Is my oscilloscope like a multimeter as far as probing voltages? - Page 1
Forum section: electronics / beginners

author: Beamin
author_rank: Super Contributor
timestamp: October 05, 2017, 01:42:31 am
content: So I don't want to low this thing up. As far as probing signals can I use it where I would use a multimeter? Could I check to see how clean the signal is on a power supply? What would happen if I probe the hot and neutral lines? Will it be like an ammeter or a volt meter? Does it allow current to pass through it and there fore need resistors?

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2017, 01:59:31 am
content: NO, don't.The reference lead of a scope probe is mains ground referenced, you can't just use it like a DMM.Furthermore scope vertical accuracy is typically+3%, much less than a good DMM.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: October 05, 2017, 02:02:44 am
content: You don't say what your oscilloscope is, so I'm assuming it's the Tek 2465. It measures voltage. Positive voltage makes the beam go up, negative makes it go down. Play with a 1.5v battery and the input mode set to "DC Coupling" to get the idea. It has a very high input impedance (1 megaohm) so it won't load the circuit you're probing much. It also has a very low 50 ohm input impedance mode: for now, don't use this mode because it's pretty easy to burn out. That's for later.The ground lead on the probe you buy is directly wired to the ground (3rd prong) on the oscilloscope's power plug. This is where it's fundamentally different from a multimeter in the blowing-stuff-up category. For example, if you connect the ground lead on the probe to a plug socket's hot line, you would short it to ground. A multimeter is isolated and floating, so connecting the black lead to a plug socket's hot would not short it out. Always ask yourself if it's safe to short what you're connecting the ground lead to to ground. Also keep in mind that all the other channels' grounds are connected together.Whether you can probe a plug socket's hot depends on the probe you buy. The 2465 allows up to 400V on it's input, but it may not be safe to do so with your probe. Learn about frequency derating with your probe before attempting any higher voltage probing. I will say that probing the mains is almost always a bad idea with no real need: you can use a transformer and see almost exactly what's going on with that.The only current that passes through the oscilloscope is what the 1 meg input impedance (or 10 meg with a common 10x probe) will allow. (This is a simplification: there is capacitive reactance too which you get into with that derating thing above. Simply put, at very high frequencies your probe stops looking like 1M or 10M resistor and rather starts looking like a very low resistance to ground.)Checking a (low voltage) power supply for cleanliness is certainly something that your oscilloscope is quite capable of. The best mode for this is with an "AC input coupling" which blocks DC so you can amplify only the AC ripple component of the power supply. Suppose you wanted to look at 100mV rippling on top of a 12V supply. In DC mode you'd have to set it for 2V/division since you only have 8 divisions to keep the trace on the screen. Your ripple would be tiny and hard to see since it would occupy only 0.1V/2V = 0.05 of a division! In AC mode the 12V DC is blocked and you can go into 100mV/division or even lower.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 05, 2017, 03:12:43 am
content: Quote from: Beamin on October 05, 2017, 01:42:31 amThere are many tutorials about using oscilloscopes. Including this famous one from Dave Jones himself:QuoteNO! There is a life and death difference between using an isolated, free-floating, battery-operated multimeter vs. using a mains ground-connected oscilloscope. We are assuming you are talking about a typical portable, battery-operated DMM. And we are assuming you are talking about a typical mains-powered (and mains-referenced) oscilloscope. Of course, there are exceptions, but unless you can identify exactly what scope you are talking about, you can assume that the exceptions DO NOT apply here.QuoteIn some cases, yes. But the tone of your questions make it sound like you aren't ready for that yet. You need to learn now to use your oscilloscope safely before you blow something up.QuoteThat is probably the #1 cause of people blowing up their oscilloscopes. Don't do it until you have A LOT more experience!QuoteIt is like a volt meter. It is NOT made for current to pass through it. If you want to measure current, you should measure the voltage across a shunt resistor of appropriate value. PLUS be very careful about where ground is in your circuit.

