Would this work as an AC probe? - Page 1
Forum section: electronics / beginners

author: Beamin
author_rank: Super Contributor
timestamp: October 14, 2018, 09:47:19 pm
content: Say I wanted to probe the outlet with my oscilloscope. What if I took a small toroid ran 10 turns of magnet wire down one side and ten turns on the other, two conductors four contacts: hot and neutral in, and 2 AC outs. The AC line side 120VAC would have a 1 mohm resister on it. That would create 0.12mA of current flowing through and not get hot and pass at most 14mW? The scope impedance would be set to what? It has a 50ohm setting or other settings (can't make out the other right now it's a tek 2465) . That always gets me is that 50 ohms like my 50 ohm inductance like a coax cable or is it 50 ohms electrical resistance? The circuit would be sealed in a small box with silicone like ghetto potting and care would be used (I use latex gloves when ever I work with AC {anything >12volts} or charged caps) handling the output wires.I always consult the internet when working with high voltage or break my most valuable equipment.That's all A/C coupling is right? Or is that when they use a cap to filter out the DC?

author: tooki
author_rank: Super Contributor
timestamp: October 15, 2018, 12:24:53 am
content: I don’t even know enough about magnetics to call myself a beginner, but 10 turns of magnet wire will just be a fuse and burn up. Why not buy an off the shelf 120V->12V transformer and just probe the output of that?

author: Beamin
author_rank: Super Contributor
timestamp: October 15, 2018, 12:37:40 am
content: Quote from: tooki on October 15, 2018, 12:24:53 amI have parts on hand and ordering transformers takes a long time. Ohms law says it shouldn't blow up at those ohm/voltages. You should be able to put a 1 mohm across the outlet unless I mixed up the decimal place.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: October 15, 2018, 01:55:08 am
content: Putting 1 milliohm across an outlet (assuming 120 V) will result in a current of 120 kA. I doubt your circuit breaker will allow that.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 15, 2018, 03:41:45 am
content: Well it is fairly obviously mistyping byBeamin, as the value of 0.12mA works out correctly for 1Megohm.My understanding is that the resistor will be in series with the transformer primary.The problem I can see is that a transformer using a random toroid may not be very useful at 60 Hz.Even if it works, it may not represent a true version of the input waveform.The error may be worse than just probing the "hot" side of the Mains with no ground clip connection,using the long connection back through the PE lead of the 'scope, the house wiring back to the PE/Neutralconnection & back again to the test point Neutral.Another way of looking at the Mains waveform is to use the " Pseudo differential " connection, using two vertical channels, & the A+B vertical function.You say you would need to order a transformer to observe the AC Mains waveform at its secondary, but you may be surprised how many transformers may be around the place.Old "wall warts" used transformers, some, indeed, have AC output.Even if you have to "open one up", bypass the rectifiers, & glue it back together, you have the advantage of a commercially made transformer.How about your soldering iron?Some feed low voltage AC to the heater, normally from a transformer.

author: Chriss
author_rank: Frequent Contributor
timestamp: October 15, 2018, 05:13:15 am
content: Why you not use a small transformer like something 120v/12v or even smaller and then you can connect it to the wall outlet on the main voltage and the secondary side of the transformer to the scope.This should do two things:1. you and your equipment will be safe because you are not touching into the main voltage directly2. The 60Hz will be shown on the scope for sure.But don't use a toroid transformer.Sent from my GT-I8260 using Tapatalk

author: Beamin
author_rank: Super Contributor
timestamp: October 15, 2018, 07:31:21 am
content: Quote from: vk6zgo on October 15, 2018, 03:41:45 amHoe did I mistype? I put in A into the formula other poster put in ma getting X1000 values out. No way 1meg would pull kA that doesn't even sound reasonable if it was true 1 ohm would be a million amps that's like lightning from your wall.Are you saying probe the neutral and the ground of the outlet and the ac will couple to the neutral?Torroids don't work at low frequencies? that's because of their flux?EDIT: Small m is milli M is Mega? I thought "mohm" was mega ohms? Do they make mili ohm? that would be a 1cm trace or shorter...

