Scope measures 300V p-p @ PCB, what's going on?! - Page 1
Forum section: electronics / testgear

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: August 20, 2014, 06:35:14 pm
content: My new rigol DS1074 scope measures ~ 300V p-p on my little perfboard circuit powered by a 5V 2A wallwart, presumably a switching mode one telling by the low weight.Now I do remember that there can fishy things be going on with those, but I'm not too knowledgeable about that.Ah, if I connect my perfboard GND line to mains earth, the effect goes away and I measure what I expected out of the circuit... (a little 2-transistor LC oscillator running at a few 100 kHz)Can you explain what's probably going on? Is this dangerous at all? Is my scope allright?

author: Creep
author_rank: Regular Contributor
timestamp: August 20, 2014, 07:14:34 pm
content: You should probably watch this.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 21, 2014, 01:23:22 am
content: Your wallwart may have a resistor between your isolated ground and live instead of isolated ground and neutral. If it has a non-polarized AC plug, flip it around and see if that "fixes" it. If it has a polarized AC plug then either the wall wart is miswired or your building/outlet is.As for how bad that is, that would depend on the exact cause and how sensitive your circuit is to ~0.2mA going through unexpected places assuming the wart's tie resistor is 1M.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: August 21, 2014, 01:54:09 am
content: That wall wart might be badly designed, but investigating that further, it seems like you were probing the circuit without a ground connection. Your circuit. Normally, the oscilloscope probe has a ground clip, which you are supposed to connect to your circuit's ground. If you don't do that, your circuit is (theoretically) isolated from the rest of the world, and changes in voltage in your circuit cannot be detected, because your scope has no way of knowing what these changes are in relation to.The oscilloscope measures things in reference to its ground, which is also normally also connected to mains earth. If you probe your circuit without a ground connection, the only thing it can measure relative to its ground is the mains voltage, which is capacitively coupled through the transformer and possibly the protection capacitor in the PSU if it exists. If you read 300 Vp-p, that's close to the theoretical value you would get if you probed the hot wire directly, so that's a bit high, but you would absolutely expect to see the mains voltage to some degree on the scope if the ground connection is missing.If you connect your circuit's ground even to mains earth, this gives the oscilloscope a reference to work from. This not the recommended method as the earth wiring is a worse conductor of high frequencies than your oscilloscope, but it will work.Things to check:* Did you connect the probe ground clip? If you've probed circuits before and it worked without it, it may because those circuits were connected through mains earth.* Is the ground clip broken? Using a multimeter, check continuity between the probe ground clip and the coax shielding of another channel, or mains earth.* Is your scope 1x, 10x or 100x? What is the oscilloscope set to? If it's set to the wrong setting, your signal may be showing as 10x or 100x the real value, and you never noticed until today.

author: andrewwong2000
author_rank: Contributor
timestamp: August 21, 2014, 02:17:41 am
content: i. Learn to use your scopeii. Check the probe settings - its manualiii. If it really is 300V on your PCB.. where's the magic smoke on your PCB, your scope, your hand ??iv. Keep one hand in your pocket until you are sure

author: nitro2k01
author_rank: Frequent Contributor
timestamp: August 21, 2014, 03:59:34 am
content: Quote from: andrewwong2000 on August 21, 2014, 02:17:41 amOP's scope is rated 300 Vrms. It should be able to handle a jolt from the mains just fine. As for fingers, if the 300V is coupled capacitively through a crap PSU, chances are you'll only feel tingles, not a cardiac arrest. In that case it's still advisable to replace the PSU with something of better quality before itactuallybecomes immediately dangerous from an internal failure. As for the circuit, the 300 V would be superimposed on both the circuit's ground and supply rails, and the circuit would never "know" about it, except as for bigger or smaller ripples on the rail. If you touch the chip, that might be another story.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: August 21, 2014, 10:18:07 am
content: Quote from: nitro2k01 on August 21, 2014, 01:54:09 amHey, I'm not *that* much of a beginner.That's the thing, that's why I was surprized:- scope is set to 10x as is the probe. That was the first thing I checked ;-)- ground clip WAS connected- it's not broken, well at least connecting it makes a 12 MHz signal focused instead of noisy as hell when the clip is not connected ;-)BUT thinking again, 2 days later I found one "crocodile clamp cable" (what are they called in english?) to be poorly conducting, because the cheap material was partially corroded (the cable itself under the insulation, not the clamps which were fine) o_O. If I used that cable to connect the scope's ground clamp, it might explain things... just didn't remember that.But what I did on that day then was to clamp a dedicated earth cable to the circuit which extinguished the effect.So in hindsight it seems very probable that I used that bad cable as an extension that night...I'm going to check that later.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 21, 2014, 01:59:36 pm
content: Quote from: nitro2k01 on August 21, 2014, 01:54:09 am.Remember 300vp-pis in fact,150vpeak,which equates to about 106vRMS,which is a bit lower than a direct connection to a 120v system,& very low for a 230v supply.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 21, 2014, 02:18:22 pm
content: Quote from: TinkeringSteve on August 21, 2014, 10:18:07 amYou were close. They are usually called alligator clips.