Electronics > Beginners
Would this work as an AC probe?
Circlotron:
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 1uF non-polarised cap 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.
Zero999:
--- Quote from: Circlotron on October 16, 2018, 11:06:26 am ---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 1uF non-polarised cap 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.
--- End quote ---
That'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.
Beamin:
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?
Zero999:
--- Quote from: Beamin on October 16, 2018, 02:26:00 pm ---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?
--- End quote ---
On 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.
AngraMelo:
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.
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