How to check live voltage value in circuits with an analog/digital meter. - Page 1
Forum section: electronics / beginners

author: Serge125
author_rank: Regular Contributor
timestamp: February 11, 2020, 02:19:52 am
content: Hi all and Mr. EEVblog. For as long as I can remember I would like learn how to check voltage on a live circuit but I've looked every where in books, vids etc.... for any detail on how to do this and I just can't find this. I remembering a long time ago I did poked on a live circuit and fried something and since then I have never poked on live circuit because I'm afraid to short something and make things worse. Mr EEVblog you have a MASSIVE vid collection on YT, but I don't know if you have one that explains how to check live voltage values on circuit boards BUT if you do can you tell me the link so that I can watch it. I wanted to fix an old Amiga HD and I asked what I could do and one guy said to check voltage at a certain part for a 5V signal and I told him that I didn't know how to do this and he never answered me. I REALY like to fix things and electronics is something that I truly enjoy doing (when I know how to do it on some problems) but when I ask people for help they just ignore me and I thought that in a blog we are all here to help and learn. I hope this place I will finally get some answers because up to now I never did find any where that wanted to help me so that I can grow my knowledge of electronic repair. Thanks for reading and hope to get news from you all!!Serge

author: andy3055
author_rank: Super Contributor
timestamp: February 11, 2020, 03:55:36 am
content: May be you can learn from one of these videos:https://www.youtube.com/results?search_query=how+to+use+a+multimeter

author: AVGresponding
author_rank: Super Contributor
timestamp: February 11, 2020, 06:52:21 pm
content: Without looking through all those videos, I can't say for sure, but there are a few things that are often not mentioned in these types of tutorial.1. Use sharp probes! Blunt probes will slip while you are trying to measure something and at best annoy you, at worst, short something out and damage it.2. Use shrouded probe tips; these minimise the risk of accidentally bridging conductors/component legs.3. On boards with a high density of components/traces (likely on something like an Amiga HD, old as it is), it might be a good idea to try and trace out the point you want to measure to a more accessible point, assuming it's not a designed test point. Failing that, it might be possible to solder a fine wire to the point you need the live measurement from, so that you can do it without risking a costly probe slip.4. Use antistatic precautions! Most components are safe from the levels of ESD you might find in a hobbyists workshop while they are in circuit,but not all of them. If you're not sure, treat it as sensitive and needing protection. Some complete circuits are static sensitive also. Technically all semiconductors are sensitive, but usually discrete components like BJTs for example have layers so thick the damage is insignificant unless extreme and/or repetitive. Still it's good practice to always follow antistatic procedures, and won't hurt the DUT.Happy testing!

author: SL4P
author_rank: Super Contributor
timestamp: February 12, 2020, 02:12:38 am
content: At a very fundamental level, any voltage you will measure is at a potential with reference to some other point of the circuit...Usually 0V, or often that’s tied to GROUND, but  you can’t guarantee that.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.

author: pcmad
author_rank: Regular Contributor
timestamp: February 12, 2020, 02:18:40 am
content: when working on live i always use an isolation transformer

author: Brumby
author_rank: Unknown
timestamp: February 12, 2020, 07:01:32 am
content: Quote from: pcmad on February 12, 2020, 02:18:40 amLet'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 amTHIS 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:Whereare 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 thereference pointfor taking measurements, but knowing where this spot is can sometimes be tricky.QuoteYes. 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

author: Serge125
author_rank: Regular Contributor
timestamp: February 12, 2020, 09:28:48 am
content: Quote from: andy3055 on February 11, 2020, 03:55:36 amOH 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

author: rstofer
author_rank: Super Contributor
timestamp: February 12, 2020, 05:36:39 pm
content: 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!

author: Serge125
author_rank: Regular Contributor
timestamp: February 12, 2020, 06:34:30 pm
content: Quote from: rstofer on February 12, 2020, 05:36:39 pmThanks 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!!!

author: AVGresponding
author_rank: Super Contributor
timestamp: February 12, 2020, 06:38:49 pm
content: Quote from: Serge125 on February 12, 2020, 06:34:30 pmAutorange 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.