Electronics > Beginners

Would this work as an AC probe?

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Chriss:
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...

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Beamin:

--- Quote from: Chriss on October 15, 2018, 12:39:10 pm ---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

--- End quote ---

She 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.

capt bullshot:

--- Quote from: Beamin on October 15, 2018, 01:49:39 pm ---She not he. I'm using it to see if a power conditioner is audio phoolery or not so it has to be somewhat accurate.

--- End quote ---
If 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.


--- Quote ---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.

--- End quote ---
If 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.

vk6zgo:

--- Quote from: Beamin on October 15, 2018, 07:31:21 am ---Are you saying probe the neutral and the ground of the outlet and the ac will couple to the neutral?

--- End quote ---
No, 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 ground
This 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.
--- Quote ---
Toroids don't work at low frequencies? that's because of their flux?

--- End quote ---
More to do with them being designed to operate efficiently at different frequencies, depending upon their intended use.

--- Quote --- 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...

--- End quote ---
Yep, 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.

Circlotron:
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.

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