Newbie Oscilloscope measuring questions - Page 1
Forum section: electronics / beginners

author: Greg123
author_rank: Newbie
timestamp: June 19, 2020, 05:29:03 pm
content: Hi, I'm Greg, I'm a beginner at electronicsI decided to buy an oscilloscope to help me learn... hope this is the right place to ask if I want to check that I'm understanding/using it correctly?If so:I was measuring a breadboard powered by an old Nintendo Entertainment System power supply unit (in: 240 V AC, 50 Hz, 17 W; out: 9V AC, 1.5 A) and an SDS1202 oscilloscope.1/ Is the oscilloscope measuring about 3.2 V vs my multimeter's 11V because the oscilloscope is measuring peak-to-peak, which is roughly 3x the RMS that the MM measures, hence it's broadly agreeing with the PSU's declared 9v and the MM's measured 11?2/ Is it possible to make a voltage multiplier using only diodes? I measured about 40v on my oscilloscope at one point, but I'm pretty sure this isn't possible to do with only diodes, and that I probably just had the x1 x10 setting incorrect, getting a voltage reading off by a factor of 10 (and so it was actually nearer 4v peak-to-peak).3/ Notice how I only said 'with diodes' and not 'with diodes and a resistor'?Any ratings I have to care about when buying replacement probe hooks, or will any generic ones do, likethis one, as mine are a bit melty now.Thanks

author: james_s
author_rank: Super Contributor
timestamp: June 19, 2020, 05:34:13 pm
content: Yes, you're in the right place, welcome!There are many different aspects of a signal that you can measure with an oscilloscope, could you describe what you are measuring? If you are looking at the peak value of an AC sine wave then the voltage should be 1.414 times the RMS value which is what a multimeter will be showing you.You do need capacitors to make a voltage multiplier however any time you have two conductors near each other you get a capacitor, this parasitic capacitance (and inductance and resistance) is unavoidable. It is very small though so if you apply any kind of load the voltage should drop right back down. Oscilloscopes and multimeters typically have very high impedance inputs so they will pick up radiated energy out of the air too, this has tripped up many beginners.Oh and those probe hooks shouldn't be too critical, as long as they fit the probe you're using. If you're melting them though you're doing something wrong.

author: Greg123
author_rank: Newbie
timestamp: June 19, 2020, 07:10:54 pm
content: Yes, I completely forgot to put a resistor in (doh).Ah, so I'm getting it backwards and a roughly 10v power supply should measure roughly 10v on a multimeter and that should be somewhere in the region of 30v peak to peak on the oscilloscope? That makes sense, as the average voltage is going to be less than the peak.I guess then that I probably had the x1 x10 setting wrong when I measured a peak-to-peak of about 3v?Thanks, I was pretty sure that you need capacitors for a significant voltage boost, but thought It'd be better to checkThanks.

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2020, 07:32:52 pm
content: The peak voltage of a sine wave is sqrt(2) times its RMS voltage. sqrt(2) is approximately 1.414*IF* the signal is close enough to a sine wave with *NO* DC offset, your scope will read peak-to-peak as DOUBLE the peak voltage, i.e. 2.828 x the RMS voltage.If its not a sinewave or has a DC offset, all bets are off - the scope wont (and shouldn't) include the offset in a peak-to-peak measurement, and the multimeter may well not be calibrated for non-sinusoidal waveforms with DC offset, so even if you know the peak to RMS ratio for the actual waveform it wont help you. A modern DSO is likely to be able to do waveform maths to calculate the actual RMS amplitude of the displayed signal, but that can be misleading if the sampling window isn't an exact multiple of the waveform period.

author: gfmucci
author_rank: Regular Contributor
timestamp: June 19, 2020, 08:13:04 pm
content: Quote from: Greg123 on June 19, 2020, 05:29:03 pmI love the "melty" part. Being at least as new as you, I expect to experience some meltiness soon. My first scope arrives Monday. And my reason for getting it is similar to yours. No better way to spend quality quarantine time than by learning by melting.

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2020, 08:27:46 pm
content: As long as the melting/burning is confined to cheap parts and a cheap breadboard, that's reasonably true for anyone who has to make hands-on mistakes to learn.To avoid doing the melty/burny thing to your scope or its probes watch:Also, don't clip a hook ended scope probe onto the lead of a power or low-ohm resistor, unless you are absolutely certain there's no way for it to get enough voltage to get hot, (remember P=V2/R ), as you risk melting the plastic round the probe tip! Unless you bought really expensive leading brand probes, spares wont be available.If in doubt, insert very short loops of bare wire into your breadboard to have good places to clip scope probes to that wont melt them!