author: Brumby
author_rank: Unknown
timestamp: October 05, 2017, 03:35:31 am
content: It is well that you ask these questions.... and from the questions you have asked, I will echo previous posters - with a little more emphasis:Is my oscilloscope like a multimeter as far as probing voltages?NO!!While the basic interface between a scope and a multimeter (in volt modes) is a similar high impedance one, there are two very significant differences that you simplycannotignore.First:Quote from: Richard Crowley on October 05, 2017, 03:12:43 amIn a nutshell - it all comes down to what is grounded and what is not. Watch Dave's video if tou're not sure what is meant by that.Second: Voltages. Generally scopes are not equipped to handle voltages you would have on the mains. Get some appropriate probes for this WHEN YOU ARE FAMILIAR with scope usage and safety.BUT, from the nature of the questions you have asked, I would suggest you are not ready to tackle mains - which is more than just elevated voltage - it is high energy. Get it wrong and you can easily have a fire or explosion.And that isnotbeing melodramatic.

author: danadak
author_rank: Super Contributor
timestamp: October 05, 2017, 10:25:24 am
content: A dual channel or greater scope is capable of measuring differentialvoltages which helps to eliminate the mains ground problem. Thatbeing said accuracy of the measurement is poor as previously men-tioned.http://www.newark.com/pdfs/techarticles/tektronix/FFM.pdfRegards, Dana.

author: Beamin
author_rank: Super Contributor
timestamp: October 05, 2017, 05:31:19 pm
content: Yes I wouldn't dare stick my new toy on the mains but I was still curious to find out. These are good suggestions. I have a fair understanding of electricity: I have wired up new rooms in my house and once worked on 440 three phase hooking up a huge air compressor. But a scope is new to me and as much as I like "Plug it in stand back and see what happens" I would rather not see this die to a stupid mistake.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: October 05, 2017, 06:10:24 pm
content: Why would you probe Live and Neutral anyway?...for seeing the "hum"?But still,Just Don't!If you want to probe around power supplies, I mean the LV DC power supplies, then be sure the probes aregrounded or ground referenced.That's just my two cents.

author: dcbrown73
author_rank: Regular Contributor
timestamp: October 05, 2017, 06:51:41 pm
content: I'm well aware of the scope and mains issue, though how does that apply to say a desktop DMM? My Rigol DM3058E has an earth ground pin on the plug.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2017, 06:54:02 pm
content: BeaminYou're obviously new to scopes and as experience grows you will use a scope a bit like a DMM, of course all measurements will be WRT mains ground.Techs that know what they're looking at (think signal fidelity) use a single remote ground connection on the DUT back to the scope and remove reference leads (no trailing clips creating shorts) and probe around the rails checking for indication that correct voltages are present. This is also a good time to use the plastic sleeve on the end of the probe to prevent the reference ring/band (mains ground referenced) coming into contact with part on the DUT and shorting things out.With this type of fault tracing definitive accuracy is not so important as we just need to see that voltages are ~correct and then start tracing signal through stages. Guys that work this way have been doing so for years and they don't use OSD measurements, only a V/div setting that they can make sense of and read values directly from the graticules.A scopesmartlyused can be the fastest fault finder but one must know the limits of it and yourself and when to grab another tool to progress further.#1 rule: think about signal reference and where it is safe to connect to. When in doubt use a DMM to ensure it's at the same potential as the scopes ground.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2017, 06:56:28 pm
content: Quote from: dcbrown73 on October 05, 2017, 06:51:41 pmIt doesn't. Period.Neither lead from a DMM is mains ground referenced.Instrument max ratings are all that apply for a DMM.

author: tggzzz
author_rank: Super Contributor
timestamp: October 05, 2017, 07:35:31 pm
content: Quote from: dcbrown73 on October 05, 2017, 06:51:41 pmIt doesn't relate; he DMM is internally isolated from the mains.Many high performance DMMs have an "earthy" section and a "floating" section; signals are typically transferred by optocouplers.