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How to check live voltage value in circuits with an analog/digital meter.

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Brumby:

--- Quote from: pcmad on February 12, 2020, 02:18:40 am ---when working on live i always use an  isolation transformer

--- End quote ---
Let's not get too carried away with the use of the word "live".  The OP is not necessarily talking about measuring mains voltages.  He is talking about a circuit that has power applied - which could be anything from a coin cell up.  He is simply asking one of the most basic questions possible - and there's only one person so far that has come close to answering it!


--- Quote from: SL4P on February 12, 2020, 02:12:38 am ---At a very fundamental level, any voltage you will measure is at a potential with reference to some other point of the circuit...
--- End quote ---
THIS is where there is a start to answering the question!

Rephrasing the above, I would say it this way.....
When measuring voltage with a DMM, once you select the appropriate range, you place the red probe on one point and the black probe on another.  As I see it, what is being asked is: Where are these two points?
Now, the point for the red probe is pretty straightforward, it will be the place where you want to measure the voltage - but this is absolutely useless unless you place the black probe on the right spot.  It is the reference point for taking measurements, but knowing where this spot is can sometimes be tricky.


--- Quote ---Usually 0V, or often that’s tied to GROUND, but [WARNING] you can’t guarantee that.
--- End quote ---
Yes.  The usual reference point can be labelled 0V - or it can be implied by use of a symbol to indicate a chassis, ground or "earth" connection.  (NOTE: The terms "earth" and "ground" do not always mean a connection to the soil under your feet through the earth pin of your mains socket - and thinking too hard around this idea can get you into some very confusing territory.  In most cases when it comes to voltage measurements, it is best to forget about that detail, as it usually doesn't help.)

Once you've got the basics sorted, the following are just some of the other points you will need to understand:

--- Quote ---Consider that some voltages may be AC, so your readings, method and range of measurement will be different from DC measurements.

And finally, do your research on WHAT you’re probing. mains-level voltages (AC or DC), can kill you in a dozen different ways.

--- End quote ---

Serge125:

--- Quote from: andy3055 on February 11, 2020, 03:55:36 am ---May be you can learn from one of these videos:

https://www.youtube.com/results?search_query=how+to+use+a+multimeter

--- End quote ---

OH WOW thanks!!! I never checked how to use a meter because I knew (at a certain point) how to use it so I never bothered to check those vids. When I saw what you sent I decided to check them out and gee I should of checked them out a long time ago. Again THANKS for the link and HELP!!!

Serge

rstofer:
Sometimes, especially in electronic circuits, 0V doesn't even enter into the conversation.  I might just want to know the voltage drop across a component and nothing about it is referenced to 0V.  Suppose I want to measure the current flow through a resistor.  I could unwire the resistor and insert an ammeter (DMM set to measure current) but this is too much work.  Better to just measure the voltage drop across the resistor and calculate the current.

I see a great future in measuring battery powered circuits.  No high voltages, no high currents, just simple electronic circuits powered by low energy sources.

Before you probe a circuit, double check the range switch.  There's nothing quite like stuffing probes into mains with the dial set on Amps and the probes in the wrong socket on the DMM.  Know what voltage/current you expect before you start probing.  If you're not sure, don't do it!

Serge125:

--- Quote from: rstofer on February 12, 2020, 05:36:39 pm ---Before you probe a circuit, double check the range switch.  There's nothing quite like stuffing probes into mains with the dial set on Amps and the probes in the wrong socket on the DMM.  Know what voltage/current you expect before you start probing.  If you're not sure, don't do it!

--- End quote ---

Thanks for the info but some DMM have auto range if I'm right. If I'm not sure of the voltage that I might expect I will set it on a high setting just to make sure and go down and recheck. Most of the time I don't have schematics so it's a guessing game. After seeing a few vids I'm gonna be more sure to check than I used to BUT if I'm not sure I'm not gonna do it and come back here and just ask for help.

THANKS!!!

AVGresponding:

--- Quote from: Serge125 on February 12, 2020, 06:34:30 pm ---
--- Quote from: rstofer on February 12, 2020, 05:36:39 pm ---Before you probe a circuit, double check the range switch.  There's nothing quite like stuffing probes into mains with the dial set on Amps and the probes in the wrong socket on the DMM.  Know what voltage/current you expect before you start probing.  If you're not sure, don't do it!

--- End quote ---

Thanks for the info but some DMM have auto range if I'm right. If I'm not sure of the voltage that I might expect I will set it on a high setting just to make sure and go down and recheck. Most of the time I don't have schematics so it's a guessing game. After seeing a few vids I'm gonna be more sure to check than I used to BUT if I'm not sure I'm not gonna do it and come back here and just ask for help.

THANKS!!!

--- End quote ---

Autorange won't save you if you have selected A/mA/uA instead of V DC/V AC.

At best, the meter's fuse will pop and/or the DUT will hiccup or fail, at worst, the meter will explode (if it's a cheap and nasty one, and there's enough energy in the circuit).

EDIT: In other words, check, then double check that you have the leads in the correct sockets of the meter, and the meter is on the correct measurement type.

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