknown
timestamp: November 24, 2013, 04:28:11 am
content: Oscilloscopes are generally specified around 3% accuracy. This of course does not account for the difference you are seeing. One possibility is that you have the 87V reading AC+DC TRMS and the oscilloscope is set to AC coupled input. This can cause a big difference in the reading depending on the shape and DC offset of the waveform. Another possibility is that you are only looking at a very small part of the waveform on the oscilloscope and the TRMS calculation is not accounting for the full cycle. It is usually best to have at least 3 - 5 full cycles on the screen to get a reasonable measurement.

author: RRobot
author_rank: Regular Contributor
timestamp: November 24, 2013, 04:56:39 am
content: Does the attenuation setting on your scope match the probe? Also is there a load on your LM317 output or is it open circuit?

author: John Coloccia
author_rank: Super Contributor
timestamp: November 24, 2013, 06:24:36 am
content: Forget where the trigger is set. Set the triggering to Auto, and set the channel to ground coupling. The basic purpose of the ground coupling setting is so you can set the vertical position to a known place without having to manually ground the probe. It just sets the probe input to ground so you can set the vertical position where you'd like 0V to end up. Setting the trigger to 0V only controls where the scope will trigger a trace if it's not in Auto...it won't affect the 0V position of the trace.Once you've done that, go back to DC coupling and you should see your 5V. By the way, when in doubt the first thing you should do is go to the test signal output and start there. You know for sure exactly what you'll expect to see there, and it's a good starting point when you've gotten into some weird configuration and you need to get back to normal.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 24, 2013, 03:37:23 pm
content: Thanks everyone,I've attached a couple of photos.The Test Signal square wave matches the drawing in the user manual.Same ~3V without a load and with a 1K resistor as a load.Horizontal sweep is set to auto triggerOscilloscope is set to DC coupled input"It is usually best to have at least 3 - 5 full cycles on the screen to get a reasonable measurement."Not sure that I understand this. It' DC, so how do I get full cycles?Attenuation setting on your scope matches the probe (Ratio:1X) and Rigol only includes the RP3300A probes which don't have a 10X switchSet ground as per John Coloccia's reply. No change in output.I'd hate to send it back just because I'm too ignorant on how to use a scope. It is my first scope and haven't used one since the mid-70's.Any other suggestions please?-Thanks !

author: RRobot
author_rank: Regular Contributor
timestamp: November 24, 2013, 04:00:09 pm
content: I'll throw out some more ideas:1) What are your trigger settings? Does it appear to trigger? Is there even an edge to trigger off? Have you tried Window trigger?2) Perhaps there is a problem with your scope probe, have you tried a different one?3) What happens if you try the same test with a house hold battery?4) Are you connecting to the same points in your circuit as your DMM?I think its as I write below.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 24, 2013, 04:05:54 pm
content: https://www.dropbox.com/s/tq57bbn0rfn2mx8/2013-11-24%2008.17.18.jpgLink to scope pic 1Photo 2:http://www.dropbox.com/s/xn3le6v7x0u2uvq/2013-11-24%2008.17.29.jpgHope this works

author: RRobot
author_rank: Regular Contributor
timestamp: November 24, 2013, 04:11:05 pm
content: Looks like its reading .5 volts. Are you sure about the attenuation on your probes?Edit: I just checked the RP3300A is a 10x fixed probe, but your attenuation is set to 1x. So set it to match and it should read as expected.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 24, 2013, 04:15:10 pm
content: RP3300A is 1:10

author: bingo600
author_rank: Super Contributor
timestamp: November 24, 2013, 04:18:42 pm
content: Quote from: JohnnyGringo on November 24, 2013, 03:37:23 pmAre you kiddingDoes the DS2072 come with "china probes now"Even my DS1052E came with switchable probes/Bingo

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 24, 2013, 04:31:48 pm
content: Quote from: RRobot on November 24, 2013, 04:00:09 pmI set trigger to Window. There is NO edge, as it just straight DC from 9vt battery (now)Channel 2 does same, with different probe9 VDC Battery gives 5 V !!Yes, same points, same results with 2nd DMM (amprobe AM-140-A)I've attached an image of 9V. Am I interpreting it correctly as reading 5V? DMM shows 8.99-ishShould I run a "Self Cal" on it?

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 24, 2013, 04:36:06 pm
content: Quote from: JohnnyGringo on November 24, 2013, 04:31:48 pmNot sure what you have here, but your 5 volt example makes perfect sense, the picture you posted indicates 0.5 volt because you set the input to 1x but the probe is 10x. If you correct that it will show 5 volt.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 24, 2013, 04:38:22 pm
content: Quote from: RRobot on November 24, 2013, 04:11:05 pmYES !!!! Setting it to 10x now shows 9V !! Thank you all very much!!!

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 24, 2013, 04:42:31 pm
content: Quote from: bingo600 on November 24, 2013, 04:18:42 pmShould I invest in a *better* set of probes? If so, what is recommended. Just general purpose for now, still in the learning (err.. curing ignorance) stage.

author: mtdoc
author_rank: Super Contributor
timestamp: November 24, 2013, 04:46:47 pm
content: Quote from: bingo600 on November 24, 2013, 04:18:42 pmYou realize everything from Rigol comes from China, right?QuoteYou seem to be assuming that switchable=better, which is not necessarily true. Many (likely most) high end probes are 10 x only. The unattenuated setting on switchable probes is only useful for small amplitude, low frequency signals in which case most inexpensive ebay "China probes" are more tnan adequate. The smaller form factor of the non-switchable probes that come with the Ds2072 has some advantages over the larger switchable probes.

author: RRobot
author_rank: Regular Contributor
timestamp: November 24, 2013, 04:48:48 pm
content: Quote from: JohnnyGringo on November 24, 2013, 04:42:31 pmI seem to remember Dave did a video on this.I think this is it, but I did not double check to see.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 24, 2013, 05:14:22 pm
content: QuoteNice explanation by Dave. I wonder if Dave's videos is the reason why Rigol stop shipping the 1x/10x probes.