author: james_s
author_rank: Super Contributor
timestamp: June 19, 2020, 09:23:04 pm
content: Just realized I made an error in my reply, I meant the peak voltage, not the peak to peak voltage which will be double that. I corrected the post to prevent confusion.You should always leave your probe set to X10 unless you have a specific reason not to, and set your scope to account for this unless it's a very old one that doesn't let you do this. I think it's stupid that so many cheap probes are switchable, using X1 when you are expecting X10 can damage the scope, and at least 95% of the time you'll want X10. It's much better IMO to have a separate X1 probe if you need one, I usually glue the switch on the switchable ones.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: June 19, 2020, 11:16:30 pm
content: Those ebay probe spring hooks may fit your probes, and they may not. Most of the probe sets I've got don't have parts that will fit other than the probe they came with. This is also true of BNC tip adapters and the little ground springy things that are used without the spring hooks.Caution: some spring hook tips require a pretty hefty push onto the probe itself to "snap" into place where the probe tip makes good contact inside with the spring hook and won't slip off in the middle of a measurement. This is definitely true of the Rigol probes they ship with the 1054z for example.I second the emotion about 1x and 10x probes. I don't put glue in them though, I just tape the switch to the 10x position. I don't want glue residue inside the little fragile switch. And _always_ confirm that the scope input attenuation is set to match the probe. I would say more than 99 percent, not 95 percent, of the time I want 10x on both probe and scope. But of course that depends on the use case.And finally... dare I say it.... RTFM. Twice, or three times even , if this is your first oscilloscope.It would be nice if the manufacturers would make the spring tip housings out of Bakelite or some other heat-resistant plastic.

author: gfmucci
author_rank: Regular Contributor
timestamp: June 20, 2020, 01:56:07 am
content: Which brings me to this question about USB isolators and differential probes. For lowish cost devices in these genres, will they suffice for the beginner within the given specs?USB isolation:https://www.amazon.com/dp/B00QU232UM/?coliid=I2K7I1FJM8W9NG&colid=3B04RKX85L5W0&psc=1&ref_=lv_ov_lig_dp_ithttps://www.amazon.com/SMAKN-Isolator-Digital-Isolation-Industrial/dp/B00XXPO4UG/ref=cm_cr_srp_d_product_top?ie=UTF8https://www.amazon.com/dp/B07GB4CQQ3/?coliid=I3DMV60RNKQJY1&colid=3B04RKX85L5W0&psc=1&ref_=lv_ov_lig_dp_itDifferential probe:https://www.amazon.com/dp/B0795XQR68/?coliid=I15ZUNV9NXKO6S&colid=3B04RKX85L5W0&psc=1&ref_=lv_ov_lig_dp_it

author: alsetalokin4017
author_rank: Super Contributor
timestamp: June 20, 2020, 03:20:49 am
content: Oh why did you have to post that?I was just enjoying having a little extra money in the bank account this month. Now I have to spend it on a differential probe.....

author: Ian.M
author_rank: Super Contributor
timestamp: June 20, 2020, 04:08:39 am
content: There are two common problems with USB isolators:All except extremely expensive ones don't support USB High Speed (480 Mbps) upwards, so make sure your test equipment doesn't need faster than USB Full Speed (12 Mbps).If bus powered, they are usually incapable of delivering the full 500mA a USB 2 host port can supply due to losses in the isolated DC-DC converter. The downstream Vbus from the DC-DC converter is also likely to be less stable and significantly more noisy than its upstream Vbus supply.The cure for the limited current problem is to get one with support for external power in on the downstream side. However the PSU you use must have a fully isolated floating output.As the common mode Y capacitor leakage current from a SMPSU such as most USB chargers and other lightweight wallwarts may well be an issue for test and measurement applications, one should use a linear supply, e.g. an old heavy line frequency transformer 9V wallwart feeding a beefy linear regulator e.g. a 78S05 on a decent sized heatsink. N.B. *CAUTION* the heatsink will be at the same potential as the isolator output!Note that the cheap 'Nobsound' USB isolator (third one) looks quite nice and has an external power connector, but it doesn't list an isolation voltage rating and I am somewhat concerned about the creepage distance under the 'speed' switch, so I wouldn't recommend its use in applications where more than 50V isolation is required.Its possible to get a really cheap USB isolator and remove the DC-DC converter, replacing it with an external supply, switched by an optocoupler (to a regulator enable pin) or a SSR, controlled by upstream Vbus so the output power shuts off when the host PC does. As long as the isolator's PCB layout has adequate creepage and clearance distances, the result is likely to be superior to many more expensive offerings.

author: gfmucci
author_rank: Regular Contributor
timestamp: June 20, 2020, 04:16:28 pm
content: Quote from: Ian.M on June 20, 2020, 04:08:39 amHow do I determine if my scope, an Instek 1054b needs a USB speed faster than 12 Mbps?

author: Greg123
author_rank: Newbie
timestamp: June 21, 2020, 08:20:15 am
content: Thanks everyone.I think I'll glue some small bits of plastic to the meltedshieldingsheath around the probe hook to fix it, as it might be hard finding one that fits.

