Where to Place Voltmeter Ground Lead

[Adam60]

So you are testing a car audio amplifier for voltages on the board. Where do you place the ground lead. Chassis? Negative side of circuit?
What if it is a home audio amplifer? AC side? DC side?
I am curious on the answers that will come out as there is not a lot on the Internet that addresses this and I see it on other forums as well? Your thoughts or practices?

[ataradov]

You place it at the point from which a measured voltage is referenced. There is no single correct answer.

[Brumby]

The correct answer is "It depends on what voltage you want to measure."

Also, the "ground" lead isn't really a ground - I would say it is more correct to call it a "reference" lead.  It is very often connected to a "ground" point (or chassis or earth as the case may be) as this is a very common reference point.

This may sound like a bit of a cop out - but the truth is, you can put either leads anywhere you like on a circuit to measure any voltage you like....

BUT

To get meaningful measurements you have to know what it is you are measuring.  First thing to understand is that a voltage measurement is a difference in potential between two points.  One point is taken as a 'Reference' point and, by definition, is given the value of zero volts.  If you take the other probe from your meter and touch the same point, it's going to read 0.00 - pretty obvious.

If you have a circuit diagram - with voltages noted - the answer to your question is simple.  Find the section of the circuit diagram that you are interested in and locate a point where it says the voltage is shown as zero.  This will be your reference point - and where you place the voltmeter "ground" lead.

If you have a circuit diagram but it does not show voltages, it is most common to use the negative rail as the reference point.  This is not a universal piece of advice - as the whole measurement paradigm gets somewhat more involved if you have dual power rails, which can be found in a number of circuit topologies.  Op-amps are a classic.

When it comes to anything in a car, you are usually safe to reference your measurements with respect to the chassis.

In your home audio, this tends to be true - but it's not always safe to rely on that.  Firstly, let me state quite clearly, that if you are doing measurements on the audio circuitry, you do not want to put your reference lead ("ground" lead) anywhere near the AC side of things.  The audio circuitry may share a common connection with the AC - and that's OK - but if you are measuring the audio circuitry, it's best to keep the voltmeter's "ground" connection closer to audio circuitry.

It can be difficult to be all-encompassing with this advice - especially when some manufacturers can get rather creative in how they physically lay things out - but hopefully this will give you an idea.  Maybe some other members have a better way of describing things....


One very important piece of advice - don't measure mains on a meter that isn't rated for it.  All you need is one unkind surge and your meter could explode.

[IanB]

So you are testing a car audio amplifier for voltages on the board. Where do you place the ground lead. Chassis? Negative side of circuit?
What if it is a home audio amplifer? AC side? DC side?
I am curious on the answers that will come out as there is not a lot on the Internet that addresses this and I see it on other forums as well? Your thoughts or practices?

To help answer your question, you should appreciate that "voltages" do not exist in a circuit, there are only voltage differences. A voltmeter then measures a voltage difference between the red lead and the black lead with a + sign or a - sign according to which way round you connect the leads.

In order to measure a voltage you need to place the two test leads between two points in the circuit where an expected voltage difference exists. The location of these two points depends entirely on what you want to measure. To do this you need to know what you are testing and why. In most cases the two test points will be on the same circuit board.

[Adam60]

Brumby, so far I like your explanation best. I wanted to see what other people do and see how people answer this question because it is one that I think a lot of people have and are afraid to ask because it sounds so simple. I think a lot of people get confused when you have say an SMPS and you have to measure voltages and frequencies, where does the black lead of the meter go? You have Ac with a chassis ground and then DC on the other side. I have never seen many diagrams with a zero reference voltage noted, although I do see other voltages given without a reference point. I want this post to go on so other newbs can see this as well because I feel it is an important point. Thank you for your candid answers so far guys. Anyone else have any thoughts on this?

[IanB]

I have never seen many diagrams with a zero reference voltage noted, although I do see other voltages given without a reference point.

I have to believe that all diagrams with annotated voltages have a zero reference point marked or indicated. Otherwise it would not be possible to measure the voltages when troubleshooting. Perhaps you did not recognize where the reference point was shown?

Do you have any diagrams where you think the reference point is not shown that you could show us?

[jeroen79]

And usually there will be a symbol for ground or common somewhere.
And if it is not explicitly indicated you could pick the low DC side of the rectifier or the - of the battery.
In- and outputs will also often have a ground pin.

But you will have to know how the circuit is designed before you can make meaningful measurements.

And not all measurements are relative to ground.
For example you could measure the voltage over a resistor to determine how much current flows through it.
Or measure the voltage between the inputs of an opamp.