author: capt bullshot
author_rank: Super Contributor
timestamp: October 15, 2018, 08:32:09 am
content: Quote from: Beamin on October 15, 2018, 07:31:21 amYes M is Mega, and m is milli. So 1MOhm is quite a different thing than 1mOhm. And, yes, one can buy 1mOhm and 1MOhm resistors.Anyway, in theory your original idea should work, but not in practice. The outcome heavily depends on the magnetic core material, doesn't matter if it is toroid or anything else. You want a transformer with quite high magnetizing inductance to work on line frequency signals, and you won't get this with 10 turns on a randomly chosen core. One can build useful toroidal core transformers working at low frequencies, but this requires the "right" material and more turns than 10. You may want to read this thread:https://www.eevblog.com/forum/blog/eevblog-1104-omicron-labs-bode-100-teardown/25/

author: tooki
author_rank: Super Contributor
timestamp: October 15, 2018, 11:52:43 am
content: Quote from: Beamin on October 15, 2018, 07:31:21 ammOhm resistances are common in current measurement shunt resistors. For example, I recently used a 10mOhm for that. And in terms of needing to measure that, you can get instruments that measure 10nA (0.00001mOhm) resolution.And though they're very specialized devices, you can go on Digikey and order a 10GOhm resistor in-stock, and higher values by special order. I know I've seen 1TOhm devices for sale. And as resistance standards (for calibrating test gear, etc), you can get 10POhm — that's 10 million GOhm or 10 billion MOhm.Now, it's absolutely true that mOhm and MOhm form the ranges that cover everything used in probably 99.99% of electronics. But if we include what's actually used in specialty fields, the range is much larger.

author: langwadt
author_rank: Super Contributor
timestamp: October 15, 2018, 12:30:34 pm
content: Quote from: Chriss on October 15, 2018, 05:13:15 amand keep in mind that some transformers are build kinda marginal so they can distort the waveform at the upper end of their voltage range

author: Chriss
author_rank: Frequent Contributor
timestamp: October 15, 2018, 12:39:10 pm
content: Yea.That is true but I assume he don't need a very high accuracy.Actually yes.In this type of measuring should be taken in count the signal deviation maybe too...Sent from my GT-I8260 using Tapatalk

author: Beamin
author_rank: Super Contributor
timestamp: October 15, 2018, 01:49:39 pm
content: Quote from: Chriss on October 15, 2018, 12:39:10 pmShe not he. I'm using it to see if a power conditioner is audio phoolery or not so it has to be somewhat accurate.How do they make Gohm? Take a metal and sprinkle sinter it into ceramic? Or take carbon and add ceramic? I imagine a Tohm is just a piece of rubber with leads epoxied to the ends. What's the resistance of air over a similar distance? Maybe they pot it? Must be for measuring power lines.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 15, 2018, 02:15:43 pm
content: Quote from: Beamin on October 15, 2018, 01:49:39 pmIf it has to be somewhat accurate, for low budget I'd recommend a small power transformer rated for a 230V primary / e.g. 12V secondary and maybe 3VA (assuming your line voltage is 120V). So you'll stay out of the core saturation effects that happen when you use a power transformer at its rated voltage.Or if you know what you're doing, connect a 100:1 probe directly to the live wire.QuoteIf you want a GOhm resistor, you want a GOhm resistor of specified accuracy, so just a piece of air or rubber won't work. Usually such large value resistors are made by applying resistive coating to a ceramic substrate meandering a really long line, or many times around a cylindrical core. If you have such a resistor, don't touch it with bare fingers, as your fingerprints will lead to leakage, messing up the accuracy.For power lines at medium and high voltage levels (1kV and up), usually transformers or capacitive voltage dividers are used.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 16, 2018, 04:01:20 am
content: Quote from: Beamin on October 15, 2018, 07:31:21 amNo, I'm saying that the Neutral & ground are already connected together at the building's "entry point".The return path between the outlet's Neutral pin & the Oscilloscope chassis ground is via all the associated wiring, accounting for quite a lot of feet.(think of it as having a very long lead on your probe ground clip)If you now probe the Active (hot), without your ground clip connected to anything, (for safety, it is best to remove it entirely when you are using this method, in case it inadvertently touches a "hot"point), you will see the supply Mains waveform.If you have the DUT open, & have actually measured between the power lead plug's ground pin, & found a point which is definitely connected to that pin, you can clip the ground clip to that, but you still have the long return path to Neutral.Both of these methods can cause error, due to noise pickup, or just the inductance & resistance of all those many feet of cable.I would not mess with trying to clip to the socket earth, as there is no real advantage.A good rule is:-Treat both the Active & Neutral connections as if they are both "hot" w.r.t groundThis is good practice for those of us in nominally 230v single phase countries, but may save your life if you ever come across a North American "Split phase" 240v outlet.In these, there is no "Neutral", both (non-ground) pins are at 120v w.r.t ground.QuoteMore to do with them being designed to operate efficiently at different frequencies, depending upon their intended use.QuoteYep, when you come across the "small m" prefix, it means the unit you are using is the base unit divided by 1000, as in millimetre, millilitre, milliamperes, millivolts, & so on.The "Large M "prefix means you are using a unit which is 1000000 times the base unit, as in Megavolt, Megahertz, Megohm,etc.