author: Ian.M
author_rank: Super Contributor
timestamp: June 21, 2020, 08:39:52 am
content: Its not 'shielding' as its not metal! The word you are looking for is sheathing, however I'd simply call it the hook shell.You could hold the hook open, and re-build the tip of the shell with epoxy, then carefully file it to shape with a set of fine needle files. To stop the epoxy sticking to the hook, lightly grease the hook with a tiny dab of Vaseline on a toothpick, then warm the hook just enough to melt the grease to spread it. If you've got a 3D printer, instead of using epoxy, hack it to feed under manual control, and use the extruder to deposit ABS on the tip of the shell by manually moving it under the extruder.@gfmucci: RTFM! or use this util:https://www.uwe-sieber.de/usbtreeview_e.html

author: gfmucci
author_rank: Regular Contributor
timestamp: June 21, 2020, 12:24:34 pm
content: Quote from: Ian.M on June 21, 2020, 08:39:52 amHuh?WTF does any of that "utility" have to do with my very simple question to your warning about how I determine that my scope, an Instek 1054b, doesn't require a USB speed faster than 12 Mbps?I RTFM! and I doubt the answer was in there, or I wouldn't have asked the FQ. RTFH of this discussion category which has words to the effect of "Beginners" and of this thread which reads "Newbie oscilloscope measuring questions."Needless to say, that was not the type of answer I was expecting nor was helpful to my question on this thread.

author: Ian.M
author_rank: Super Contributor
timestamp: June 21, 2020, 01:51:22 pm
content: Its interface requirements *should* be listed in its table of specifications, which *should* be in its manual, hence RTFM. If not, the utility I linked can, among other things, display the properties of any connected USB device, including the connection speed. Heck, you can see 'Device Bus Speed' discriminating between '0x02 (High-Speed)' and '0x01 (Full-Speed)' in the first two screenshots at my link.Quote from: gfmucci on June 21, 2020, 12:24:34 pmIts not *YOUR* thread, you hijacked Greg123's thread when you switched from being a contributor with a general question about suitability of isolators and differential probes to a specific question about hardware you own (and Greg presumably doesn't), so you certainly aren't owed anything, not even this explanation! I apologise for my terseness while responding (with the intention of being helpful) to an off topic threadjacker.

author: gfmucci
author_rank: Regular Contributor
timestamp: June 21, 2020, 02:40:11 pm
content: Quote from: Ian.M on June 21, 2020, 01:51:22 pmNo "hijacking" here. My question was right on point. What is the title of the OP? Oh yeah, "Newbie Oscilloscope measuring questions". The OP asked about "melting things", or how not to. Subsequent posts did the same, as did mine, asking what can we do, what devices might help, to minimize the excitement of melting things. You answered. I didn't understand your "RTFM" answer. I responded.You then provided a MUCH MORE HELPFUL response (thank you) with the scopes "interface requirements *should* be listed in its table of specifications, which *should* be in its manual." Yes, look at "interface requirements" in the FM. That is much more helpful than RTFM.Thanks, I'll look there as I memorize the other 212 pages of the FM.

author: Ian.M
author_rank: Super Contributor
timestamp: June 21, 2020, 03:10:10 pm
content: Using the USB Device Tree Viewer utility with your scope connected is almost certainly an easier way to find the speed than wading through a manual. Although the OEM *should* have tabulated the interface requirements in the specifications section of the manual, it may be buried elsewhere, or even be in an obscure readme file if they offer proprietary PC software for your scope.USB speed would be more likely to be a problem if you had a PC based USB scope (i.e. just a pod with BNCs), rather than a standalone DSO with USB interfacing, especially if it transferrs raw samples and most of the data processing for the scope display is done on the PC.

author: gfmucci
author_rank: Regular Contributor
timestamp: June 21, 2020, 03:22:16 pm
content: Well, I looked in my FM, page 205 under "Interface" and there is nothing posted to address the question at hand, which was:If I use one of the isolating USB adapters I linked above on my Instek 1054b scope, will my scope need a faster than 12 Mbps (USB Full Speed) ?whatever?, which was the posted caution to my question.The FM mentions the USB 2.0 High-speed host port, which I won't be using. It mentions the Ethernet port, that I won't be using. It mentions the Go-NoGo BNC port, which I won't be using.I'll be using the test probes. There is nothing about USB speed under "Probe Spedifications."So I'll close out this dead end, purportedly "highjacking" inquiry on this thread, and start a new thread on the specific topic of "compatibility of various isolating USB adapters with the Instek 1054b scope."We will now resume our regular programming.

author: Ian.M
author_rank: Super Contributor
timestamp: June 21, 2020, 05:12:31 pm
content: To prevent confusion: >99.9% of full standalone DSOs will have a three wire mains lead and BNC connectors with grounded outer shells. A USB isolator on their USB PC interface wont do diddly squat to protect them unless the PC's ground is at a significantly different potential to the scope ground, which is easily mitigated by powering both from the same power strip.Where using a USB isolator with a scope comes in to its own is if you need to float*a PC hosted USB scope 'pod', or need to break the ground to a D.U.T. that is a USB device that needs an active host connection, (e.g. any Arduino with a sketch that uses the terminal)* Floating a scope is generally a very bad idea - floating scopes have killed competent fully qualified engineers, but if you have no other choice, floating the 'pod' of a USB scope may be the least unsafe option.