author: Circlotron
author_rank: Super Contributor
timestamp: October 16, 2018, 04:20:28 am
content: Yeah, I'd use a 240V 100VA or so rated toroid across 120V mains, with a resistor load across the secondary dissipating about 1 watt for a bit of damping. Toroids like this often have a frequency response beyond 50KHz and very little phase shift so you'll get quite a good picture of what's going on. EI transformers with a split bobbin and segregated windings have a relatively poor frequency response.

author: Circlotron
author_rank: Super Contributor
timestamp: October 16, 2018, 11:06:26 am
content: If you want to listen for the presence or absence of power line noise, why not get a moderate size toroid transformer as mentioned, with a secondary voltage of say 20-40VAC and feed the secondary to a loudspeaker with a 1uFnon-polarisedcap in series with the speaker wire. The capacitor will stop most of the low frequency passing through to the speaker and only let through any high frequency noise and clicks and plops and tones. Experiment with the 1uF value. You might be surprised what you hear.

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2018, 02:06:49 pm
content: Quote from: Circlotron on October 16, 2018, 11:06:26 amThat's a good idea. If you don't have a non-polarised 1µF capacitor, then make one with two 2.2µF aluminium electrolytic capacitors connected back-to-back.A polarised capacitor, can be made by connecting two of the same value aluminium capacitors in series, with opposite polarities. The total capacitance is equal to half the total.

author: Beamin
author_rank: Super Contributor
timestamp: October 16, 2018, 02:26:00 pm
content: I thought about connecting the probe directly to the hot lead being mindful not to let the ground clip near anything but I thought for good practice in order to not pick up noise you used the ground lead. Therefore negating the results of my test.I should have asked this first: The current path through a scope is not high impedance like through a multimeter? When the scope is on 50 ohm setting is it like a 50 ohm resistor or like 50 ohm feedline? what is the scope at when not in the 50ohm setting? So hooking scope probe directly to hot and neutral, or inadvertently your ESD mats ground and the hot side of a power supply you are probing is like creating a dead short?

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2018, 03:39:28 pm
content: Quote from: Beamin on October 16, 2018, 02:26:00 pmOn all settings the oscilloscope's 0V at the same potential as mains earth. With the oscilloscope in the high impedance setting, it's possible to probe the phase conductor, with the ground clip disconnected, as long as the probe and oscilloscope both have the appropriate CAT rating.When the oscilloscope's input is set to 50Ohm, then its imped impedance is 50Ohms which is useful because a length of 50Ohms co-axial cable can be connected to its input, giving an input impedance of 50Ohms, up to a certain frequency beyond that of most instruments. It enables a low-Z probe to be made from a length of 50R co-axial cable and a 450R resistor, giving a total input impedance of 500R. This may seem like a lower impedance, than a standard x10 probe, with a DC impedance of 10M, but it has less capacitance and a higher impedance, above a few tens of MHz.The 50R termination resistor will only have a low power rating of a couple of Watts at most and will be damaged if it's connected to too higher voltage.

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 23, 2018, 05:29:24 pm
content: It does look like the best way you can get out of an ugly scenario like shorting the scope is to follow the idea above and look for comercially made transformers in your gear.As for future projects and experiments put an isolation transformer in your "to buy" list, I bet mine save my life more times than I can realize.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 24, 2018, 06:15:47 am
content: Quote from: Hero999 on October 16, 2018, 03:39:28 pmThe most common use for the 50ohm setting on a 'scope (where provided) is so you can look at the output of devices which have a 50ohm output impedance.This can be a device under test (DUT) the signal source, or a monitor point provided on a device with a different output impedance.Built in 50ohm terminations were not particularly common in the past, as a lot of Oscilloscopes were used with systems where the standard impedance was not 50ohm.For instance, analog video, where the standard impedance was 75ohm.In most cases, the 'scope as supplied had a 1Megohm input impedance, & external terminations were used.QuoteFor this reason, leave your 'scope in the high impedance mode when messing with possible high energy sources.

author: Beamin
author_rank: Super Contributor
timestamp: October 28, 2018, 10:49:33 am
content: Scope is a tek 2465 That I'm still learning...